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Tom
 
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Default 11degC across Boiler, why?

Most manufacturers seem to recommend setting flow rates in CH systems to
achieve a rise of 11deg across boiler, why is this? is it the most efficient
setting for heat transfer?
Tom


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Ian Stirling
 
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Tom wrote:
Most manufacturers seem to recommend setting flow rates in CH systems to
achieve a rise of 11deg across boiler, why is this? is it the most efficient
setting for heat transfer?


It's a compromise.
For condensing boilers, you want the exhaust output temperature to be as low
as possible, otherwise the efficiency will drop as it won't condense properly.

On the other hand, you've got to get temperature high enough to heat
radiators and central heating.
So, the smaller the temperature rise, the higher the efficiency.
Even for non-condensing boilers, for non-pressurised systems, the
boiler can't get past some 90C or so, otherwise you get localised
boiling, which does all sorts of nasty things.

Both of these mean that you want the temperature drop across the
boiler to be relatively small.
  #3   Report Post  
Andy Hall
 
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Default

On Sat, 18 Sep 2004 16:17:55 +0100, "Tom"
wrote:

Most manufacturers seem to recommend setting flow rates in CH systems to
achieve a rise of 11deg across boiler, why is this? is it the most efficient
setting for heat transfer?
Tom



This is the case for conventional, non-condensing boilers and
represents the optimum for the heat exchanger. In fact the general
design rule is to have 82 degree flow and 70 degree return.
This also avoids the risk of the boiler entering condensing mode,
which for a conventional boiler would result in corrosion.

For condensing boilers the heat exchanger can work with a temperature
drop of 20 or even 25 degrees and is optimal at as low a temperature
as possible -i.e. efficiiency is maximised at lower temperatures.
..andy

To email, substitute .nospam with .gl
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Tom
 
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"Andy Hall" wrote in message
...
On Sat, 18 Sep 2004 16:17:55 +0100, "Tom"
wrote:

Most manufacturers seem to recommend setting flow rates in CH systems to
achieve a rise of 11deg across boiler, why is this? is it the most

efficient
setting for heat transfer?
Tom



This is the case for conventional, non-condensing boilers and
represents the optimum for the heat exchanger. In fact the general
design rule is to have 82 degree flow and 70 degree return.
This also avoids the risk of the boiler entering condensing mode,
which for a conventional boiler would result in corrosion.

snipped

Thanks for that, The background to my question is that I'm contemplating
redesigning my CH system in a bungalow I moved into some 6 Months ago; since
4 of the 6 rooms are subjected to high solar gain at different times of the
day I'm attracted towards using TRVs on the rads in the 4 rooms which make
up approx 80% of the total heat load.
I'm further minded to install a Grundfos Alpha pump, however, I'm undecided
as to the control mechanism i.e. a flow switch as advocated by IMM in a
previous thread or to rely on the boiler stat set to 82degC as you stated.
I'm anxious to avoid short cycling of the boiler.
I'd be gratefull for your counsel.
Regards Tom
..


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IMM
 
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Default


"Tom" wrote in message
...

Most manufacturers seem to recommend setting flow rates in CH systems to
achieve a rise of 11deg across boiler, why is this? is it the most

efficient
setting for heat transfer?
Tom


The prime aim in regular boilers is to avoid condensation in the burner box.
Regular boilers should always be set as high as possible, and "never" below
65C flow temp. Some regular boilers only had a high limit stat and the flow
temp pre-set.





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IMM
 
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"Tom" wrote in message
...

Most manufacturers seem to recommend setting flow rates in CH systems to
achieve a rise of 11deg across boiler, why is this? is it the most

efficient
setting for heat transfer?


It is the most efficient for the heat exchanger design which will be a low
priced one. Having the heat exchanger within a tight temp range is cheaper
and quicker to produce. That is why the expensive condensing boilers have
25C across the heat exchanger as the it is made to a higher spec and with
superior manufacture and materials. They can stretch the temp range across
it.


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Andy Hall
 
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On Sun, 19 Sep 2004 08:32:57 +0100, "Tom"
wrote:


"Andy Hall" wrote in message
.. .
On Sat, 18 Sep 2004 16:17:55 +0100, "Tom"
wrote:

Most manufacturers seem to recommend setting flow rates in CH systems to
achieve a rise of 11deg across boiler, why is this? is it the most

efficient
setting for heat transfer?
Tom



This is the case for conventional, non-condensing boilers and
represents the optimum for the heat exchanger. In fact the general
design rule is to have 82 degree flow and 70 degree return.
This also avoids the risk of the boiler entering condensing mode,
which for a conventional boiler would result in corrosion.

snipped

Thanks for that, The background to my question is that I'm contemplating
redesigning my CH system in a bungalow I moved into some 6 Months ago; since
4 of the 6 rooms are subjected to high solar gain at different times of the
day I'm attracted towards using TRVs on the rads in the 4 rooms which make
up approx 80% of the total heat load.
I'm further minded to install a Grundfos Alpha pump, however, I'm undecided
as to the control mechanism i.e. a flow switch as advocated by IMM in a
previous thread or to rely on the boiler stat set to 82degC as you stated.
I'm anxious to avoid short cycling of the boiler.
I'd be gratefull for your counsel.
Regards Tom
.

With an Alpha pump together with TRVs the water flow aspect will be
taken care of.

You would need anyway to put a room thermostat in a room or space
where the radiator has no TRV (or you could fit one and turn it fully
on). If this is to be a living room, then it mustn't have another
source of heat like a gas fire.

The aim is tha tthe TRVs in the other rooms should begin to close and
reduce the heat load. Remember that these are quite analogue in
nature - they don't just suddenly cut off.
The room thermostat is then set just above this point, the idea being
that the other rooms come up to temperature slightly before the heat
source is cut off.
You can tweak the flow to the radiator in the room where the
thermostat is, to adjust all of this as well.

As the load requirement falls, the boiler will cycle or cycle more,
inevitably. The trick is to make sure that this does not go on for
too long.


It does take some experimentation, so one good way if you are not sure
of which room to use for the thermostat is to use an RF one and move
it around.


The flow switch idea does have its uses in that it could be used to
shut down the boiler and pump when the flow reduces. However, the
inexpensive ones with a magnet and reed switch are not very accurate.
You can adjust their effect a bit by putting a bypass around the
valve, but really they are intended for sensing little flow or no
flow. For example I have one for my secondary heating circuit for
the workshop. The pump and controls are in the workshop and I
didn't want any electrical signalling connections to the house. The
workshop circuit is heated via a heat exchanger from the main system
and has a zone valve to allow water from the main system to the
primary side of the exchanger. There is a flow switch in the
secondary side in the house which detects when the pump is running (at
all) and opens the valve. I'm not trying to sense low flow with it.

So this could be useful, but I would try the thermostat idea first.


..andy

To email, substitute .nospam with .gl
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Tom
 
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Default


"Andy Hall" wrote in message
...
On Sun, 19 Sep 2004 08:32:57 +0100, "Tom"
wrote:

snipped


The flow switch idea does have its uses in that it could be used to
shut down the boiler and pump when the flow reduces. However, the
inexpensive ones with a magnet and reed switch are not very accurate.
You can adjust their effect a bit by putting a bypass around the
valve, but really they are intended for sensing little flow or no
flow. For example I have one for my secondary heating circuit for
the workshop. The pump and controls are in the workshop and I
didn't want any electrical signalling connections to the house. The
workshop circuit is heated via a heat exchanger from the main system
and has a zone valve to allow water from the main system to the
primary side of the exchanger. There is a flow switch in the
secondary side in the house which detects when the pump is running (at
all) and opens the valve. I'm not trying to sense low flow with it.


Much obliged for your detailed reply,

I've been mulling over this flow switch idea and had mused on using it to
shut down the boiler and not the Alpha in the hope that when a TRV next
opened, the Alpha would increase it's flow against the reduced resistance,
the flow switch consequently switching the boiler back on. I assume the
Alpha uses little power when throttled down so to speak.
I realise that this would require careful balancing of the system
perhaps incorporating a permanently connected pressure gauge on the pump
outlet and manual throttling valve(s) to facilitate balancing.
What do you think?
Interesting use of your flow switch to separate the electrics from the shed,
I like it.
Thanks
Tom




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Set Square
 
Posts: n/a
Default

In an earlier contribution to this discussion,
Tom wrote:

Most manufacturers seem to recommend setting flow rates in CH systems
to achieve a rise of 11deg across boiler, why is this? is it the most
efficient setting for heat transfer?
Tom


As others have stated, many systems are designed to run with a flow
temperature of 82 degC and a return temperature of 71 degC.

But why these particular values, I hear you cry? Well, it just so happens
that these correspond to 180 and 160 degF - with a 20 degree drop - all nice
round numbers!

So the whole thing dates from the time when temperatures were measured in
Fahrenheit, and energy in good old Britsh Thermal Units. [Wait for the
reaction to that one!] The values have simply been converted to Celsius.
--
Cheers,
Set Square
______
Please reply to newsgroup. Reply address is invalid.


  #10   Report Post  
Malcolm Reeves
 
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Default

On Sun, 19 Sep 2004 08:32:57 +0100, "Tom"
wrote:

I'm attracted towards using TRVs on the rads in the 4 rooms which make
up approx 80% of the total heat load.
I'm further minded to install a Grundfos Alpha pump, however, I'm undecided
as to the control mechanism i.e. a flow switch as advocated by IMM in a
previous thread or to rely on the boiler stat set to 82degC as you stated.
I'm anxious to avoid short cycling of the boiler.



You should have TRV on at least all the bedroom rads which is the new
rules AFAIR. Ideally all rooms should have TRVs except one (the
lounge?) and that room should have the thermostat that turns the
system on/off.

The optimum system is where the water comes back to the boiler at just
above the condensing level for non-condensing boilers or below for
condensing. If condensing boiler return temperature is above then it
doesn't condense and so the extra money you paid for it is wasted as
it's no more efficient than a standard boiler. If the temperature is
too low on a non-condensing boiler then you get condensing which
corrodes the heat exchanger. I have been told that 56C is the
condensing threshold.

The normal flow and return is 80C out, 70C back (in round numbers,
82C, 71C if you want, in F it was 180F out, 160F back, rads 100F above
room temperature). So rads will be at 75C, about 55C above room
temperature. If you run which a larger drop, say 80C out, 60C back,
then the rads are at 70C. That 5C difference is significant (see rad
makers for curves). The rad could be 10% down on output. Hence for a
condensing boiler system you need larger radiators.

I wouldn't go with an alpha pump with a standard boiler. Think about
it. An alpha pump slows down the flow so as the TRVs cut in the flow
back to the boiler is reduced. Water spends more time in the rads so
comes back colder. The risk is you get condensing. What I would go
for is a standard pump + an automatic bypass (which is cheaper
anyway). Then as the TRVs cut in the hot water gets fed back with the
cold from the rads and the return temperature flow goes up - no risk
of condensing.

