I am about to replace our 1960's Thorn M gas boiler
with a Condensing boiler..

Evidently condensing boilers run more efficiently
than non-condensing boilers. But my understanding
is that they rely on a return temperature of 55/60C ish
(I'm not sure what a typical flow temp for a
condensing boiler is - same as non condensing
boiler ?)

I believe that my system runs with a flow 82C of
and a return of about 71C. Not checked but that
would be typical.

So what sort of efficiency am I going to get out
of my brad new Condensing boiler? Surely not
much better than my old Thorn M?

Is there anything I can do to my system to help the
situation. I had a plumber come in recently who
told me that I should be fitting thermostatic
controls to all radiators. Is that a regs requirement?
Isn't that going to make the situation worse?

Roy

Evidently condensing boilers run more efficiently
than non-condensing boilers.

Yes.

But my understanding is that they rely on a return temperature of
55/60C ish

Nope. Whilst condensing boilers do like a low return temp, they do not
require one.

Although larger radiators will enable you to maintain output at lower
temperatures, the electronics within the boiler are capable of reducing the
temperature when it is not very cold, anyway, so your small rads can still
run cooler. Even at exactly the same temperature, the condensing boiler will
eat your old Thorn for breakfast in the efficiency stakes.

So what sort of efficiency am I going to get out
of my brad new Condensing boiler? Surely not
much better than my old Thorn M?

Loads better.

I had a plumber come in recently who told me that I should be fitting
thermostatic controls to all radiators. Is that a regs requirement?

It is one way of meeting the regs requirement. The cheapest way, in fact.

Isn't that going to make the situation worse?

No, it will make it considerably better.

Christian.

On Wed, 7 Sep 2005 17:00:58 +0100, "RzB"
wrote:

I am about to replace our 1960's Thorn M gas boiler
with a Condensing boiler..

Evidently condensing boilers run more efficiently
than non-condensing boilers. But my understanding
is that they rely on a return temperature of 55/60C ish
(I'm not sure what a typical flow temp for a
condensing boiler is - same as non condensing
boiler ?)

It will depend on the behaviour of the boiler controls. One thing to
remember is that condensing boilers usually modulate, meaning that the
burn rate is reduced when the heat requirement is less in warmer
weather.

One thing to realise is that the 54 degree dew point is not like
finding the Holy Grail. On a condensing boiler, the efficiency
improves with reducing return temperature. Above the dew point this
happens at one rate with respect to temperature. It happens at a
greater rate below the dew point. There is a graph on the data sheet
of the Celsius boiler on Keston's web site which shows this.

The second point is that the existing radiator sizings will have been
designed on the basis of 82 flow, 70 return and a worst case outside
temperature of -1 or -3 degrees. However, these days are relatively
rare, and most of the time, the output needed to keep the place warm
is a lot less. So for at least 9 months plus of a typical year, the
boiler will modulate down as will temperatures to more efficient
ranges. When needed, the boiler will be able to deliver 82/70. Even
though it won't be condensing, because it will have a better heat
exchanger anyway, it will be more efficient boiler than the old one.



I believe that my system runs with a flow 82C of
and a return of about 71C. Not checked but that
would be typical.

So what sort of efficiency am I going to get out
of my brad new Condensing boiler? Surely not
much better than my old Thorn M?


The old boiler is likely to have a seasonal efficiency in the 50-60%
range. Seasonal efficiency is described on the SEDBUK web site, but
is basically the behaviour characterised for the typical UK climate
over the whole year rather than simple lab optimised figures.

Prior to the change in rules mandating condensing boilers in most
cases, non-condensing boilers were reaching 78-80% on this scale,
partly because of better heat exchangers, partly the use of fanned
flues and partly absence of pilots (possibly others). Decent
condensing boilers all achieve 90-91%.

So, if you do nothing with radiators you could expect to achieve
better than 80% and not 90%.




Is there anything I can do to my system to help the
situation.


You could replace radiators with larger ones or ones having more
panels, or do what I did and move some round and replace some.


I had a plumber come in recently who
told me that I should be fitting thermostatic
controls to all radiators.

This is worth doing anyway and is likely to provide a better ROI than
radiator changing, so if you do one thing, that is it.



Is that a regs requirement?
Isn't that going to make the situation worse?

No because the boiler will modulate output down when the heat isn't
required. You leave one radiator where the room thermostat is located
without a TRV




Roy


--

..andy

To email, substitute .nospam with .gl

On Wed, 07 Sep 2005 18:39:33 +0100, Andy Hall wrote:

| The second point is that the existing radiator sizings will have been
| designed on the basis of 82 flow, 70 return and a worst case outside
| temperature of -1 or -3 degrees. However, these days are relatively
| rare, and most of the time, the output needed to keep the place warm
| is a lot less. So for at least 9 months plus of a typical year, the
| boiler will modulate down as will temperatures to more efficient
| ranges. When needed, the boiler will be able to deliver 82/70. Even
| though it won't be condensing, because it will have a better heat
| exchanger anyway, it will be more efficient boiler than the old one.

