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Mike Clarke
 
Posts: n/a
Default TRVs, flow switches and boiler cycling

In article , Christian
McArdle wrote:

Is retrofitting a flow switch a practical option without modifying the
internal boiler wiring?


Correct. You may find it works with an additional external pump set to
minimum, if the boiler doesn't mind the extra help! The flow switch output
would connect to the thermostat input on the combi.

Whether this would work or not depends on the internal design on the combi.


That's an interesting possibility but I'm not sure if it would be
possible with our boiler. After living in a house with ducted warm air
heating for the last 35 years I'm seriously lacking in experience with
radiator systems so any comments and advice will be welcome.

To summarize my original post, we've just bought a house where there is
no room stat and only the bedrooms have TRVs. The "standard" approach of
a room stat in the living room and TRVs everywhere else doesn't appeal
since there will be times when we'll want to heat parts of the house
without heating the living room. Separate zones would obviously be the
best solution but I don't fancy the task of ripping up floors and
modifying the pipework layout to achieve this so I was considering
fitting TRVs to all the radiators and relying on the boiler's internal
bypass.

I was back at the "new" house last week and picked up the manual for the
boiler. It's a Worcester Bosch "350 Combi" (HC350.FSN), probably about 4
years old. The boiler has a single heat exchanger in the combustion
chamber, this normally feeds the CH circuit unless there's demand for
DHW when the CH pump stops and a separate pump starts up to divert the
flow to a water to water heat exchanger to provide DHW. In case my
description isn't clear I've put a copy of the boiler water flow diagram
on http://milibyte.co.uk/boiler.gif. As I said I'm a newbie to
radiator systems so I don't know if this is an unusual design or not.

It looks to me that an extra CH pump would have a detrimental impact on
the DHW mode by stealing some of the flow intended for the heat
exchanger unless it was connected across the flow and return lines with
a motorized valve (or a couple of non return valves) and configured to
cut in only when the flow valve sensed that all the TRVs had closed.
Since the extra pump would always be pumping into a "blocked" line I
suppose it would also need it's own bypass valve. I suspect I'd end up
with an overly complex setup just to avoid a bit of boiler cycling.

The boiler manual claims that "the electronic controls prevent rapid
cycling of the appliance in the central heating mode" but fails to go
into any detail. I suspect this is achieved by an imposed minimum "off"
cycle of 3 minutes which is mentioned elsewhere in the manual.

I suppose it all boils down to whether the cost of the fuel wasted in
cycling the boiler is sufficient to justify the cost of installing a
more complex control system.

--
Mike Clarke
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Malcolm Reeves
 
Posts: n/a
Default

On Sun, 19 Sep 2004 15:45:12 +0100, Mike Clarke
wrote:


To summarize my original post, we've just bought a house where there is
no room stat and only the bedrooms have TRVs. The "standard" approach of
a room stat in the living room and TRVs everywhere else doesn't appeal
since there will be times when we'll want to heat parts of the house
without heating the living room. Separate zones would obviously be the
best solution but I don't fancy the task of ripping up floors and
modifying the pipework layout to achieve this so I was considering
fitting TRVs to all the radiators and relying on the boiler's internal
bypass.


That won't work and probably is against building regs which now cover
heating systems. If you had a modulating boiler it could work but you
would need a bypass or an alpha pump since when all the TRVs closed
the pressure would rocket and most likely the TRVs make an awful
racket.

Have you thought about a radio thermostat? Not cheap but you could
take it from room to room. Then in the room that you want to keep
constant turn the TRVs to full and in the old thermostat room down to
some level.

I was back at the "new" house last week and picked up the manual for the
boiler. It's a Worcester Bosch "350 Combi" (HC350.FSN), probably about 4
years old. The boiler has a single heat exchanger in the combustion
chamber, this normally feeds the CH circuit unless there's demand for
DHW when the CH pump stops and a separate pump starts up to divert the
flow to a water to water heat exchanger to provide DHW. In case my
description isn't clear I've put a copy of the boiler water flow diagram
on http://milibyte.co.uk/boiler.gif. As I said I'm a newbie to
radiator systems so I don't know if this is an unusual design or not.