Of course as the return flow temperature rises the efficiency goes
down but the trade off is that you can set the normal flow for a
larger drop (assuming you have the rad area to cope) and know that the
return temperature can only go up.

The effect for short cycling is the same. For a bypass the flow is
constant so temperature rise across the boiler is constant. It's the
return temperature that rises. Exceed the limit before the thermostat
cuts in and the boiler short cycles. For an alpha the flow drops so
the rise across the boiler increases. Again exceed the limit and the
boiler short cycles. It's all down to the boiler outputting X kW and
the house only needing X kW. The way to lengthen the cycle time is
to have enough heat capacity (water and rad metal) to absorb the X kW
until the main thermostat cuts in. That's all about having a balanced
system.

BTW this assumes that you don't have a modulating boiler (i.e. one
that can vary the output power and so doesn't short cycle). I think
some as boilers are as it is easier to do on gas. Most oil boilers
aren't.


--

Malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
, or ).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

NEW - Desktop ToDo/Reminder program (free)


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Tom
 
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"Malcolm Reeves" wrote in message
...

You should have TRV on at least all the bedroom rads which is the new
rules AFAIR. Ideally all rooms should have TRVs except one (the
lounge?) and that room should have the thermostat that turns the
system on/off.


Fine, Ok to that, existing thermostat in hallway, a non-TRV area.
Lounge has additional heat source(SWMBO requirement) + high Solar Gain
,hence TRV.

The optimum system is where the water comes back to the boiler at just
above the condensing level for non-condensing boilers or below for
condensing. If condensing boiler return temperature is above then it
doesn't condense and so the extra money you paid for it is wasted as
it's no more efficient than a standard boiler. If the temperature is
too low on a non-condensing boiler then you get condensing which
corrodes the heat exchanger. I have been told that 56C is the
condensing threshold.


Agreed, I remember in the dim and distant past a HVAC consultant quoting
that figure at me in work.

The normal flow and return is 80C out, 70C back (in round numbers,
82C, 71C if you want, in F it was 180F out, 160F back, rads 100F above
room temperature). So rads will be at 75C, about 55C above room
temperature. If you run which a larger drop, say 80C out, 60C back,
then the rads are at 70C. That 5C difference is significant (see rad
makers for curves). The rad could be 10% down on output. Hence for a
condensing boiler system you need larger radiators.


Agreed, no problem with that, makes sense.

I wouldn't go with an alpha pump with a standard boiler. Think about
it. An alpha pump slows down the flow so as the TRVs cut in the flow
back to the boiler is reduced. Water spends more time in the rads so
comes back colder. The risk is you get condensing.


Yessss, I hadn't thought about that, getting too old in the tooth I think!!!

What I would go for is a standard pump + an automatic bypass (which is

cheaper
anyway). Then as the TRVs cut in the hot water gets fed back with the
cold from the rads and the return temperature flow goes up - no risk
of condensing.


OK.--The automatic bypass, do these work in an analogue fashion, do they
have differing pressure/resistance curves or are they on/off devices.

Of course as the return flow temperature rises the efficiency goes
down but the trade off is that you can set the normal flow for a
larger drop (assuming you have the rad area to cope) and know that the
return temperature can only go up.

The effect for short cycling is the same. For a bypass the flow is
constant

Implying analogue characteristicts?

so temperature rise across the boiler is constant. It's the
return temperature that rises. Exceed the limit before the thermostat
cuts in and the boiler short cycles. For an alpha the flow drops so
the rise across the boiler increases. Again exceed the limit and the
boiler short cycles. It's all down to the boiler outputting X kW and
the house only needing X kW. The way to lengthen the cycle time is
to have enough heat capacity (water and rad metal) to absorb the X kW
until the main thermostat cuts in. That's all about having a balanced
system.

Agreed again.

BTW this assumes that you don't have a modulating boiler (i.e. one
that can vary the output power and so doesn't short cycle). I think
some as boilers are as it is easier to do on gas. Most oil boilers
aren't.

Unfortunately I don't have a modulating or condensing boiler.

Much obliged to you for your detailed critique, I will be looking at bypass
systems I think. I inherited the boiler (Gloworm Hideaway 60B FF) with the
bungalow I moved into 6 Months ago, I shall use the existing pump which will
save me a bit.


Best Regards
Tom


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Andy Hall
 
Posts: n/a
Default

On Sun, 19 Sep 2004 12:55:42 +0100, Malcolm Reeves
wrote:

On Sun, 19 Sep 2004 08:32:57 +0100, "Tom"
wrote:

I'm attracted towards using TRVs on the rads in the 4 rooms which make
up approx 80% of the total heat load.
I'm further minded to install a Grundfos Alpha pump, however, I'm undecided
as to the control mechanism i.e. a flow switch as advocated by IMM in a
previous thread or to rely on the boiler stat set to 82degC as you stated.
I'm anxious to avoid short cycling of the boiler.



You should have TRV on at least all the bedroom rads which is the new
rules AFAIR. Ideally all rooms should have TRVs except one (the
lounge?) and that room should have the thermostat that turns the
system on/off.

The optimum system is where the water comes back to the boiler at just
above the condensing level for non-condensing boilers or below for
condensing.


That isn't true. Non-condensing boilers are designed to operate most
efficiently at 82/70 flow return.


If condensing boiler return temperature is above then it
doesn't condense and so the extra money you paid for it is wasted as
it's no more efficient than a standard boiler.


That isn't quite true either.

A condensing boiler's efficiency doesn't suddenly improve as if a Holy
Grail has been found at the dew point.

WHat happens is that the efficiency increases with decrease in
temperature both above and below the dew point. When condensing
commences at the dew point, all that happens is that the *rate* of
increase of efficiency increases with falling temperature.

At a flow of 82 degrees, a condensing boiler will be more efficient
than a conventional one anyway because the heat exchanger is larger.

If the temperature is
too low on a non-condensing boiler then you get condensing which
corrodes the heat exchanger. I have been told that 56C is the
condensing threshold.

The normal flow and return is 80C out, 70C back (in round numbers,
82C, 71C if you want, in F it was 180F out, 160F back, rads 100F above
room temperature). So rads will be at 75C, about 55C above room
temperature. If you run which a larger drop, say 80C out, 60C back,
then the rads are at 70C. That 5C difference is significant (see rad
makers for curves). The rad could be 10% down on output. Hence for a
condensing boiler system you need larger radiators.


That is also untrue.

You can replace a conventional boiler with a condensing one. When
required, in the depths of winter the condensing boiler can run up to
the 82 degree level of the conventional one and still be more
efficient. For most of the heating season, the condensing boiler will
modulate down to a lower output and into an even more efficient
working range.

It is true that system efficency can be improved by using larger
radiators, but it isn't essential.

In terms of derating, the manufacturers tables are too high for
typical UK boilers.

The normal derating factors are to multiply by 0.89 for an 82/70
boiler and by 0.6 for a 70/50 design with a condensing boiler.




I wouldn't go with an alpha pump with a standard boiler. Think about
it. An alpha pump slows down the flow so as the TRVs cut in the flow
back to the boiler is reduced. Water spends more time in the rads so
comes back colder. The risk is you get condensing. What I would go
for is a standard pump + an automatic bypass (which is cheaper
anyway). Then as the TRVs cut in the hot water gets fed back with the
cold from the rads and the return temperature flow goes up - no risk
of condensing.


This is precisely where an Alpha pump is intended to be used. An
automatic bypass is intended to deal with the case where both CH and
DHW demand has ceased altogether and the boiler has been firing at
full tilt. It simply prevents the water from boiling and nothing
more.

The TRVs do not stop the flow completely if adjusted correctly in
relation to the radiator in the non-TRV room. The object is to turn
off the boiler via the room thermostat as all rooms have arrived at
set point, not for the TRVs to have closed completely.
Provided that the boiler thermostat is adjusted correctly, operating
temperatures will also be correctly in range.



Of course as the return flow temperature rises the efficiency goes
down but the trade off is that you can set the normal flow for a
larger drop (assuming you have the rad area to cope) and know that the
return temperature can only go up.

The effect for short cycling is the same. For a bypass the flow is
constant so temperature rise across the boiler is constant. It's the
return temperature that rises. Exceed the limit before the thermostat
cuts in and the boiler short cycles. For an alpha the flow drops so
the rise across the boiler increases. Again exceed the limit and the
boiler short cycles. It's all down to the boiler outputting X kW and
the house only needing X kW. The way to lengthen the cycle time is
to have enough heat capacity (water and rad metal) to absorb the X kW
until the main thermostat cuts in. That's all about having a balanced
system.

BTW this assumes that you don't have a modulating boiler (i.e. one
that can vary the output power and so doesn't short cycle). I think
some as boilers are as it is easier to do on gas. Most oil boilers
aren't.



..andy

To email, substitute .nospam with .gl
  #13   Report Post  
Andy Hall
 
Posts: n/a
Default

On Sun, 19 Sep 2004 10:38:33 +0100, "Tom"
wrote:


"Andy Hall" wrote in message
.. .
On Sun, 19 Sep 2004 08:32:57 +0100, "Tom"
wrote:

snipped


The flow switch idea does have its uses in that it could be used to
shut down the boiler and pump when the flow reduces. However, the
inexpensive ones with a magnet and reed switch are not very accurate.
You can adjust their effect a bit by putting a bypass around the
valve, but really they are intended for sensing little flow or no
flow. For example I have one for my secondary heating circuit for
the workshop. The pump and controls are in the workshop and I
didn't want any electrical signalling connections to the house. The
workshop circuit is heated via a heat exchanger from the main system
and has a zone valve to allow water from the main system to the
primary side of the exchanger. There is a flow switch in the
secondary side in the house which detects when the pump is running (at
all) and opens the valve. I'm not trying to sense low flow with it.


Much obliged for your detailed reply,

I've been mulling over this flow switch idea and had mused on using it to
shut down the boiler and not the Alpha in the hope that when a TRV next
opened, the Alpha would increase it's flow against the reduced resistance,
the flow switch consequently switching the boiler back on. I assume the
Alpha uses little power when throttled down so to speak.
I realise that this would require careful balancing of the system
perhaps incorporating a permanently connected pressure gauge on the pump
outlet and manual throttling valve(s) to facilitate balancing.
What do you think?


I doubt if you need to make it that complicated. As long as the
boiler output is not hugely larger than the house requirement, simply
using a room thermostat, TRVs and an Alpha should give a good result.

If you get the radiator without the TRV adjusted to the correct
operating point such that the TRVs are not completely closing as the
room thermostat operates, you will get the desired effect.

