What gas boiler would you recommend for a two bedroom, two storey
terrace house? There is no central heating or back boiler at the moment.

I am quite attracted to the idea of a modulating boiler that conforms to
the Opentherm standard. Though buying programmable thermostats that
could control such a boiler appears impossible. There is the Honeywell
CM937 but it does not appear to be sold in the UK.

Given that a combi boiler is likely to be much more powerful than needed
to heat the house, is there any good reason for not over sizing the
radiators so that the house can be heated quickly?


--
Michael Chare

"Michael Chare" mUNDERSCOREnews@chareDOTorgDOTuk wrote in message
o.uk...
What gas boiler would you recommend for a two bedroom, two storey terrace
house? There is no central heating or back boiler at the moment.

I am quite attracted to the idea of a modulating boiler that conforms to
the Opentherm standard. Though buying programmable thermostats that could
control such a boiler appears impossible. There is the Honeywell CM937 but
it does not appear to be sold in the UK.

Given that a combi boiler is likely to be much more powerful than needed
to heat the house, is there any good reason for not over sizing the
radiators so that the house can be heated quickly?

If you oversize them too much they hold a lot of water and take longer to
heat and cool causing large temperature cycles.

The larger the rads are the lower the temperature they can be run at, making
condensing boilers more efficient (assuming they are well designed).

On 28/08/2011 17:43, Michael Chare wrote:

What gas boiler would you recommend for a two bedroom, two storey
terrace house? There is no central heating or back boiler at the moment.

What are the water heating requirements? How well insulated is the place?

I am quite attracted to the idea of a modulating boiler that conforms to
the Opentherm standard. Though buying programmable thermostats that
could control such a boiler appears impossible. There is the Honeywell
CM937 but it does not appear to be sold in the UK.

Given that a combi boiler is likely to be much more powerful than needed
to heat the house, is there any good reason for not over sizing the
radiators so that the house can be heated quickly?

Oversized rads can work well with condensors - particularly when they
have weather compensation since that can boost efficiency of the boiler.


--
Cheers,

John.

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On 28/08/2011 18:57, John Rumm wrote:
On 28/08/2011 17:43, Michael Chare wrote:

What gas boiler would you recommend for a two bedroom, two storey
terrace house? There is no central heating or back boiler at the moment.

What are the water heating requirements? How well insulated is the place?



The short answer is I don't know the heat requirement but one would have
to make assumptions about the heat loss into the adjacent properties anyway.

I was assuming that any combi would be easily powerful enough since they
are generally 25kw or more and the house needs much less than that.


--
Michael Chare

In article ,
Michael Chare mUNDERSCOREnews@chareDOTorgDOTuk writes:
What gas boiler would you recommend for a two bedroom, two storey
terrace house? There is no central heating or back boiler at the moment.

I am quite attracted to the idea of a modulating boiler that conforms to
the Opentherm standard. Though buying programmable thermostats that
could control such a boiler appears impossible. There is the Honeywell
CM937 but it does not appear to be sold in the UK.

Given that a combi boiler is likely to be much more powerful than needed
to heat the house, is there any good reason for not over sizing the
radiators so that the house can be heated quickly?

This can work very well. I well-oversized the radiators (partly due
to a fault in the beta version of Myson's heatloss calculator I used),
but I'm really pleased I did. I can keep the house warm running the
rads at 45/40 (flow/return), which means the boiler is running more
efficiently than it normally would. If you have young or elderly,
this also means they can't burn themselves on the rads/pipes.
Also, the energy stored in the rads is lower when running at low
temperature, and I don't have to allow for any room temperature
overshoot after demand for heat is cut, unlike a system running
at more normal higher temperatures.

My heating demand is calculated as 11kW at -3C outside. The boiler
can output 25kW max, so I can use this to generate a very fast heat
up of the house from stone cold. In theory the boiler can run at up
to 83C, but the radiator sizing means the radiators reach 25kW
output before the boiler gets to 83C, so my system never runs that
hot for full output.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On 28/08/2011 20:58, Michael Chare wrote:
On 28/08/2011 18:57, John Rumm wrote:
On 28/08/2011 17:43, Michael Chare wrote:

What gas boiler would you recommend for a two bedroom, two storey
terrace house? There is no central heating or back boiler at the moment.

What are the water heating requirements? How well insulated is the place?



The short answer is I don't know the heat requirement but one would have
to make assumptions about the heat loss into the adjacent properties
anyway.

I meant more in terms of showers / baths etc and tank or instant hot water.

I was assuming that any combi would be easily powerful enough since they
are generally 25kw or more and the house needs much less than that.

If you are planning on a combi, then yup most will be 24kW or better
(although many of the "or better" only give full output on hot water
heating and quite often limit max rad power to 25kW or thereabouts).

A wide modulation range will let the boiler match the actual heat loss
better than a narrow one (i.e. allowing longer lower powered burns and
less cycling with more accurate temperature tracking).

(Although this is less of an issue with modern low water content boilers
anyway).


--
Cheers,

John.

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| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 29/08/2011 04:21, John Rumm wrote:
On 28/08/2011 20:58, Michael Chare wrote:
On 28/08/2011 18:57, John Rumm wrote:
On 28/08/2011 17:43, Michael Chare wrote:

What gas boiler would you recommend for a two bedroom, two storey
terrace house? There is no central heating or back boiler at the
moment.

What are the water heating requirements? How well insulated is the
place?



The short answer is I don't know the heat requirement but one would have
to make assumptions about the heat loss into the adjacent properties
anyway.

I meant more in terms of showers / baths etc and tank or instant hot water.

Thanks for your reply.

There will be just one bathroom with a bath. It would be nice if the
bath was not to slow to fill.