Are you sure the CH stops? Usually both pumps run and the flow is
shared. It's the same as one pump + 3 port valve only with 2 pumps
instead. Boiler drives DHW, CH or CH+DHW.

I suppose it all boils down to whether the cost of the fuel wasted in
cycling the boiler is sufficient to justify the cost of installing a
more complex control system.


Don't forget that the entire house is a system. Consider what happens
when the thermostat switches on. The water in the rads is at X C.
This is pumped through the boiler raising its temperature, heating the
rads up, heating the room air up. Eventually room stat clicks off.
Water in rads cools, air cools, room stat clicks on. Cycle repeats.
So the cycle time is a function of boiler output, amount of water in
the rads, amount of air in the room, etc.

If the boiler flow water exceeds its set temp before the room stat
clicks off then the boiler short cycles. But as you can see from
above this is function of the boiler output, rads etc. You can't
really stop it with controls unless you vary something above (i.e.
increase rad size etc.).

Basically the boiler is stuffing out X kW. It has to go somewhere,
the water, the house fabric, etc. If the boiler output is too high
then it will exceed temperature before the room has warmed up and the
stat turned off. The solution is to increase the output in the stat
room so that the stat turns off first (or increase the power sinking,
more rads say). Clearly balancing the system so each rad gets its
share of the power is a must.



--

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)
  #3   Report Post  
Mike Clarke
 
Posts: n/a
Default

In article , Malcolm Reeves
wrote:

On Sun, 19 Sep 2004 15:45:12 +0100, Mike Clarke
wrote:


To summarize my original post, we've just bought a house where there is
no room stat and only the bedrooms have TRVs. The "standard" approach of
a room stat in the living room and TRVs everywhere else doesn't appeal
since there will be times when we'll want to heat parts of the house
without heating the living room. Separate zones would obviously be the
best solution but I don't fancy the task of ripping up floors and
modifying the pipework layout to achieve this so I was considering
fitting TRVs to all the radiators and relying on the boiler's internal
bypass.


That won't work and probably is against building regs which now cover
heating systems. If you had a modulating boiler it could work but you
would need a bypass or an alpha pump since when all the TRVs closed
the pressure would rocket and most likely the TRVs make an awful
racket.


It is a modulating boiler and has an internal bypass so it works OK but
I am conscious of the energy wasted when all the TRVs are shut as the
boiler cuts in from time to time to keep the water circulating inside
itself up to temperature. Since this is an existing system it doesn't
have to be modified to comply with the more recent regs. OTOH I do need
to make some improvements over the current situation where 7 of the 12
radiators have no thermostatic control at all.

Have you thought about a radio thermostat? Not cheap but you could
take it from room to room. Then in the room that you want to keep
constant turn the TRVs to full and in the old thermostat room down to
some level.


That looks like a possible option to go on the list.

I was back at the "new" house last week and picked up the manual for the
boiler. It's a Worcester Bosch "350 Combi" (HC350.FSN), probably about 4
years old. The boiler has a single heat exchanger in the combustion
chamber, this normally feeds the CH circuit unless there's demand for
DHW when the CH pump stops and a separate pump starts up to divert the
flow to a water to water heat exchanger to provide DHW. In case my
description isn't clear I've put a copy of the boiler water flow diagram
on http://milibyte.co.uk/boiler.gif. As I said I'm a newbie to
radiator systems so I don't know if this is an unusual design or not.


Are you sure the CH stops? Usually both pumps run and the flow is
shared. It's the same as one pump + 3 port valve only with 2 pumps
instead. Boiler drives DHW, CH or CH+DHW.


Well, the manual states that demand for DHW will override the CH
requirement and when DHW is no longer required the burner will
extinguish for a waiting period of about 3 minutes before returning to
CH state. I can certainly confirm that the burners always shut down for
a few minutes after drawing hot water.