Another approach would be to get one of the optimising room
thermostats which controls the boiler and prevents temperature
overshoot - e.g. CM67 or TP75. This will give a more controlled
result than a simple thermostat.


..andy

To email, substitute .nospam with .gl
  #14   Report Post  
Andy Hall
 
Posts: n/a
Default

On Sun, 19 Sep 2004 13:59:34 +0100, "Tom"
wrote:


"Malcolm Reeves" wrote in message
.. .

You should have TRV on at least all the bedroom rads which is the new
rules AFAIR. Ideally all rooms should have TRVs except one (the
lounge?) and that room should have the thermostat that turns the
system on/off.


Fine, Ok to that, existing thermostat in hallway, a non-TRV area.
Lounge has additional heat source(SWMBO requirement) + high Solar Gain
,hence TRV.


You might find that you get smoother results if you put it on the
upstairs landing. It will be less suscepitble to the effects of
opening the front door.



The optimum system is where the water comes back to the boiler at just
above the condensing level for non-condensing boilers or below for
condensing. If condensing boiler return temperature is above then it
doesn't condense and so the extra money you paid for it is wasted as
it's no more efficient than a standard boiler. If the temperature is
too low on a non-condensing boiler then you get condensing which
corrodes the heat exchanger. I have been told that 56C is the
condensing threshold.


Agreed, I remember in the dim and distant past a HVAC consultant quoting
that figure at me in work.


It's around 52-54 degrees.



The normal flow and return is 80C out, 70C back (in round numbers,
82C, 71C if you want, in F it was 180F out, 160F back, rads 100F above
room temperature). So rads will be at 75C, about 55C above room
temperature. If you run which a larger drop, say 80C out, 60C back,
then the rads are at 70C. That 5C difference is significant (see rad
makers for curves). The rad could be 10% down on output. Hence for a
condensing boiler system you need larger radiators.


Agreed, no problem with that, makes sense.


See separate note on derating of radiator specs.


I wouldn't go with an alpha pump with a standard boiler. Think about
it. An alpha pump slows down the flow so as the TRVs cut in the flow
back to the boiler is reduced. Water spends more time in the rads so
comes back colder. The risk is you get condensing.


Yessss, I hadn't thought about that, getting too old in the tooth I think!!!

What I would go for is a standard pump + an automatic bypass (which is

cheaper
anyway). Then as the TRVs cut in the hot water gets fed back with the
cold from the rads and the return temperature flow goes up - no risk
of condensing.


OK.--The automatic bypass, do these work in an analogue fashion, do they
have differing pressure/resistance curves or are they on/off devices.


They are pretty much on/off and are intended to deal with the case
where DHW and CH both stop demanding and the boiler is in full burn.
The setting should be arranged so that the valve is closed in all
cases exept where this happens. The idea is *not* to allow any
feedback of flow water to the boiler except under these conditions
when the only purpose is to keep a flow going and to prevent boiling.



Of course as the return flow temperature rises the efficiency goes
down but the trade off is that you can set the normal flow for a
larger drop (assuming you have the rad area to cope) and know that the
return temperature can only go up.

The effect for short cycling is the same. For a bypass the flow is
constant

Implying analogue characteristicts?

so temperature rise across the boiler is constant. It's the
return temperature that rises. Exceed the limit before the thermostat
cuts in and the boiler short cycles. For an alpha the flow drops so
the rise across the boiler increases. Again exceed the limit and the
boiler short cycles. It's all down to the boiler outputting X kW and
the house only needing X kW. The way to lengthen the cycle time is
to have enough heat capacity (water and rad metal) to absorb the X kW
until the main thermostat cuts in. That's all about having a balanced
system.

Agreed again.

BTW this assumes that you don't have a modulating boiler (i.e. one
that can vary the output power and so doesn't short cycle). I think
some as boilers are as it is easier to do on gas. Most oil boilers
aren't.

Unfortunately I don't have a modulating or condensing boiler.

Much obliged to you for your detailed critique, I will be looking at bypass
systems I think. I inherited the boiler (Gloworm Hideaway 60B FF) with the
bungalow I moved into 6 Months ago, I shall use the existing pump which will
save me a bit.


You can do that provided that the pump is set to compromise with the
requirements of full heat demand and at the point when the TRVs are
closing. You also need to arrange that the room thermostat is
turning off the boiler when there is still some flow left through the
TRVs.


Best Regards
Tom


..andy

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  #15   Report Post  
Tom
 
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"Andy Hall" wrote in message
...
On Sun, 19 Sep 2004 13:59:34 +0100, "Tom"
wrote:


"Malcolm Reeves" wrote in message
.. .

You should have TRV on at least all the bedroom rads which is the new
rules AFAIR. Ideally all rooms should have TRVs except one (the
lounge?) and that room should have the thermostat that turns the
system on/off.


Fine, Ok to that, existing thermostat in hallway, a non-TRV area.
Lounge has additional heat source(SWMBO requirement) + high Solar Gain
,hence TRV.


You might find that you get smoother results if you put it on the
upstairs landing. It will be less suscepitble to the effects of
opening the front door.

It's a Bungalow, and because of the layout of the dwelling we hardly ever
use the front door in practice.



The optimum system is where the water comes back to the boiler at just
above the condensing level for non-condensing boilers or below for
condensing. If condensing boiler return temperature is above then it
doesn't condense and so the extra money you paid for it is wasted as
it's no more efficient than a standard boiler. If the temperature is
too low on a non-condensing boiler then you get condensing which
corrodes the heat exchanger. I have been told that 56C is the
condensing threshold.


Agreed, I remember in the dim and distant past a HVAC consultant quoting
that figure at me in work.


It's around 52-54 degrees.



The normal flow and return is 80C out, 70C back (in round numbers,
82C, 71C if you want, in F it was 180F out, 160F back, rads 100F above
room temperature). So rads will be at 75C, about 55C above room
temperature. If you run which a larger drop, say 80C out, 60C back,
then the rads are at 70C. That 5C difference is significant (see rad
makers for curves). The rad could be 10% down on output. Hence for a
condensing boiler system you need larger radiators.


Agreed, no problem with that, makes sense.


See separate note on derating of radiator specs.


I wouldn't go with an alpha pump with a standard boiler. Think about
it. An alpha pump slows down the flow so as the TRVs cut in the flow
back to the boiler is reduced. Water spends more time in the rads so
comes back colder. The risk is you get condensing.


Yessss, I hadn't thought about that, getting too old in the tooth I

think!!!

What I would go for is a standard pump + an automatic bypass (which is

cheaper
anyway). Then as the TRVs cut in the hot water gets fed back with the
cold from the rads and the return temperature flow goes up - no risk
of condensing.


OK.--The automatic bypass, do these work in an analogue fashion, do they
have differing pressure/resistance curves or are they on/off devices.


They are pretty much on/off and are intended to deal with the case
where DHW and CH both stop demanding and the boiler is in full burn.

Since my query I found the specs on the honeywell DU145 and whereas they
claim that it is designed to operate as a boiler bypass *or* where radiator
thermostats are fitted, the curves they publish are very flat indicating
that they open over a very small pressure change, in the absence of detailed
info on the actual pressure variations which occur in a typical TRV domestic
CH system it's difficult for me to make a reasoned judgment.
Are you aware of any other Automatic Bypass Valves which have steeper
curves.
The setting should be arranged so that the valve is closed in all
cases exept where this happens. The idea is *not* to allow any
feedback of flow water to the boiler except under these conditions
when the only purpose is to keep a flow going and to prevent boiling.



Of course as the return flow temperature rises the efficiency goes
down but the trade off is that you can set the normal flow for a
larger drop (assuming you have the rad area to cope) and know that the
return temperature can only go up.

The effect for short cycling is the same. For a bypass the flow is
constant

Implying analogue characteristicts?

so temperature rise across the boiler is constant. It's the
return temperature that rises. Exceed the limit before the thermostat
cuts in and the boiler short cycles. For an alpha the flow drops so
the rise across the boiler increases. Again exceed the limit and the
boiler short cycles. It's all down to the boiler outputting X kW and
the house only needing X kW. The way to lengthen the cycle time is
to have enough heat capacity (water and rad metal) to absorb the X kW
until the main thermostat cuts in. That's all about having a balanced
system.

Agreed again.

BTW this assumes that you don't have a modulating boiler (i.e. one
that can vary the output power and so doesn't short cycle). I think
some as boilers are as it is easier to do on gas. Most oil boilers
aren't.

Unfortunately I don't have a modulating or condensing boiler.

Much obliged to you for your detailed critique, I will be looking at

bypass
systems I think. I inherited the boiler (Gloworm Hideaway 60B FF) with

the
bungalow I moved into 6 Months ago, I shall use the existing pump which

will
save me a bit.


You can do that provided that the pump is set to compromise with the
requirements of full heat demand and at the point when the TRVs are
closing. You also need to arrange that the room thermostat is
turning off the boiler when there is still some flow left through the
TRVs.

Thanks Andy, would you agree that the Alpha might induce condensing
conditions in my standard boiler, assuming that thermostats and TRV are
susceptible to frequent adjustments in my household :-(

Cheers
Tom




  #16   Report Post  
Malcolm Reeves
 
Posts: n/a
Default

On Sun, 19 Sep 2004 14:53:59 +0100, Andy Hall
wrote:

The optimum system is where the water comes back to the boiler at just
above the condensing level for non-condensing boilers or below for
condensing.


That isn't true. Non-condensing boilers are designed to operate most
efficiently at 82/70 flow return.


Not according to HRM technical department. It also stands to reason
that all other things being equal the lower the boiler average water
temperature the more energy you get from the fuel, i.e. the cooler the
flue gas. SEDBUK cap standard boiler efficiencies specifically to
stop boiler makers designing standard boilers on the edge of
condensing and thus improving their efficiency at the risk of shorter
life (due to condensing causing corrosion). Hence the sedbuk figures
need to be viewed in the light of being capped when comparing
condensing and non-condensing boilers :-/.

If condensing boiler return temperature is above then it
doesn't condense and so the extra money you paid for it is wasted as
it's no more efficient than a standard boiler.


That isn't quite true either.

A condensing boiler's efficiency doesn't suddenly improve as if a Holy
Grail has been found at the dew point.

WHat happens is that the efficiency increases with decrease in
temperature both above and below the dew point. When condensing
commences at the dew point, all that happens is that the *rate* of
increase of efficiency increases with falling temperature.


So what you are saying is that a larger difference than 82/70 does
improve efficiency. Like I said. The specific latent heat extracted
at the dew point is a lump of energy, either you have it or you don't.
If you are running above the dew point you don't have it and the
boiler is working in standard mode not condensing.

At a flow of 82 degrees, a condensing boiler will be more efficient
than a conventional one anyway because the heat exchanger is larger.