I was assuming that any combi would be easily powerful enough since they
are generally 25kw or more and the house needs much less than that.

If you are planning on a combi, then yup most will be 24kW or better
(although many of the "or better" only give full output on hot water
heating and quite often limit max rad power to 25kW or thereabouts).

I did wonder whether the heat output to DHW and CH would be the same.


A wide modulation range will let the boiler match the actual heat loss
better than a narrow one (i.e. allowing longer lower powered burns and
less cycling with more accurate temperature tracking).


If you use a programmable thermostat which probably cycles the boiler
several times an hour, how does the boiler know to modulate down? - and
does this modulating down confuse the thermostat?


(Although this is less of an issue with modern low water content boilers
anyway).




--
Michael Chare

On 29/08/2011 10:03, Michael Chare wrote:
On 29/08/2011 04:21, John Rumm wrote:
On 28/08/2011 20:58, Michael Chare wrote:
On 28/08/2011 18:57, John Rumm wrote:
On 28/08/2011 17:43, Michael Chare wrote:

What gas boiler would you recommend for a two bedroom, two storey
terrace house? There is no central heating or back boiler at the
moment.

What are the water heating requirements? How well insulated is the
place?



The short answer is I don't know the heat requirement but one would have
to make assumptions about the heat loss into the adjacent properties
anyway.

I meant more in terms of showers / baths etc and tank or instant hot
water.

Thanks for your reply.

There will be just one bathroom with a bath. It would be nice if the
bath was not to slow to fill.



I was assuming that any combi would be easily powerful enough since they
are generally 25kw or more and the house needs much less than that.

If you are planning on a combi, then yup most will be 24kW or better
(although many of the "or better" only give full output on hot water
heating and quite often limit max rad power to 25kW or thereabouts).

I did wonder whether the heat output to DHW and CH would be the same.


A wide modulation range will let the boiler match the actual heat loss
better than a narrow one (i.e. allowing longer lower powered burns and
less cycling with more accurate temperature tracking).


If you use a programmable thermostat which probably cycles the boiler
several times an hour, how does the boiler know to modulate down? - and
does this modulating down confuse the thermostat?

The boiler can basically sense the load by one of a number of
approaches. It can either monitor the temperature of the return water
and and modulate to keep this at desired difference from the flow, or it
can just attempt to keep the flow at a set temperature.

As the return temp rises (i.e. the heating load is reducing; TRVs are
reaching their set temp and shutting down, the heatloss from the primary
water is reducing) the boiler needs to reduce the heat input to prevent
the flow temperature exceeding the set point[1]. Hence it winds back the
power. If it reaches minimum power, and that still does not limit the
flow temp, then it will have to cycle off.

The room stat is only really interested in the end result. Once the room
is up to temperature then it stops calling for heat[2]. So it won't get
confused. The only difficulty is when you have a TRV on the rad warming
the room with the stat. Then there is a possibility that the TRV limits
the room temperature to lower than the set temp on the room stat. Hence
it can't be "satisfied" at its current set temp, and keeps the heating
running - even if the house is actually warm enough.

[1] The set point may be as simple as a rad flow temperature control on
the boiler - manually set to say "70" or whatever, or it could be
controlled by a more elaborate weather sensor that adjusts the flow temp
based on the outside temperature combined with a set of heat loss curves
that are appropriate for the construction of the building.

[2] "optimising" ones may introduce a bit of extra "hunting" when used
with weather compensated systems - since they will attempt to adjust
actual start times to ensure the house is at the desired temp at the
pre-programmed times. (i.e. if you say I want it to be 20 degrees at
7am, and its currently reading 15 degrees, they may "learn" that means
turning the heating on at 6 am to be at 20 by 7 am). Needless to say if
there is another control system that is affecting the rate of heat input
based on the temperature, it may mean the room stat's adjustments are
excessive.

(Some of the posher setups may use integrated room stats and weather
compensators that talk to each other to get round these issues).


--
Cheers,

John.

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On 29/08/2011 04:21, John Rumm wrote:

[...]
(although many of the "or better" only give full output on hot water
heating and quite often limit max rad power to 25kW or thereabouts).

Both the (other people's) combis that I've anything to do with have a
pre-set pot adjustment on the PCB for setting the max o/p to the heating
circuit. These are Baxi and Vokera models, so hardly qualify as
"better" grades. The adjustment is part of the commissioning procedure
- you decide on the heating o/p required, then look-up the corresponding
burner pressure in the manual, then twiddle the pot to set the desired
gas pressure - simples. However in both cases the 'professional'
installers had left the adjustments at max, making for a rather fierce
heating start-up.

--
Andy

On 02/09/2011 16:18, Andy Wade wrote:
On 29/08/2011 04:21, John Rumm wrote:

[...]
(although many of the "or better" only give full output on hot water
heating and quite often limit max rad power to 25kW or thereabouts).

Both the (other people's) combis that I've anything to do with have a
pre-set pot adjustment on the PCB for setting the max o/p to the heating
circuit. These are Baxi and Vokera models, so hardly qualify as "better"
grades. The adjustment is part of the commissioning procedure - you

Sorry, perhaps I was not being clear there. The "or better" I was
referring to was not the boiler quality, but the power output. I.e. 24kW
or better (i.e. higher).

decide on the heating o/p required, then look-up the corresponding
burner pressure in the manual, then twiddle the pot to set the desired
gas pressure - simples. However in both cases the 'professional'
installers had left the adjustments at max, making for a rather fierce
heating start-up.

Mo there is a surprise ;-)

I think the last one I owned did 35kW to water, and upto 24kW to rads
(but could modulate down to under half that)


--
Cheers,

John.

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