[Snip - explanation of room stat operation]

If the boiler flow water exceeds its set temp before the room stat
clicks off then the boiler short cycles. But as you can see from
above this is function of the boiler output, rads etc. You can't
really stop it with controls unless you vary something above (i.e.
increase rad size etc.).


Yes I appreciate that the boiler will cycle in this way when it can't
modulate down low enough. This form of cycling is OK, it's saving
energy. I was more concerned about the wasteful cycling when all the rad
valves are shut and it's just circulating through the bypass. Perhaps
I'm just being paranoid and the total heat wasted will be very low
compared to the heat used in warming the house at other times.

--
Mike Clarke
  #4   Report Post  
Andy Hall
 
Posts: n/a
Default

On Sun, 19 Sep 2004 22:45:15 +0100, Mike Clarke
wrote:



It is a modulating boiler and has an internal bypass so it works OK but
I am conscious of the energy wasted when all the TRVs are shut as the
boiler cuts in from time to time to keep the water circulating inside
itself up to temperature. Since this is an existing system it doesn't
have to be modified to comply with the more recent regs. OTOH I do need
to make some improvements over the current situation where 7 of the 12
radiators have no thermostatic control at all.


Given that situation, there is not a lot that you need to do other
than to fit TRVs on all of the remaining radiators bar one and then
arrange for the thermostat to be in the area of the non-TRV one.




Well, the manual states that demand for DHW will override the CH
requirement and when DHW is no longer required the burner will
extinguish for a waiting period of about 3 minutes before returning to
CH state. I can certainly confirm that the burners always shut down for
a few minutes after drawing hot water.


That's fairly typical behaviour.



[Snip - explanation of room stat operation]

If the boiler flow water exceeds its set temp before the room stat
clicks off then the boiler short cycles. But as you can see from
above this is function of the boiler output, rads etc. You can't
really stop it with controls unless you vary something above (i.e.
increase rad size etc.).


Yes I appreciate that the boiler will cycle in this way when it can't
modulate down low enough. This form of cycling is OK, it's saving
energy. I was more concerned about the wasteful cycling when all the rad
valves are shut and it's just circulating through the bypass. Perhaps
I'm just being paranoid and the total heat wasted will be very low
compared to the heat used in warming the house at other times.


It is very little energy but if you can avoid it it is better to do
so. You will probably find that the boiler adopts a different
on/off pattern at the bottom of its range to avoid rapid cycling.
It knows from how it has controlled the burner that the requirement is
low, and it also knows that this is because there is little or no
demand in the heating circuit.

The room thermostat hookup would eliminate it as long as you set the
temperature of it and the flow in the radiator appropriately.




..andy

To email, substitute .nospam with .gl
  #5   Report Post  
Christian McArdle
 
Posts: n/a
Default

That's an interesting possibility but I'm not sure if it would be
possible with our boiler. After living in a house with ducted warm air
heating for the last 35 years I'm seriously lacking in experience with
radiator systems so any comments and advice will be welcome.


One method that could be used is to separate the heating circuit from the
combi boiler primary, using a plate heat exchanger. Then, the new circuit
can have a full time alpha pump, flow switch or whatever, with all TRVs. The
flow switch, which detects flow through any radiator is used connected to
the combi as the call for heat. The system would work and be efficient, but
viability depends on obtaining a cheap plate heat exchanger.

+--------+ PHE
| -------------++-------+-FS----- to rads
| Boiler | || BP
| -------------++---------+--------
+--------+

= external pump, run continuously

BP = bypass (p. not required if pump is alpha)
FS = flow switch, demands heat from boiler
PHE = plate heat exchanger


Christian.




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

On Mon, 20 Sep 2004 10:03:15 +0100, "Christian McArdle"
wrote:

That's an interesting possibility but I'm not sure if it would be
possible with our boiler. After living in a house with ducted warm air
heating for the last 35 years I'm seriously lacking in experience with
radiator systems so any comments and advice will be welcome.