True a larger heat exchanger will get more heat out of the fuel and
thus be more efficient. However, condensing boilers are $$$ more
money and if the only benefit is the larger heat exchanger (i.e. not
condensing) would the savings be worth it?

I'm just changing my heating to oil fire standard boiler not a
condensing since on consideration of the extra boiler cost, problems
(smells, plume etc.) vs the potential savings (perhaps GBP20 pa) I
have chosen a standard oil boiler. I'm fitting this now in case they
change the rules in April 2005.

Hence for a
condensing boiler system you need larger radiators.


That is also untrue.

You can replace a conventional boiler with a condensing one. When
required, in the depths of winter the condensing boiler can run up to
the 82 degree level of the conventional one and still be more
efficient. For most of the heating season, the condensing boiler will
modulate down to a lower output and into an even more efficient
working range.


How does a non modulating boiler modulate? The water comes out of the
boiler at 80C goes through the radiators which cools it by 10C (if
they are exactly right for the room loss) and returns to the boiler at
70C. If you want it to return cooler the radiators have to be bigger
since the heat they give up to the room is dependent on their size and
the (fixed) room temperature. You could run with a lower flow
temperature but then you'd have less drop across the boiler which as
we have already agreed is less efficient. In the case of a condensing
boiler the condensing gain would probably exceed the standard heat
exchange section drop in efficiency but it still won't be as good as
if it was running at the design figures.


I wouldn't go with an alpha pump with a standard boiler. Think about
it. An alpha pump slows down the flow so as the TRVs cut in the flow
back to the boiler is reduced. Water spends more time in the rads so
comes back colder. The risk is you get condensing. What I would go
for is a standard pump + an automatic bypass (which is cheaper
anyway). Then as the TRVs cut in the hot water gets fed back with the
cold from the rads and the return temperature flow goes up - no risk
of condensing.


This is precisely where an Alpha pump is intended to be used. An
automatic bypass is intended to deal with the case where both CH and
DHW demand has ceased altogether and the boiler has been firing at
full tilt. It simply prevents the water from boiling and nothing
more.


No. An automatic bypass says it is for TRVs. In any case what will a
bypass do in the situation you have above. If you are trying to get
heat out of boiler after the fuel has cut off you need a pump over
run. That is something that runs the pump for a few minutes after the
thermostats have cut off and the boiler stopped firing. Usually a
thermostat on the boiler I believe rather than a timer but the effect
is the same. Some boilers don't need it as the cut off quick enough
that the water can absorb the extra heat.

A three way valve must be open to at least one way and you must have a
path (non TRV rads or tank). What stops the flow is the pump not
running. If you have flow but not enough then that's where an
automatic bypass comes in. Pump head (pressure) rises and the valve
opens.

The TRVs do not stop the flow completely if adjusted correctly in
relation to the radiator in the non-TRV room. The object is to turn
off the boiler via the room thermostat as all rooms have arrived at
set point, not for the TRVs to have closed completely.
Provided that the boiler thermostat is adjusted correctly, operating
temperatures will also be correctly in range.


If the TRVs are even partially closed then the flow is reduced. An
alpha will keep the head constant-ish resulting in less flow and thus
lower return temperature. A standard + bypass will result in constant
head (the setting the bypass is set to), and a constant-ish flow


--

Malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
, or ).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

NEW - Desktop ToDo/Reminder program (free)
  #17   Report Post  
Andy Hall
 
Posts: n/a
Default

On Sun, 19 Sep 2004 16:01:36 +0100, "Tom"
wrote:



It's a Bungalow, and because of the layout of the dwelling we hardly ever
use the front door in practice.


OK, so I would suggest positioning the thermostat away from areas
where there can be sudden increases or decreases of heat such as the
kitchen or if you have supplementary heating, the living room.



Since my query I found the specs on the honeywell DU145 and whereas they
claim that it is designed to operate as a boiler bypass *or* where radiator
thermostats are fitted, the curves they publish are very flat indicating
that they open over a very small pressure change, in the absence of detailed
info on the actual pressure variations which occur in a typical TRV domestic
CH system it's difficult for me to make a reasoned judgment.
Are you aware of any other Automatic Bypass Valves which have steeper
curves.


I am pretty sure that all are fairly similar. Danfoss is another make.




Thanks Andy, would you agree that the Alpha might induce condensing
conditions in my standard boiler, assuming that thermostats and TRV are
susceptible to frequent adjustments in my household :-(


I think that it's unlikely provided that you set the boiler thermostat
high enough.

I would take it one step at a time.

- Give the system a good clean out as you fit the TRVs.
This is a good opportunity to take the radiators outside and give them
a good hose through. Don't forget to note the lockshield valve
positions.

Afterwards a flush with a system cleanser is a good plan - run hot for
the recommended period of time.

Flush, Refill and add inhibitor.

- Balance the non TRV radiator so that the TRVs are starting to close
as the room thermostat cuts off the boiler.
You will know the rough point because if the boiler is cut off too
early, the other rooms will be too cool. If much later, the radiators
with TRVs will start to cool and the boiler will run for shorter
periods.

I would leave the pump unless you find that the system becomes noisy
as the TRVs are starting to close. You will know if the existing
pump is a problem if you find that it gets noisy and you have to turn
it down but that on that setting when all TRVs are open, the boiler
runs for short periods.

The other thinkg is that you won't really be able to do this until the
weather turns cooler.




Cheers
Tom


..andy

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  #18   Report Post  
Malcolm Reeves
 
Posts: n/a
Default

On Sun, 19 Sep 2004 13:59:34 +0100, "Tom"
wrote:


What I would go for is a standard pump + an automatic bypass (which is

cheaper
anyway). Then as the TRVs cut in the hot water gets fed back with the
cold from the rads and the return temperature flow goes up - no risk
of condensing.


OK.--The automatic bypass, do these work in an analogue fashion, do they
have differing pressure/resistance curves or are they on/off devices.


Does it matter? The combined system has a pump driving the head and
the flow. Even if the bypass valve has a very sharp response the pump
doesn't. The sharp or soft response of a bypass combines with the
pump curve. Or, if you like, a sharp response bypass will keep the
maximum head at say 3m (if is it set to 3m), whereas a soft response
one will keep the maximum head at say 2.5m to 3.5m depending on how
much it has to open. In either case the bypass will open enough to
let the flow stay constant-ish.

BTW a bypass is just a spring loaded valve. A knob on the top (or
similar) allows you to set the maximum head. When that is reached the
valve opens, just enough so that the head stays at that level. The
increase in flow then allows the pump head to decrease and the system
settles to a stable value.


--

Malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
, or ).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

NEW - Desktop ToDo/Reminder program (free)
  #19   Report Post  
Andy Hall
 
Posts: n/a
Default

On Sun, 19 Sep 2004 19:02:39 +0100, Malcolm Reeves
wrote:

On Sun, 19 Sep 2004 14:53:59 +0100, Andy Hall
wrote:

The optimum system is where the water comes back to the boiler at just
above the condensing level for non-condensing boilers or below for
condensing.


That isn't true. Non-condensing boilers are designed to operate most
efficiently at 82/70 flow return.


Not according to HRM technical department. It also stands to reason
that all other things being equal the lower the boiler average water
temperature the more energy you get from the fuel, i.e. the cooler the
flue gas.


The operating range possible with a conventional boiler is so narrow
anyway that this makes very little difference. The heat exchangers
used are relatively small.


SEDBUK cap standard boiler efficiencies specifically to
stop boiler makers designing standard boilers on the edge of
condensing and thus improving their efficiency at the risk of shorter
life (due to condensing causing corrosion). Hence the sedbuk figures
need to be viewed in the light of being capped when comparing
condensing and non-condensing boilers :-/.


The SEDBUK figures really confuse the issue because of the factors
used. For the purposes of looking at boiler behaviour it is much
more useful to use the measuring methods used in the rest of Europe.

These specify certain temperatures and firing rates.



If condensing boiler return temperature is above then it
doesn't condense and so the extra money you paid for it is wasted as
it's no more efficient than a standard boiler.


That isn't quite true either.

A condensing boiler's efficiency doesn't suddenly improve as if a Holy
Grail has been found at the dew point.

WHat happens is that the efficiency increases with decrease in
temperature both above and below the dew point. When condensing
commences at the dew point, all that happens is that the *rate* of
increase of efficiency increases with falling temperature.


So what you are saying is that a larger difference than 82/70 does
improve efficiency. Like I said.


I don't think you did, but the point is that a larger temperature
difference together with a larger heat exchanger certainly does help.

The specific latent heat extracted
at the dew point is a lump of energy, either you have it or you don't.
If you are running above the dew point you don't have it and the
boiler is working in standard mode not condensing.


You are missing the point. While it is true that latent heat is
extracted below the dew point and results in a greater efficiency,
this is not a step function as people seem to believe -it is an
increase in the rate of change of efficiency with falling
temperature.. You also have to account for the burn rate of the
boiler and the heat delivery to the load.




At a flow of 82 degrees, a condensing boiler will be more efficient
than a conventional one anyway because the heat exchanger is larger.


True a larger heat exchanger will get more heat out of the fuel and
thus be more efficient. However, condensing boilers are $$$ more
money and if the only benefit is the larger heat exchanger (i.e. not
condensing) would the savings be worth it?


The answer is that both aspects are useful, as is the control system
running the boiler. Generally these are more sophisticated than with
conventional boilers and are able to match the heat generated with the
load and to minimise the temperature of operation. All of these
factors improve efficiency.

The difference in cost is becoming less and less, and from next year
the whole issue will be academic since changes to the Building
Regulations will mandate condensing boilers anyway.

In terms of cost saving, I have found it to be in the predicted band
of 25-30%


I'm just changing my heating to oil fire standard boiler not a
condensing since on consideration of the extra boiler cost, problems
(smells, plume etc.) vs the potential savings (perhaps GBP20 pa) I
have chosen a standard oil boiler. I'm fitting this now in case they
change the rules in April 2005.


You must be spending very little on energy if £20 represents 25% of
your energy cost.

There are no real problems with modern condensing models in terms of
pluming. Much improvement has been made over early designs. Where
you get the idea that there are smells, I have no idea.



Hence for a
condensing boiler system you need larger radiators.


That is also untrue.

You can replace a conventional boiler with a condensing one. When
required, in the depths of winter the condensing boiler can run up to
the 82 degree level of the conventional one and still be more
efficient. For most of the heating season, the condensing boiler will
modulate down to a lower output and into an even more efficient
working range.


How does a non modulating boiler modulate? The water comes out of the
boiler at 80C goes through the radiators which cools it by 10C (if
they are exactly right for the room loss) and returns to the boiler at
70C. If you want it to return cooler the radiators have to be bigger
since the heat they give up to the room is dependent on their size and
the (fixed) room temperature. You could run with a lower flow
temperature but then you'd have less drop across the boiler which as
we have already agreed is less efficient.