My house is so well insulated that it stays nice and warm on radiators
rated 3kw downstairs to which I switch another 1.5 kw upstairs in very cold
weather. These are similar to Stelrad Elite P1
http://www.stelrad.com/UK/elite.html from which I cribbed the ratings. I
did the heating calcs for these 25 years ago, since which I have improved
the insulation, so I believe that I could now use smaller radiators. I
have uses a 10 kw conventional boiler, for Central heating and domestic hot
water, without problems for 25 years.

My problem is that the smallest combi boiler I can find is the eco Hometec
EC16S which is rated by SEBUK at 22.0kw regulating down to 3kw. The
companies literature rates it at 16kw ish modulating down to 2.5 or 3.1kw.
Clearly much too big for my house.

Surely through most of the year, on minimum modulation this will cycle on
and off and loose most of its condensing advantages. DHW is OK

--
Dave Fawthrop dave hyphenologist co uk
"Intelligent Design?" my knees say *not*.
"Intelligent Design?" my back says *not*.

Dave Fawthrop wrote:

My problem is that the smallest combi boiler I can find is the eco Hometec
EC16S which is rated by SEBUK at 22.0kw regulating down to 3kw. The
companies literature rates it at 16kw ish modulating down to 2.5 or 3.1kw.
Clearly much too big for my house.

Not really. If you replace it and don't go the combi route, you will
need to replace the hot water storage cylinder as well with a fast
recoivery one to comply with the building regs. That will swallow the
20kW output nicely and give fast recovery times.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

My problem is that the smallest combi boiler I can find is the eco
Hometec
EC16S which is rated by SEBUK at 22.0kw regulating down to 3kw. The
companies literature rates it at 16kw ish modulating down to 2.5 or
3.1kw.
Clearly much too big for my house.

Not really. If you replace it and don't go the combi route, you will
need to replace the hot water storage cylinder as well with a fast
recoivery one to comply with the building regs. That will swallow the
20kW output nicely and give fast recovery times.

I wasn't aware that the cylinder needed to be changed, only that if it was
changed, it would have to be Part L1 compliant. Any existing cylinder kept
would need to be fully pumped, with a thermostat added, though. Obviously,
an ancient scaled up cylinder with only a crappy add on jacket is probably
worth replacing anyway.

Christian.

"John Rumm" wrote in message

Not really. If you replace it and don't go the combi route, you will need
to replace the hot water storage cylinder as well with a fast recoivery
one to comply with the building regs.

Do you have a link to this requirement please John?
I have seen people advise the use of fast recovery cylinders, but never
stipulated as a requirement for Building Regs!

TIA

Phil

Do you have a link to this requirement please John?
I have seen people advise the use of fast recovery cylinders, but never
stipulated as a requirement for Building Regs!

L1 compliant cylinders must have a certain amount of insulation and a
certain size of heat exchanger (so the boiler can heat it quickly and turn
off, reducing primary circuit losses). A true rapid recovery cylinder will
actually have a much larger heat exchanger than that mandated for L1,
although the L1 is much better than the standard cylinders installed 10-20
years ago.

I believe you only need to fit one when installing a new one, or replacing
an old one.

Christian.

TheScullster wrote:

Do you have a link to this requirement please John?

http://www.odpm.gov.uk/stellent/grou...reg_600288.pdf

I have seen people advise the use of fast recovery cylinders, but never
stipulated as a requirement for Building Regs!

Yup sorry, I might have overstated the requirements slightly, reading
the detail :

"1.43 For systems incorporating integral or
separate hot water storage vessels, ways of
meeting the requirement include:
a) arranging for hot water storage systems to
meet the insulation requirements of BS 1566,
BS 699, BS 3198, or BS 7206 (as appropriate);
or
b) in ordinary cases, insulating vessels with a
35mm thick, factory-applied coating of
PU-foam having a minimum density of 30kg/m3.
(For unvented hot water systems additional
insulation should be provided to control the
heat losses through the safety fittings and
pipework but without impeding safe operation
and visibility of warning discharges. (See
Approved Document G.)"

And:

"1.44 Provisions should enable efficient
operation without excessive boiler firing and
primary circuit losses. A way of demonstrating
compliance for indirectly heated hot water
storage systems would be for the size of the
heat exchanger to be at least that
recommended in BS 1566, BS 3198, or
BS 7206 (as appropriate) and for them to be
served by a pumped primary system."

I have not looked at BS1566, or 3198, but it looks like there is a
rquirement to meet L1, but that does not need to be as fast as a "fast
recovery" cylinder. Just well insulated and fast ish!

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Thu, 08 Sep 2005 13:01:58 +0100, John Rumm
wrote:

| Dave Fawthrop wrote:
|
| My problem is that the smallest combi boiler I can find is the eco Hometec
| EC16S which is rated by SEBUK at 22.0kw regulating down to 3kw. The
| companies literature rates it at 16kw ish modulating down to 2.5 or 3.1kw.
| Clearly much too big for my house.
|
| Not really. If you replace it and don't go the combi route, you will
| need to replace the hot water storage cylinder as well with a fast
| recoivery one to comply with the building regs. That will swallow the
| 20kW output nicely and give fast recovery times.