One method that could be used is to separate the heating circuit from the
combi boiler primary, using a plate heat exchanger. Then, the new circuit
can have a full time alpha pump, flow switch or whatever, with all TRVs. The
flow switch, which detects flow through any radiator is used connected to
the combi as the call for heat. The system would work and be efficient, but
viability depends on obtaining a cheap plate heat exchanger.



Cost is in the £80-100 range......


+--------+ PHE
| -------------++-------+-FS----- to rads
| Boiler | || BP
| -------------++---------+--------
+--------+

= external pump, run continuously

BP = bypass (p. not required if pump is alpha)
FS = flow switch, demands heat from boiler
PHE = plate heat exchanger


Christian.


..andy

To email, substitute .nospam with .gl
  #7   Report Post  
Malcolm Reeves
 
Posts: n/a
Default

On Sun, 19 Sep 2004 22:45:15 +0100, Mike Clarke
wrote:

It is a modulating boiler and has an internal bypass.


I keep forgetting that modulating boilers are common on gas. I don't
have gas so I'm oil centric :-), and modulating oil burners are very
rare (and expensive).


--

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)
  #8   Report Post  
Mike Clarke
 
Posts: n/a
Default

In article , Andy Hall
wrote:

On Mon, 20 Sep 2004 10:03:15 +0100, "Christian McArdle"
wrote:


[Snip]

One method that could be used is to separate the heating circuit from the
combi boiler primary, using a plate heat exchanger. Then, the new circuit
can have a full time alpha pump, flow switch or whatever, with all TRVs. The
flow switch, which detects flow through any radiator is used connected to
the combi as the call for heat. The system would work and be efficient, but
viability depends on obtaining a cheap plate heat exchanger.


Cost is in the £80-100 range......



+--------+ PHE
| -------------++-------+-FS----- to rads
| Boiler | || BP
| -------------++---------+--------
+--------+

= external pump, run continuously

BP = bypass (p. not required if pump is alpha)
FS = flow switch, demands heat from boiler
PHE = plate heat exchanger


Christian.


.andy


Thanks, this looks a much more elegant approach than my idea for an
extra pump and valves. Either way it's going to be quite a lot more
expensive than just adding a flow switch if that had been feasible.

I'm reassured by Andy's earlier comment about the losses from cycling
when the TRVs are all off being low, so I'll probably just go ahead with
adding TRVs to the rest of the rads and see how things go. I'll try to
get a feel for the losses by monitoring the gas meter for a couple of
hours or so with all the rads shut down.

--
Mike Clarke
  #9   Report Post  
Christian McArdle
 
Posts: n/a
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I'll try to get a feel for the losses by monitoring the gas meter for a
couple of hours or so with all the rads shut down.


Remember that such a system would not comply with building regulations,
though. If you really do want to continue with such a system, it is
essential that the entire primary circuit is insulated to reduce losses.

Christian.


  #10   Report Post  
Mike Clarke
 
Posts: n/a
Default

In article , Christian
McArdle wrote:

Remember that such a system would not comply with building regulations,


Agreed, but it will be a big improvement on the existing situation where
almost half of the radiators have no thermostatic control at all. Since
the system was installed about 4 years ago I assume it doesn't have to
comply with the latest regs though I do want to make it reasonably
economical to run, providing the cost of any improvements can be
recouped from the energy savings.

though. If you really do want to continue with such a system, it is
essential that the entire primary circuit is insulated to reduce
losses.


The pipework does seem to be quite well insulated but I don't suppose
this is relevant to the situation where all TRVs are shut and the only
flow is through the internal bypass in the boiler.

--
Mike Clarke


  #11   Report Post  
Christian McArdle
 
Posts: n/a
Default

The pipework does seem to be quite well insulated but I don't suppose
this is relevant to the situation where all TRVs are shut and the only
flow is through the internal bypass in the boiler.


The problem comes when the radiators are mostly shut off. In this situation,
you would probably want the flow switch to cut the power, as the boiler will
be well below modulating minimum and will short cycle. Better to cut totally
and wait until the rooms require real levels of heat.