You are missing the point. The temperature difference across a heat
exchanger is limited by the heat exchanger itself. So on a
conventional boiler, arranging a greater heating load by having more
emission will simply result in the flow being cooler. It doesn't
buy you anything.

In the case of a condensing
boiler the condensing gain would probably exceed the standard heat
exchange section drop in efficiency but it still won't be as good as
if it was running at the design figures.

For a modulating condensing boiler, there is no specific "design
temperature". They will work at 82/70 if put into a standard
system and the level of heat demand is high.
It is typical to run at 70/50 with new radiator systems but you don't
have to do so. You can run UFH systems at 50/30 if the boiler will
do it, and some will.
The lower the temperature, the greater the efficiency and below the
dew point the rate of change increases.



I wouldn't go with an alpha pump with a standard boiler. Think about
it. An alpha pump slows down the flow so as the TRVs cut in the flow
back to the boiler is reduced. Water spends more time in the rads so
comes back colder. The risk is you get condensing. What I would go
for is a standard pump + an automatic bypass (which is cheaper
anyway). Then as the TRVs cut in the hot water gets fed back with the
cold from the rads and the return temperature flow goes up - no risk
of condensing.


This is precisely where an Alpha pump is intended to be used. An
automatic bypass is intended to deal with the case where both CH and
DHW demand has ceased altogether and the boiler has been firing at
full tilt. It simply prevents the water from boiling and nothing
more.


No. An automatic bypass says it is for TRVs. In any case what will a
bypass do in the situation you have above.


You are assuming that TRVs are digital devices that suddenly cut off
the flow when the temperature is reached. They are not, and are
simple wax expansion devices that gradually reduce the flow.

The value in a bypass is if you get to a situation where all radiators
have been turned off.

If you are trying to get
heat out of boiler after the fuel has cut off you need a pump over
run. That is something that runs the pump for a few minutes after the
thermostats have cut off and the boiler stopped firing.


Yes, I'm fully aware of that.

Usually a
thermostat on the boiler I believe rather than a timer but the effect
is the same. Some boilers don't need it as the cut off quick enough
that the water can absorb the extra heat.

A three way valve must be open to at least one way and you must have a
path (non TRV rads or tank). What stops the flow is the pump not
running. If you have flow but not enough then that's where an
automatic bypass comes in. Pump head (pressure) rises and the valve
opens.


Obviously.


The TRVs do not stop the flow completely if adjusted correctly in
relation to the radiator in the non-TRV room. The object is to turn
off the boiler via the room thermostat as all rooms have arrived at
set point, not for the TRVs to have closed completely.
Provided that the boiler thermostat is adjusted correctly, operating
temperatures will also be correctly in range.


If the TRVs are even partially closed then the flow is reduced. An
alpha will keep the head constant-ish resulting in less flow and thus
lower return temperature.


No because when this happens the heat emission will have dropped.
The purpose is to reduce the power to the pump as the flow resistance
increases.

A standard + bypass will result in constant
head (the setting the bypass is set to), and a constant-ish flow


No it won't because the bypass opens in effect digitally at a certain
head. After that, it has a pressure/flow characteristic.




..andy

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  #20   Report Post  
IMM
 
Posts: n/a
Default


"Malcolm Reeves" wrote in message
...
On Sun, 19 Sep 2004 08:32:57 +0100, "Tom"
wrote:

I'm attracted towards using TRVs on the rads in the 4 rooms which make
up approx 80% of the total heat load.
I'm further minded to install a Grundfos Alpha pump, however, I'm

undecided
as to the control mechanism i.e. a flow switch as advocated by IMM in a
previous thread or to rely on the boiler stat set to 82degC as you

stated.
I'm anxious to avoid short cycling of the boiler.



You should have TRV on at least all the bedroom rads which is the new
rules AFAIR.


Nothing says it is madatory in the regs.

Ideally all rooms should have TRVs except one (the
lounge?) and that room should have the thermostat that turns the
system on/off.


The stat should be in the coolest room which is usualy the hall.

The optimum system is where the water comes back to the boiler at just
above the condensing level for non-condensing boilers or below for
condensing. If condensing boiler return temperature is above then it
doesn't condense and so the extra money you paid for it is wasted as
it's no more efficient than a standard boiler.


Condensing boilers are far more efficient as they have a larger heat
exchanger, condensing or not.

If the temperature is
too low on a non-condensing boiler then you get condensing which
corrodes the heat exchanger. I have been told that 56C is the
condensing threshold.


Not so.

The normal flow and return is 80C out, 70C back (in round numbers,
82C, 71C if you want, in F it was 180F out, 160F back, rads 100F above
room temperature). So rads will be at 75C, about 55C above room
temperature. If you run which a larger drop, say 80C out, 60C back,
then the rads are at 70C. That 5C difference is significant (see rad
makers for curves). The rad could be 10% down on output. Hence for a
condensing boiler system you need larger radiators.


Modern condensing boilers have load compensation control which is a great be
benefit on part load. Most systems are on part load for most of the time.

I wouldn't go with an alpha pump with a standard boiler.


I would use a condesning boiler.

Think about it. An alpha pump slows down the flow so as the TRVs cut in

the flow
back to the boiler is reduced. Water spends more time in the rads so
comes back colder. The risk is you get condensing.


You only get condensing when the return temp is far to low.

What I would go for is a standard pump +
an automatic bypass (which is cheaper
anyway).


And difficult to get right. Do not use one of these with a condensing
boiler.

Then as the TRVs cut in the hot water gets fed back with the
cold from the rads and the return temperature flow goes up - no risk
of condensing.

Of course as the return flow temperature rises the efficiency goes
down but the trade off is that you can set the normal flow for a
larger drop (assuming you have the rad area to cope) and know that the
return temperature can only go up.

The effect for short cycling is the same. For a bypass the flow is
constant so temperature rise across the boiler is constant. It's the
return temperature that rises. Exceed the limit before the thermostat
cuts in and the boiler short cycles. For an alpha the flow drops so
the rise across the boiler increases. Again exceed the limit and the
boiler short cycles. It's all down to the boiler outputting X kW and
the house only needing X kW. The way to lengthen the cycle time is
to have enough heat capacity (water and rad metal) to absorb the X kW
until the main thermostat cuts in. That's all about having a balanced
system.

BTW this assumes that you don't have a modulating boiler (i.e. one
that can vary the output power and so doesn't short cycle). I think
some as boilers are as it is easier to do on gas. Most oil boilers
aren't.


You need to know more about regular v condensing boilers and much, much
more.




  #21   Report Post  
IMM
 
Posts: n/a
Default


"Andy Hall" wrote in message
...
On Sun, 19 Sep 2004 13:59:34 +0100, "Tom"
wrote:


"Malcolm Reeves" wrote in message
.. .

You should have TRV on at least all the bedroom rads which is the new
rules AFAIR. Ideally all rooms should have TRVs except one (the
lounge?) and that room should have the thermostat that turns the
system on/off.


Fine, Ok to that, existing thermostat in hallway, a non-TRV area.
Lounge has additional heat source(SWMBO requirement) + high Solar Gain
,hence TRV.


You might find that you get smoother results if you put it on the
upstairs landing. It will be less suscepitble to the effects of
opening the front door.



The optimum system is where the water comes back to the boiler at just
above the condensing level for non-condensing boilers or below for
condensing. If condensing boiler return temperature is above then it
doesn't condense and so the extra money you paid for it is wasted as
it's no more efficient than a standard boiler. If the temperature is
too low on a non-condensing boiler then you get condensing which
corrodes the heat exchanger. I have been told that 56C is the
condensing threshold.


Agreed, I remember in the dim and distant past a HVAC consultant quoting
that figure at me in work.


It's around 52-54 degrees.



The normal flow and return is 80C out, 70C back (in round numbers,
82C, 71C if you want, in F it was 180F out, 160F back, rads 100F above
room temperature). So rads will be at 75C, about 55C above room
temperature. If you run which a larger drop, say 80C out, 60C back,
then the rads are at 70C. That 5C difference is significant (see rad
makers for curves). The rad could be 10% down on output. Hence for a
condensing boiler system you need larger radiators.


Agreed, no problem with that, makes sense.


See separate note on derating of radiator specs.


I wouldn't go with an alpha pump with a standard boiler. Think about
it. An alpha pump slows down the flow so as the TRVs cut in the flow
back to the boiler is reduced. Water spends more time in the rads so
comes back colder. The risk is you get condensing.


Yessss, I hadn't thought about that, getting too old in the tooth I

think!!!

What I would go for is a standard pump + an automatic bypass (which is

cheaper
anyway). Then as the TRVs cut in the hot water gets fed back with the
cold from the rads and the return temperature flow goes up - no risk
of condensing.


OK.--The automatic bypass, do these work in an analogue fashion, do they
have differing pressure/resistance curves or are they on/off devices.


They are pretty much on/off


They are not. The valve opens according to pressure.the higher the more it
opens.

and are intended to deal with the case
where DHW and CH both stop demanding and the boiler is in full burn.
The setting should be arranged so that the valve is closed in all
cases exept where this happens. The idea is *not* to allow any
feedback of flow water to the boiler except under these conditions
when the only purpose is to keep a flow going and to prevent boiling.



Of course as the return flow temperature rises the efficiency goes
down but the trade off is that you can set the normal flow for a
larger drop (assuming you have the rad area to cope) and know that the
return temperature can only go up.

The effect for short cycling is the same. For a bypass the flow is
constant

Implying analogue characteristicts?

so temperature rise across the boiler is constant. It's the
return temperature that rises. Exceed the limit before the thermostat
cuts in and the boiler short cycles. For an alpha the flow drops so
the rise across the boiler increases. Again exceed the limit and the
boiler short cycles. It's all down to the boiler outputting X kW and
the house only needing X kW. The way to lengthen the cycle time is
to have enough heat capacity (water and rad metal) to absorb the X kW
until the main thermostat cuts in. That's all about having a balanced
system.

Agreed again.

BTW this assumes that you don't have a modulating boiler (i.e. one
that can vary the output power and so doesn't short cycle). I think
some as boilers are as it is easier to do on gas. Most oil boilers
aren't.

Unfortunately I don't have a modulating or condensing boiler.

Much obliged to you for your detailed critique, I will be looking at

bypass
systems I think. I inherited the boiler (Gloworm Hideaway 60B FF) with

the
bungalow I moved into 6 Months ago, I shall use the existing pump which

will
save me a bit.


To save money that is fine. For anew system go Alpha and flow stitch and
condensing boiler.

You can do that provided that the pump is set to compromise with the
requirements of full heat demand and at the point when the TRVs are
closing. You also need to arrange that the room thermostat is
turning off the boiler when there is still some flow left through the
TRVs.