I do not need hot water storage, the EC16S will give more DHW flow than I
have at the moment.

--
Dave Fawthrop dave hyphenologist co uk
"Intelligent Design?" my knees say *not*.
"Intelligent Design?" my back says *not*.

On Thu, 08 Sep 2005 11:48:25 +0100, Dave Fawthrop
wrote:

On Wed, 07 Sep 2005 18:39:33 +0100, Andy Hall wrote:

| The second point is that the existing radiator sizings will have been
| designed on the basis of 82 flow, 70 return and a worst case outside
| temperature of -1 or -3 degrees. However, these days are relatively
| rare, and most of the time, the output needed to keep the place warm
| is a lot less. So for at least 9 months plus of a typical year, the
| boiler will modulate down as will temperatures to more efficient
| ranges. When needed, the boiler will be able to deliver 82/70. Even
| though it won't be condensing, because it will have a better heat
| exchanger anyway, it will be more efficient boiler than the old one.

My house is so well insulated that it stays nice and warm on radiators
rated 3kw downstairs to which I switch another 1.5 kw upstairs in very cold
weather. These are similar to Stelrad Elite P1
http://www.stelrad.com/UK/elite.html from which I cribbed the ratings. I
did the heating calcs for these 25 years ago, since which I have improved
the insulation, so I believe that I could now use smaller radiators. I
have uses a 10 kw conventional boiler, for Central heating and domestic hot
water, without problems for 25 years.

My problem is that the smallest combi boiler I can find is the eco Hometec
EC16S which is rated by SEBUK at 22.0kw regulating down to 3kw. The
companies literature rates it at 16kw ish modulating down to 2.5 or 3.1kw.
Clearly much too big for my house.

Surely through most of the year, on minimum modulation this will cycle on
and off and loose most of its condensing advantages. DHW is OK

This is actually the MAN Heiztechnik Micromat. I have one of the
larger versions, but of course not a combi.

The control system is more sophisticated than most boilers in that
there is also weather compensation and slope control.

This means that it will modulate down such that the flow temperature
is at around 40 degrees. The pump also reduces in power to as low as
20% of full run rate.

Once you fall below the minimum 3kW modulation level, the boiler will
cycle, but at this low temperature on a very long time constant.

Since your existing boiler is a 10kW one, it will be cycling on the
thermostat at full power and 82 degree operation. I would suspect
that there is quite an overshoot on flow temperature as well.

Running the burner at 3kW and 40 degrees means that the boiler is
operating well into the high efficiency slope anyway and spare heat is
distributed within the building. Much of the efficiency loss through
cycling with conventional boilers cycling is flue loss when the boiler
no longer should be providing heat.



--

..andy

To email, substitute .nospam with .gl

RzB wrote:

I am about to replace our 1960's Thorn M gas boiler
with a Condensing boiler..

Evidently condensing boilers run more efficiently
than non-condensing boilers. But my understanding
is that they rely on a return temperature of 55/60C ish
(I'm not sure what a typical flow temp for a
condensing boiler is - same as non condensing
boiler ?)

"Rely" is too strong a word in this circumstance. A modern boiler will
be significantly more efficient than an older counterpart by virtue of
more sophisticated control systems, and better design anyway. A
condensing boiler even moreso as a result of its larger heat exchanger
and its ability to recover what would otherwise be lost heat.

I believe that my system runs with a flow 82C of
and a return of about 71C. Not checked but that
would be typical.

Probably about right...

So what sort of efficiency am I going to get out
of my brad new Condensing boiler? Surely not
much better than my old Thorn M?

I would expect it to be significantly better - since the old boiler will
be fairly poor by modern standards even if comparing against a non
condenser.

Is there anything I can do to my system to help the
situation. I had a plumber come in recently who

The boiler will do much of the work for you, since it will use its
ability to modulate to actually match the heat it provides to the load.
It can also use this capability to maximise the amount of time it spends
condensing. In many cases it will run the system cooler, but for longer
(which is preferable to bursts of full output, which result in
overshooting the desired temperature, followed by gaps of zero output)

told me that I should be fitting thermostatic
controls to all radiators. Is that a regs requirement?

Yes, the controls on the whole system will need to be up to spec. So
TRVs on all reads except the one in the room with the room stat (which
provides the interlock to turn the whole lot off when the demand is
reacted (can also do it with all TRVs and a flow sensor)). Fast recovery
cylinder, fully pumped, and zoned if it is a big house.

Isn't that going to make the situation worse?

Nope they will help. As the TRVs close down, the rate of flow through
the rads falls, giving bigger temperature differentials across the rads
and lower return temps - this improves boiler efficiency still further.
As the system as a whole comes up to temp, there will be a time when the
boiler sees a rise in the overall return temp (since the heat being
dumped to the house is falling) this will allow the boiler to modulate
down further.



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Christian/Andy/John,

That sounds good news.

Many thanks for your advice,
Roy