Without the interlock, enough water may leak past the TRVs to keep the
primary circuit hot at all times, with the short cycling boiler, so you
really need the entire circuit insulated.

Christian.


  #12   Report Post  
IMM
 
Posts: n/a
Default


"Mike Clarke" wrote in message
...
In article , Christian
McArdle wrote:

Is retrofitting a flow switch a practical option without modifying the
internal boiler wiring?


Correct. You may find it works with an additional external pump set to
minimum, if the boiler doesn't mind the extra help! The flow switch

output
would connect to the thermostat input on the combi.

Whether this would work or not depends on the internal design on the

combi.

That's an interesting possibility but I'm not sure if it would be
possible with our boiler. After living in a house with ducted warm air
heating for the last 35 years I'm seriously lacking in experience with
radiator systems so any comments and advice will be welcome.

To summarize my original post, we've just bought a house where there is
no room stat and only the bedrooms have TRVs. The "standard" approach of
a room stat in the living room and TRVs everywhere else doesn't appeal
since there will be times when we'll want to heat parts of the house
without heating the living room. Separate zones would obviously be the
best solution but I don't fancy the task of ripping up floors and
modifying the pipework layout to achieve this so I was considering
fitting TRVs to all the radiators and relying on the boiler's internal
bypass.

I was back at the "new" house last week and picked up the manual for the
boiler. It's a Worcester Bosch "350 Combi" (HC350.FSN), probably about 4
years old. The boiler has a single heat exchanger in the combustion
chamber, this normally feeds the CH circuit unless there's demand for
DHW when the CH pump stops and a separate pump starts up to divert the
flow to a water to water heat exchanger to provide DHW. In case my
description isn't clear I've put a copy of the boiler water flow diagram
on http://milibyte.co.uk/boiler.gif. As I said I'm a newbie to
radiator systems so I don't know if this is an unusual design or not.

It looks to me that an extra CH pump would have a detrimental impact on
the DHW mode by stealing some of the flow intended for the heat
exchanger unless it was connected across the flow and return lines with
a motorized valve (or a couple of non return valves) and configured to
cut in only when the flow valve sensed that all the TRVs had closed.
Since the extra pump would always be pumping into a "blocked" line I
suppose it would also need it's own bypass valve. I suspect I'd end up
with an overly complex setup just to avoid a bit of boiler cycling.

The boiler manual claims that "the electronic controls prevent rapid
cycling of the appliance in the central heating mode" but fails to go
into any detail. I suspect this is achieved by an imposed minimum "off"
cycle of 3 minutes which is mentioned elsewhere in the manual.

I suppose it all boils down to whether the cost of the fuel wasted in
cycling the boiler is sufficient to justify the cost of installing a
more complex control system.


Your gif is too fuzzy for me to see, so I can't see if there is an internal
by-pass. If not, then install one between the flow and return. On the
outlet side of the by-pass, bwteen the two tees, fit a flow switch which
switches on-off the boiler. When the by-pass opens there will be flow, when
it opens further the flowswitch cuts in and cuts out the boiler.



  #13   Report Post  
IMM
 
Posts: n/a
Default


"Andy Hall" wrote in message
...
On Mon, 20 Sep 2004 10:03:15 +0100, "Christian McArdle"
wrote:

That's an interesting possibility but I'm not sure if it would be
possible with our boiler. After living in a house with ducted warm air
heating for the last 35 years I'm seriously lacking in experience with
radiator systems so any comments and advice will be welcome.


One method that could be used is to separate the heating circuit from the
combi boiler primary, using a plate heat exchanger. Then, the new circuit
can have a full time alpha pump, flow switch or whatever, with all TRVs.

The
flow switch, which detects flow through any radiator is used connected to
the combi as the call for heat. The system would work and be efficient,

but
viability depends on obtaining a cheap plate heat exchanger.



Cost is in the £80-100 range......