Best Regards
Tom


.andy

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  #22   Report Post  
IMM
 
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"Tom" wrote in message
...

Thanks Andy, would you agree that the Alpha might induce condensing
conditions in my standard boiler, assuming that thermostats and TRV are
susceptible to frequent adjustments in my household :-(


As long at the boiler is set to 80C this is highly unlikely when the system
is warmed up.

You may require a minimum flow through the boiler. Then I would forget the
Alpha a go for am open rad and room stat.



  #23   Report Post  
Andy Hall
 
Posts: n/a
Default

On Sun, 19 Sep 2004 23:45:34 +0100, "IMM" wrote:


They are not. The valve opens according to pressure.the higher the more it
opens.


In fact if you look at the data sheet you will see that for a given
setting, they are roughly a constant pressure drop device over a range
of flows.


..andy

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  #24   Report Post  
Andy Hall
 
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Default

On Sun, 19 Sep 2004 23:50:08 +0100, "IMM" wrote:


"Tom" wrote in message
...

Thanks Andy, would you agree that the Alpha might induce condensing
conditions in my standard boiler, assuming that thermostats and TRV are
susceptible to frequent adjustments in my household :-(


As long at the boiler is set to 80C this is highly unlikely when the system
is warmed up.

You may require a minimum flow through the boiler. Then I would forget the
Alpha a go for am open rad and room stat.


You can set the nominal power level on the Alpha above or below
nominal setting.

Even so, I agree that if the existing pump is OK, it makes more sense
to try that first and only switch to an Alpha if there is a problem
with noise in the radiators closed down case (i.e. reduce pump
setting), but that that is not enough wit everything open.


..andy

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  #25   Report Post  
IMM
 
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"Malcolm Reeves" wrote in message
...
On Sun, 19 Sep 2004 14:53:59 +0100, Andy Hall
wrote:

The optimum system is where the water comes back to the boiler at just
above the condensing level for non-condensing boilers or below for
condensing.


That isn't true. Non-condensing boilers are designed to operate most
efficiently at 82/70 flow return.


Not according to HRM technical department.
It also stands to reason that all other things
being equal the lower the boiler average water
temperature the more energy you get from the fuel,
i.e. the cooler the flue gas.


It is true that "generally" the lower the operating temperature the higher
the efficiency. But regular boilers are designed to operate at maximum
efficiency at higher temperatures. Any gains at running at lower temp is
very marginal.

You could fit a blending valve on the flow return to ensure that only water
higher than 55C is returned back to the boiler. Then you would need to set
the boiler temp 11C higher to ensure the delta T. Exceeding the Delta T for
long periods can harm the heat exchanger.

SEDBUK cap standard boiler efficiencies specifically to
stop boiler makers designing standard boilers on the edge of
condensing and thus improving their efficiency at the risk of shorter
life (due to condensing causing corrosion). Hence the sedbuk figures
need to be viewed in the light of being capped when comparing
condensing and non-condensing boilers :-/.


Sedbuk is being phased out.

If condensing boiler return temperature is above then it
doesn't condense and so the extra money you paid for it is wasted as
it's no more efficient than a standard boiler.


That isn't quite true either.

A condensing boiler's efficiency doesn't suddenly improve as if a Holy
Grail has been found at the dew point.

WHat happens is that the efficiency increases with decrease in
temperature both above and below the dew point. When condensing
commences at the dew point, all that happens is that the *rate* of
increase of efficiency increases with falling temperature.


So what you are saying is that a larger difference than 82/70 does
improve efficiency. Like I said. The specific latent heat extracted
at the dew point is a lump of energy, either you have it or you don't.
If you are running above the dew point you don't have it and the
boiler is working in standard mode not condensing.

At a flow of 82 degrees, a condensing boiler will be more efficient
than a conventional one anyway because the heat exchanger is larger.


True a larger heat exchanger will get more heat out of the fuel and
thus be more efficient. However, condensing boilers are $$$ more
money


These days not that much and in April they will be standard, except for the
odd boiler here and there where pluming is a problem. Condensing boilers
can be made not to plume.

and if the only benefit is the larger heat exchanger (i.e. not
condensing) would the savings be worth it?


Using a large quick recovery coil in a cylinder enhances condensing and in
most newish homes DHW now used more gas than CH. When the new insulation
regs bite in a few years CH will use far more than CH.

I'm just changing my heating to oil fire
standard boiler not a
condensing since on consideration of
the extra boiler cost, problems
(smells, plume etc.) vs the potential savings
(perhaps GBP20 pa) I
have chosen a standard oil boiler. I'm fitting this now in case they
change the rules in April 2005.


Standard oil boilers make more sense economically. They are much cheaper and
less latent heat is produced by oil. As most people use n gas condensing
makes much more sense as more heat is extracted and they are much cheaper.

When all boilers are condensers, prices will drop. You may have jumped the
gun.

Hence for a
condensing boiler system you need larger radiators.


That is also untrue.

You can replace a conventional boiler with a condensing one. When
required, in the depths of winter the condensing boiler can run up to
the 82 degree level of the conventional one and still be more
efficient. For most of the heating season, the condensing boiler will
modulate down to a lower output and into an even more efficient
working range.


How does a non modulating boiler modulate? The water comes out of the
boiler at 80C goes through the radiators which cools it by 10C (if
they are exactly right for the room loss) and returns to the boiler at
70C. If you want it to return cooler the radiators have to be bigger
since the heat they give up to the room is dependent on their size and
the (fixed) room temperature. You could run with a lower flow
temperature but then you'd have less drop across the boiler which as
we have already agreed is less efficient. In the case of a condensing
boiler the condensing gain would probably exceed the standard heat
exchange section drop in efficiency but it still won't be as good as
if it was running at the design figures.


I wouldn't go with an alpha pump with a standard boiler. Think about
it. An alpha pump slows down the flow so as the TRVs cut in the flow
back to the boiler is reduced. Water spends more time in the rads so
comes back colder. The risk is you get condensing. What I would go
for is a standard pump + an automatic bypass (which is cheaper
anyway). Then as the TRVs cut in the hot water gets fed back with the
cold from the rads and the return temperature flow goes up - no risk
of condensing.


This is precisely where an Alpha pump is intended to be used. An
automatic bypass is intended to deal with the case where both CH and
DHW demand has ceased altogether and the boiler has been firing at
full tilt. It simply prevents the water from boiling and nothing
more.


No. An automatic bypass says it is for TRVs. In any case what will a
bypass do in the situation you have above. If you are trying to get
heat out of boiler after the fuel has cut off you need a pump over
run. That is something that runs the pump for a few minutes after the
thermostats have cut off and the boiler stopped firing. Usually a
thermostat on the boiler I believe rather than a timer but the effect
is the same. Some boilers don't need it as the cut off quick enough
that the water can absorb the extra heat.

A three way valve must be open to at least one way and you must have a
path (non TRV rads or tank). What stops the flow is the pump not
running. If you have flow but not enough then that's where an
automatic bypass comes in. Pump head (pressure) rises and the valve
opens.

The TRVs do not stop the flow completely if adjusted correctly in
relation to the radiator in the non-TRV room. The object is to turn
off the boiler via the room thermostat as all rooms have arrived at
set point, not for the TRVs to have closed completely.
Provided that the boiler thermostat is adjusted correctly, operating
temperatures will also be correctly in range.


If the TRVs are even partially closed then the flow is reduced. An
alpha will keep the head constant-ish resulting in less flow and thus
lower return temperature. A standard + bypass will result in constant
head (the setting the bypass is set to), and a constant-ish flow


--

Malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
, or ).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and

Power
electronics. More details plus freeware, Win95/98 DUN and Pspice tips,

see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

NEW - Desktop ToDo/Reminder program (free)





  #26   Report Post  
IMM
 
Posts: n/a
Default


"Andy Hall" wrote in message
...
On Sun, 19 Sep 2004 23:50:08 +0100, "IMM" wrote:


"Tom" wrote in message
...

Thanks Andy, would you agree that the Alpha might induce condensing
conditions in my standard boiler, assuming that thermostats and TRV are
susceptible to frequent adjustments in my household :-(


As long at the boiler is set to 80C this is highly unlikely when the

system
is warmed up.

You may require a minimum flow through the boiler. Then I would forget

the
Alpha a go for am open rad and room stat.


You can set the nominal power level on the Alpha above or below
nominal setting.


If all TRVs are close, no flow. It is best to get a flow switch rated at
the minum boiler flow. below this flow the boiler burner is cut out. The
pump can still pump through the boiler.

http://www.crydom.co.uk/
http://www.axdistribution.com/FluidM...ow_switchs.htm

Even so, I agree that if the existing pump is OK, it makes more sense
to try that first and only switch to an Alpha if there is a problem
with noise in the radiators closed down case (i.e. reduce pump
setting), but that that is not enough wit everything open.


.andy

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  #27   Report Post  
Andy Hall
 
Posts: n/a
Default

On Mon, 20 Sep 2004 00:19:40 +0100, "IMM" wrote:


"Andy Hall" wrote in message
.. .
On Sun, 19 Sep 2004 23:50:08 +0100, "IMM" wrote:


"Tom" wrote in message
...

Thanks Andy, would you agree that the Alpha might induce condensing
conditions in my standard boiler, assuming that thermostats and TRV are
susceptible to frequent adjustments in my household :-(

As long at the boiler is set to 80C this is highly unlikely when the

system
is warmed up.

You may require a minimum flow through the boiler. Then I would forget

the
Alpha a go for am open rad and room stat.


You can set the nominal power level on the Alpha above or below
nominal setting.


If all TRVs are close, no flow. It is best to get a flow switch rated at
the minum boiler flow. below this flow the boiler burner is cut out. The
pump can still pump through the boiler.

http://www.crydom.co.uk/
http://www.axdistribution.com/FluidM...ow_switchs.htm


I see your point, but these are far from being precision devices.
For example, the Crydom ones have a 2:1 or 3:1 switch on/off flow
rate, and these are max and min figures.

One would have to put some valves around and in series with the
switch to tune it to the correct behaviour.

I suppose a more accurate flow sensor that measures the flow coupled
to some electronics sould be used, but this is going to cost a bit.


Even so, I agree that if the existing pump is OK, it makes more sense
to try that first and only switch to an Alpha if there is a problem
with noise in the radiators closed down case (i.e. reduce pump
setting), but that that is not enough wit everything open.


.andy

To email, substitute .nospam with .gl



..andy

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  #28   Report Post  
IMM
 
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Default


"Andy Hall" wrote in message
...
On Mon, 20 Sep 2004 00:19:40 +0100, "IMM" wrote:


"Andy Hall" wrote in message
.. .
On Sun, 19 Sep 2004 23:50:08 +0100, "IMM" wrote:


"Tom" wrote in message
...

Thanks Andy, would you agree that the Alpha might induce condensing
conditions in my standard boiler, assuming that thermostats and TRV

are
susceptible to frequent adjustments in my household :-(

As long at the boiler is set to 80C this is highly unlikely when the

system
is warmed up.

You may require a minimum flow through the boiler. Then I would forget

the
Alpha a go for am open rad and room stat.


You can set the nominal power level on the Alpha above or below
nominal setting.


If all TRVs are close, no flow. It is best to get a flow switch rated at
the minum boiler flow. below this flow the boiler burner is cut out.

The
pump can still pump through the boiler.

http://www.crydom.co.uk/
http://www.axdistribution.com/FluidM...ow_switchs.htm


I see your point, but these are far from being precision devices.
For example, the Crydom ones have a 2:1 or 3:1 switch on/off flow
rate, and these are max and min figures.

One would have to put some valves around and in series with the
switch to tune it to the correct behaviour.


That is the cheap way. Precise flow switchs, even adjustable, are
available, but not cheap as they tend to be for commercial applications.

I suppose a more accurate flow sensor that measures the flow coupled
to some electronics sould be used, but this is going to cost a bit.


Even so, I agree that if the existing pump is OK, it makes more sense
to try that first and only switch to an Alpha if there is a problem
with noise in the radiators closed down case (i.e. reduce pump
setting), but that that is not enough wit everything open.


.andy

To email, substitute .nospam with .gl



.andy

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  #29   Report Post  
Malcolm Reeves
 
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On Sun, 19 Sep 2004 23:40:39 +0100, "IMM" wrote:


You should have TRV on at least all the bedroom rads which is the new
rules AFAIR.


Nothing says it is madatory in the regs.


Building regs part L 2002 guidelines say:

"If a whole heating system is replaced, the controls must permit
independent temperature control in two separate zones: the living and
sleeping areas. This can be achieved with a single room thermostat and
thermostatic radiator valves."

Ideally all rooms should have TRVs except one (the
lounge?) and that room should have the thermostat that turns the
system on/off.


The stat should be in the coolest room which is usualy the hall.


Opinions on this differ. Mine is that I want the room control where I
am, the lounge (being couch potato :-)). I don't care if the hall is
at 17 or 18 or 19. The lounge is where I care about the temperature
so that's what I want to control.

In any case, as you can see from above, the building regs now suggest
the lounge for the stat.

Condensing boilers are far more efficient as they have a larger heat
exchanger, condensing or not.


We are probably talking about oil (me), gas (you). Yes condensing
boilers have a bigger heat exchanger but "far" more efficient? For
oil, a standard boiler on sedbuk rates at about 85% (capped).
Condensing range is 90-95%. Saving is thus 5-10%.

In my case I want a wall mount oil fired and from sedbuk there is only
one oil fired wall mounted (and it's not available anymore). That one
is only 90% and a lot more than GBP200 more expensive! Sedbuk reckons
running costs are 400 pa for my sort of house which I'd agree with.
So 5% saving is 20 pa. Pay back is 10yrs

I wouldn't go with an alpha pump with a standard boiler.


I would use a condesning boiler.


For a condensing boiler an alpha would be good, which is what I said.
With an alpha the return temperature goes down. Bad news for a
standard boiler but just what you want for a condensing.

What I would go for is a standard pump +
an automatic bypass (which is cheaper
anyway).


And difficult to get right.


What is difficult? As the TRVs cut in the flow resistance increases
and the pump head rises. You just need to have the bypass set higher
than the fully open condition. After that everything is automatic.

You need to know more about regular v condensing boilers and much, much
more.


From someone who doesn't seem to know about part L or the performance
figures it seems very patronising.


--

Malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
, or ).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

NEW - Desktop ToDo/Reminder program (free)
  #30   Report Post  
Malcolm Reeves
 
Posts: n/a
Default

On Sun, 19 Sep 2004 20:19:21 +0100, Andy Hall
wrote:

The operating range possible with a conventional boiler is so narrow
anyway that this makes very little difference. The heat exchangers
used are relatively small.


Perhaps we are talking about gas (you) and oil (me). Condensing
boilers do have a larger heat exchanger. However, if you look at the
sedbuk figures you'll see best standard is 85% (capped), condensing
range 90-95%. So the efficiency difference is only 5-10% and with a
lower return temp on a standard boiler, or a high return on a
condensing, the difference is less again.

The SEDBUK figures really confuse the issue because of the factors
used. For the purposes of looking at boiler behaviour it is much
more useful to use the measuring methods used in the rest of Europe.

These specify certain temperatures and firing rates.


If the figures are available. The only database I know of with all
the uk boiler. If you have a URL for another database please post it.
Also, for oil, isn't it a problem that UK use kerosene and europe uses
gas oil so europe figures will be different anyway.

So what you are saying is that a larger difference than 82/70 does
improve efficiency. Like I said.


I don't think you did, but the point is that a larger temperature
difference together with a larger heat exchanger certainly does help.


A = Large Heat Exchanger + Large difference
B = Large Heat Exchanger + Small difference
C = Small Heat Exchanger + Large difference
D = Small Heat Exchanger + Small difference

A is better than B. C is better than D. A is better than C (but
boiler cost is more). B is better than D (but boiler cost is more).


The difference in cost is becoming less and less, and from next year
the whole issue will be academic since changes to the Building
Regulations will mandate condensing boilers anyway.


Not clear if that will cover oil, but just in case I'm putting mine in
now.

In terms of cost saving, I have found it to be in the predicted band
of 25-30%


For oil that flies in the face of the sedbuk figures which suggest
5%-10% for floor mounted. 5% for wall mounted (which I need).

You must be spending very little on energy if £20 represents 25% of
your energy cost.


GBP20 is 5% of GBP400 my estimated fuel cost.

There are no real problems with modern condensing models in terms of
pluming. Much improvement has been made over early designs. Where
you get the idea that there are smells, I have no idea.


Oil smells. I suspect you are talking more about gas.

A condensing boiler with its lower flue gas temperature is much more
likely to accumulate fumes at low level (that's why they plume after
all). The advice on siting says to take account of the plume
possibility. My flue will face my neighbour. Do I want to risk the
possibility I might be dropping smelly fumes into his garden (and over
his washing line). For, perhaps GBP20 pa when I won't break even on
the boiler costs for 10yrs. NO.

If the TRVs are even partially closed then the flow is reduced. An
alpha will keep the head constant-ish resulting in less flow and thus
lower return temperature.


No because when this happens the heat emission will have dropped.
The purpose is to reduce the power to the pump as the flow resistance
increases.


We are talking about non-modulating boilers aren't we? I am since 99%
of oil boilers are not modulating. I don't know how common modulation
is on gas. So on a non modulating boiler how does the heat emission
drop? Come to that since we were talking about short cycling then how
does a modulating boiler short cycle since they would modulate before
that and thus very unlikely to short cycle.

A standard + bypass will result in constant
head (the setting the bypass is set to), and a constant-ish flow


No it won't because the bypass opens in effect digitally at a certain
head. After that, it has a pressure/flow characteristic.


It's a valve with a spring. It will have some slope to its
characteristic. See http://www.sunvic.co.uk/bypass.html which repeats
what I said.

I would say the sunvic curve is soft. For my house I have calculated
a head of about 1m (about 0.1 bar) and a flow of 0.8 m3/hr. So I'd
set my valve at #3 (this assumes the pump matches this, in practice
the head would be higher to get the flow as I can only restrict the
flow). The valve would start to open at 0.15 bar (1.5m) and by 2 bar
(2m) all my flow would be via the bypass. The head range is thus
1.5-2m. Not exactly digital.


--

Malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
, or ).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

NEW - Desktop ToDo/Reminder program (free)


  #31   Report Post  
Andy Hall
 
Posts: n/a
Default

On Mon, 20 Sep 2004 10:25:39 +0100, Malcolm Reeves
wrote:

On Sun, 19 Sep 2004 20:19:21 +0100, Andy Hall
wrote:

The operating range possible with a conventional boiler is so narrow
anyway that this makes very little difference. The heat exchangers
used are relatively small.


Perhaps we are talking about gas (you) and oil (me).


Predominantly I was.

Condensing
boilers do have a larger heat exchanger. However, if you look at the
sedbuk figures you'll see best standard is 85% (capped), condensing
range 90-95%. So the efficiency difference is only 5-10% and with a
lower return temp on a standard boiler, or a high return on a
condensing, the difference is less again.


The trouble is that the SEDBUK figures tend to obfuscate what is
happening with the boiler.


The SEDBUK figures really confuse the issue because of the factors
used. For the purposes of looking at boiler behaviour it is much
more useful to use the measuring methods used in the rest of Europe.

These specify certain temperatures and firing rates.


If the figures are available. The only database I know of with all
the uk boiler. If you have a URL for another database please post it.
Also, for oil, isn't it a problem that UK use kerosene and europe uses
gas oil so europe figures will be different anyway.


The manufacturers should have them if they are selling their products
outside the UK.

I'm not sure whether gas oil and kerosene would result in different
efficiency figures - different calorific values possibly.



So what you are saying is that a larger difference than 82/70 does
improve efficiency. Like I said.


I don't think you did, but the point is that a larger temperature
difference together with a larger heat exchanger certainly does help.


A = Large Heat Exchanger + Large difference
B = Large Heat Exchanger + Small difference
C = Small Heat Exchanger + Large difference
D = Small Heat Exchanger + Small difference

A is better than B. C is better than D. A is better than C (but
boiler cost is more). B is better than D (but boiler cost is more).

OK.


The difference in cost is becoming less and less, and from next year
the whole issue will be academic since changes to the Building
Regulations will mandate condensing boilers anyway.


Not clear if that will cover oil, but just in case I'm putting mine in
now.

In terms of cost saving, I have found it to be in the predicted band
of 25-30%


For oil that flies in the face of the sedbuk figures which suggest
5%-10% for floor mounted. 5% for wall mounted (which I need).


I was talking about gas.



You must be spending very little on energy if £20 represents 25% of
your energy cost.


GBP20 is 5% of GBP400 my estimated fuel cost.

There are no real problems with modern condensing models in terms of
pluming. Much improvement has been made over early designs. Where
you get the idea that there are smells, I have no idea.


Oil smells. I suspect you are talking more about gas.


It does anyway though, doesn't it?


A condensing boiler with its lower flue gas temperature is much more
likely to accumulate fumes at low level (that's why they plume after
all). The advice on siting says to take account of the plume
possibility. My flue will face my neighbour. Do I want to risk the
possibility I might be dropping smelly fumes into his garden (and over
his washing line). For, perhaps GBP20 pa when I won't break even on
the boiler costs for 10yrs. NO.