+--------+ PHE
| -------------++-------+-FS----- to rads
| Boiler | || BP
| -------------++---------+--------
+--------+

= external pump, run continuously

BP = bypass (p. not required if pump is alpha)
FS = flow switch, demands heat from boiler
PHE = plate heat exchanger


All so complicated.


  #14   Report Post  
IMM
 
Posts: n/a
Default


"Mike Clarke" wrote in message
...

It is a modulating boiler and has
an internal bypass so it works OK but
I am conscious of the energy wasted
when all the TRVs are shut as the
boiler cuts in from time to time
to keep the water circulating inside
itself up to temperature.


Ah! Simple. Just fit a flowswitch on the return. When all TVRs are closed
the boiler will protect itself by opening its own by-pass and the flow
switch will simply turn the boiler off as there is no flow.

What is all the fuss on this thread. My, oh, my!



  #15   Report Post  
IMM
 
Posts: n/a
Default


"IMM" wrote in message
...

"Mike Clarke" wrote in message
...

It is a modulating boiler and has
an internal bypass so it works OK but
I am conscious of the energy wasted
when all the TRVs are shut as the
boiler cuts in from time to time
to keep the water circulating inside
itself up to temperature.


Ah! Simple. Just fit a flowswitch on the return. When all TVRs are

closed
the boiler will protect itself by opening its own by-pass and the flow
switch will simply turn the boiler off as there is no flow.

What is all the fuss on this thread. My, oh, my!


Just re-read what I wrote. To expand. Use an external by-pass and
flowswitch and a timer to switch back on to check if there is still flow
(TVRs opening), if no flow then if immediately switches off. If flow it stay
on until the flowswitch cuts in again.





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

On Fri, 5 Nov 2004 17:42:59 -0000, "IMM" wrote:




Your gif is too fuzzy for me to see, so I can't see if there is an internal
by-pass.


Are you sure that's the reason? I can see it all perfectly easily.

You may want to check for hairs on the palms of your hands.......




--

..andy

To email, substitute .nospam with .gl
  #17   Report Post  
IMM
 
Posts: n/a
Default


"Andy Hall" wrote in message
...
On Fri, 5 Nov 2004 17:42:59 -0000, "IMM" wrote:


Your gif is too fuzzy for me to see, so I can't see if there is an

internal
by-pass.


Are you sure that's the reason? I can see it all perfectly easily.

You may want to check for hairs on the palms of your hands.......


Will they make the screen go clear?


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

On Fri, 5 Nov 2004 23:14:33 -0000, "IMM" wrote:


"Andy Hall" wrote in message
.. .
On Fri, 5 Nov 2004 17:42:59 -0000, "IMM" wrote:


Your gif is too fuzzy for me to see, so I can't see if there is an

internal
by-pass.


Are you sure that's the reason? I can see it all perfectly easily.

You may want to check for hairs on the palms of your hands.......


Will they make the screen go clear?

Doubtful. You'd have to clean it off first.



--

..andy

To email, substitute .nospam with .gl
  #19   Report Post  
Mike Clarke
 
Posts: n/a
Default

In article , IMM wrote:


"IMM" wrote in message
...

"Mike Clarke" wrote in message
...

It is a modulating boiler and has
an internal bypass so it works OK but
I am conscious of the energy wasted
when all the TRVs are shut as the
boiler cuts in from time to time
to keep the water circulating inside
itself up to temperature.


Ah! Simple. Just fit a flowswitch on the return. When all TVRs are

closed
the boiler will protect itself by opening its own by-pass and the flow
switch will simply turn the boiler off as there is no flow.

What is all the fuss on this thread. My, oh, my!


Just re-read what I wrote. To expand. Use an external by-pass and
flowswitch and a timer to switch back on to check if there is still flow
(TVRs opening), if no flow then if immediately switches off. If flow it stay
on until the flowswitch cuts in again.


But it shouldn't switch off immediately, we'll need a few seconds for
the flow to build up. So we're talking about a bit of custom timer logic
here or a couple of cascaded timeout switches, probably getting as
complicated as previous ideas.

--
Mike Clarke
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