Well again, gas ones don't have this issue or don't have to have this
issue.


If the TRVs are even partially closed then the flow is reduced. An
alpha will keep the head constant-ish resulting in less flow and thus
lower return temperature.


No because when this happens the heat emission will have dropped.
The purpose is to reduce the power to the pump as the flow resistance
increases.


We are talking about non-modulating boilers aren't we? I am since 99%
of oil boilers are not modulating. I don't know how common modulation
is on gas.


Nowadays very - on gas condensing boilers.

o on a non modulating boiler how does the heat emission
drop? Come to that since we were talking about short cycling then how
does a modulating boiler short cycle since they would modulate before
that and thus very unlikely to short cycle.


Right. I was referring to conventional gas boilers without
modulation.


A standard + bypass will result in constant
head (the setting the bypass is set to), and a constant-ish flow


No it won't because the bypass opens in effect digitally at a certain
head. After that, it has a pressure/flow characteristic.


It's a valve with a spring. It will have some slope to its
characteristic. See http://www.sunvic.co.uk/bypass.html which repeats
what I said.

I would say the sunvic curve is soft. For my house I have calculated
a head of about 1m (about 0.1 bar) and a flow of 0.8 m3/hr. So I'd
set my valve at #3 (this assumes the pump matches this, in practice
the head would be higher to get the flow as I can only restrict the
flow). The valve would start to open at 0.15 bar (1.5m) and by 2 bar
(2m) all my flow would be via the bypass. The head range is thus
1.5-2m. Not exactly digital.


If you look at data sheets for devices like the Honeywell it shows it
as a set of almost straight lines depending on setting. Once the
valve opens it presents as a virtually constant pressure drop device.



..andy

To email, substitute .nospam with .gl
  #32   Report Post  
Tony Bryer
 
Posts: n/a
Default

In article , Malcolm
Reeves wrote:
The SEDBUK figures really confuse the issue because of the
factors used. For the purposes of looking at boiler behaviour
it is much more useful to use the measuring methods used in the
rest of Europe. These specify certain temperatures and firing
rates.


If the figures are available. The only database I know of with
all the uk boiler. If you have a URL for another database
please post it.


I would guess that what Andy is referring to the fact that the
SEDBUK efficiencies are derived from formulae which can be found in
the [free] SAP-2001 book Appendix D. For an on/off boiler this is
0.5(Efull+EPart)-2.5, for a modulating boiler 0.5(Efull+EPart)-2.0,
less 4% if the boiler has a pilot light - other formulae apply to
combis of various types. Efull and Epart are the efficiencies at
full and 30% load.

Using 50/50 part and full load figures is reckoned to give a fairly
accurate measure of likely efficiency under UK climatic conditions.
It would be quite possible that some other EU country uses a
different mix and if you were comparing two boilers with
significantly different part and full load performance their
relative assumed in-use efficiencies would be different to that
shown by SEDBUK.

--
Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk
Free SEDBUK boiler database browser
http://www.sda.co.uk/qsedbuk.htm


  #33   Report Post  
Malcolm Reeves
 
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On Mon, 20 Sep 2004 10:46:27 +0100, Andy Hall
wrote:

The manufacturers should have them if they are selling their products
outside the UK.


I'm not sure that they do. The oil difference is quite significant I
think. In uk only kerosene can be low level flue so its all kerosene.
Europe is all gas oil I think (and I guess they allow gas oil low
level flues).

I'm not sure whether gas oil and kerosene would result in different
efficiency figures - different calorific values possibly.


Lots of differences. The nozzle at least needs to change, perhaps
more. Certainly a retest.

Oil smells. I suspect you are talking more about gas.


It does anyway though, doesn't it?


I'd have gas if I could :-). Anyway a good reason IMO to have a
higher flue temperature so exhaust dissipates upwards.

If you look at data sheets for devices like the Honeywell it shows it
as a set of almost straight lines depending on setting. Once the
valve opens it presents as a virtually constant pressure drop device.


Mine is a sunvic, perhaps they work better?


--

Malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
, or ).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

NEW - Desktop ToDo/Reminder program (free)
  #34   Report Post  
Christian McArdle
 
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I'm further minded to install a Grundfos Alpha pump, however, I'm
undecided
as to the control mechanism i.e. a flow switch as advocated by IMM in a
previous thread or to rely on the boiler stat set to 82degC as you stated.


Unfortunately, relying on the boiler stat would not meet building
regulations. You must have a flow switch, a room thermostat (in a room
without a TRV) or other similar mechanism that entirely cuts the boiler when
no heat is required.

Christian.



  #35   Report Post  
Neil Jones
 
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Malcolm Reeves wrote:
On Sun, 19 Sep 2004 23:40:39 +0100, "IMM" wrote:


You should have TRV on at least all the bedroom rads which is the
new rules AFAIR.


Nothing says it is madatory in the regs.


Building regs part L 2002 guidelines say:

"If a whole heating system is replaced, the controls must permit
independent temperature control in two separate zones: the living and
sleeping areas. This can be achieved with a single room thermostat and
thermostatic radiator valves."

*can* be achieved...

It can also be achieved by using zoning with 2 thermostats or (I can't
believe I'm writing this) 2 boilers, one running the living areas, and
one for the bedrooms.

Neil




  #36   Report Post  
Malcolm Reeves
 
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On Mon, 20 Sep 2004 14:04:41 +0100, "Neil Jones"
wrote:

Malcolm Reeves wrote:
On Sun, 19 Sep 2004 23:40:39 +0100, "IMM" wrote:


You should have TRV on at least all the bedroom rads which is the
new rules AFAIR.

Nothing says it is madatory in the regs.


Building regs part L 2002 guidelines say:

"If a whole heating system is replaced, the controls must permit
independent temperature control in two separate zones: the living and
sleeping areas. This can be achieved with a single room thermostat and
thermostatic radiator valves."

*can* be achieved...

It can also be achieved by using zoning with 2 thermostats or (I can't
believe I'm writing this) 2 boilers, one running the living areas, and
one for the bedrooms.


True, but as I read it the post was saying you didn't have to do that,
NOT, you didn't have to do that IF, you did something else.

It is mandatory to do some zone control - which for most TRVs are the
easiest route. The original poster has TRVs AFAIR, I was just adding
that he needed to keep them.

--

Malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
, or ).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

NEW - Desktop ToDo/Reminder program (free)
  #37   Report Post  
IMM
 
Posts: n/a
Default


"Neil Jones" wrote in message
...
Malcolm Reeves wrote:
On Sun, 19 Sep 2004 23:40:39 +0100, "IMM" wrote:


You should have TRV on at least all the bedroom rads which is the
new rules AFAIR.

Nothing says it is madatory in the regs.


Building regs part L 2002 guidelines say:

"If a whole heating system is replaced, the controls must permit
independent temperature control in two separate zones: the living and
sleeping areas. This can be achieved with a single room thermostat and
thermostatic radiator valves."

*can* be achieved...

It can also be achieved by using zoning with 2 thermostats or (I can't
believe I'm writing this) 2 boilers, one running the living areas, and
one for the bedrooms.


It is on about "logical" zones, not physical zones.


  #38   Report Post  
IMM
 
Posts: n/a
Default


"Malcolm Reeves" wrote in message
...
On Mon, 20 Sep 2004 14:04:41 +0100, "Neil Jones"
wrote:

Malcolm Reeves wrote:
On Sun, 19 Sep 2004 23:40:39 +0100, "IMM" wrote:


You should have TRV on at least all the bedroom rads which is the
new rules AFAIR.

Nothing says it is madatory in the regs.

Building regs part L 2002 guidelines say:

"If a whole heating system is replaced, the controls must permit
independent temperature control in two separate zones: the living and
sleeping areas. This can be achieved with a single room thermostat and
thermostatic radiator valves."

*can* be achieved...

It can also be achieved by using zoning with 2 thermostats or (I can't
believe I'm writing this) 2 boilers, one running the living areas, and
one for the bedrooms.


True, but as I read it the post was saying you didn't have to do that,
NOT, you didn't have to do that IF, you did something else.

It is mandatory to do some zone control


Where does it say that. A zone is an independently controlled, in time and
temp, section of the heating system. A TVR does not converts a room into a
zone.

- which for most TRVs are the
easiest route. The original poster
has TRVs AFAIR, I was just adding
that he needed to keep them.

--

Malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
, or ).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and

Power
electronics. More details plus freeware, Win95/98 DUN and Pspice tips,

see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

NEW - Desktop ToDo/Reminder program (free)



  #39   Report Post  
Malcolm Reeves
 
Posts: n/a
Default

On Mon, 20 Sep 2004 17:43:57 +0100, "IMM" wrote:


"Malcolm Reeves" wrote in message
.. .
On Mon, 20 Sep 2004 14:04:41 +0100, "Neil Jones"
wrote:

Malcolm Reeves wrote:
On Sun, 19 Sep 2004 23:40:39 +0100, "IMM" wrote:


You should have TRV on at least all the bedroom rads which is the
new rules AFAIR.

Nothing says it is madatory in the regs.

Building regs part L 2002 guidelines say:

"If a whole heating system is replaced, the controls must permit
independent temperature control in two separate zones: the living and
sleeping areas. This can be achieved with a single room thermostat and
thermostatic radiator valves."

*can* be achieved...

It can also be achieved by using zoning with 2 thermostats or (I can't
believe I'm writing this) 2 boilers, one running the living areas, and
one for the bedrooms.


True, but as I read it the post was saying you didn't have to do that,
NOT, you didn't have to do that IF, you did something else.

It is mandatory to do some zone control


Where does it say that. A zone is an independently controlled, in time and
temp, section of the heating system. A TVR does not converts a room into a
zone.

- which for most TRVs are the
easiest route. The original poster
has TRVs AFAIR, I was just adding
that he needed to keep them.


Eh? I just quoted Building regs guide for part L above. That says
you have to do it which makes it mandatory. They are only considering
zones to be at different control temperatures not different on/off
times. But of course you can go that far if you want.


--

Malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
, or ).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

NEW - Desktop ToDo/Reminder program (free)
  #40   Report Post  
Andy Hall
 
Posts: n/a
Default

On Mon, 20 Sep 2004 20:29:52 +0100, Malcolm Reeves
wrote:



Eh? I just quoted Building regs guide for part L above. That says
you have to do it which makes it mandatory. They are only considering
zones to be at different control temperatures not different on/off
times. But of course you can go that far if you want.



The Approved Documents to the Building Regulations are not of
themselves legally binding. It says as much in the introduction of
most.

They represent *a* way of complying with the statute but are not the
*only* way.

If you can propose an alternative to the suggestion, this can be just
as valid, although it may be necessary to agree the point with the
BCO.


..andy

To email, substitute .nospam with .gl
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