CH Boiler Sizing

Can anyone point me towards an Internet resource which gives information about boiler size to square footage of property for a domestic CH system? Or alternatively comment on the following?
I visited a property today to investigate the CH system not giving out enough heat when it gets really cold. One of the things I found immediately was that the radiators weren't balanced properly (the return valves were full open on all that I checked), so that's one job I've got to do.
However, the CH boiler looks like it's well underspecced for the size of property. It's a Potterton Profile - I didn't think to get the model number but it just looked too puny to deliver the heat required for what is a large property - the property has been extended with an extra floor. Physically it looked like it was the Potterton Profile 40L jobbie:
http://inspiredheating.co.uk/acatalog/POTTERTON_PROFILE__FAN_FLUE_.html
If so then it's a 40,000 BTU unit.
What I'd really like to get information on is what size boiler needs to be installed for a given square footage, if such info is available anywhere.
Clues anyone?
PoP
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In order to do a proper job, you need to take account of far more factors than just the floor area. The wall and roof construction/insulation and the amount and type of glazing all have a profound effect on the heat losses.
There are various bits of software (such as http://www.barlo.co.uk/pages/download.cfm ) which you can download, which will do the calcs for you - but they do require some reasonable input data. Otherwise, its garbage in, garbarge out!
HTH, Roger
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wrote:

As a quick solution, download one of the heating calculators like the Myson one. Treat the entire house as one enormous room - i.e. all room temperatures the same at 21 degrees and use the exterior dimensions of the house.
You do need to take into account the materials used for walls and windows (use the areas of those or estimate). There is anything up to a 5:1 difference in heat loss through walls depending on the material used, so a guess based on area is going to be wildly inaccurate. Generally the walls represent the largest heat loss.
Maybe you could do a proper measurement when you balance the radiators, but a 12kW boiler is likely to be too small for a house of any size.
In that case a new or an additional one is needed and that renders the balancing exercise pointless until it's fixed.
.andy
To email, substitute .nospam with .gl
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wrote:

Exactly what I told the guy.
Thanks for the additional comments BTW. I think I was just looking for an answer to "is 40000 BTU too small for a large 4 bed house". It might have been a better question anyway ;)
Too little beer last night I fear.....
Talking of which, when shall we arrange our next Screwfix sponsored event?
PoP
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Boiler No 6 (or is it 7, I'm losing count :-) coming up? ;-)

Heh heh!
-- John Stumbles -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -+ Bob the builder / it'll cost yer Bob the builder / loadsa dosh
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On Wed, 5 Nov 2003 21:08:19 -0000, "John Stumbles"

I can offer your name if you want. Changing the boiler is not something I'm going to take on, but flushing the system and refilling is no problem.
PoP
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PoP wrote:

Could we stick to kW by the way? Its easier to visualise. e.g. say you have 24kW boiler and 10 rooms thats average 2.4kW /room which sounds enough in a very crude sense. 12kW just doesn't sound enough does it when looked at this way?
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wrote:

Perhaps someone could kindly advise how to convert BTU to kW?
PoP
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http://www.simetric.co.uk/sibtu.htm
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http://www.ex.ac.uk/cimt/dictunit/ccthcony.htm
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From memory, 1Kw = approx 3,500 BThU/Hr (but doubtless someone will supply the exact figure)
Roger
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BillR wrote:

My kitchen has an aga in it that runs day and night at about 600W. In winter it has probably 12x50W lights in it burning most of teh time. It also has U/F heating but I have that bit balanced down to bugger all. Its 30 square meters of room. Thats 1.2Kw over 30 sq m, or about 40W per square. The whole house of about 200sq meter was calculated by experts to need just 10Kw, which is 50W.sq meter.
My U/F heating guide reckoned that a well insulated house needed peak demands of 50W/sq meter, and a poorly insulated one maybe up to 200W/sq meter, with 125W/sq meter being a good working average.
Apart from the living rooms, which usually get an open fire in winter, that level (50w/sq meter) of heating works. Its higher in the bathrooms. Of course there is a lot of heat from lighting in the winter - probabaly after dark each room is kicking in another 400W per 30 sq meter room - so maybe I have up to 65W/sq meter.
Contrast with my old single brick cottage, which used to burn exactly the same amount of oil as I do now, to heat (badly) four rooms and a porch.
And where a 3Kw fan heater was not adequate to heat a single room.

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PoP wrote:

The answer is emphatically NO, its NOT, if its well insulated, and assuming its not a combi and has to heat three independent showers and a bath simultaneously.

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Andy Hall wrote:

My expenively subcontracted heatloss calculations came up with 10Kw as peak demand. On a very big house.
I stuck in a bigger boiler than that tho - but its not HUGE. Even at oeak cold it doesn't run flat out - maybe 50% duty cycle at the worst.

Thats a clue to an emirical way to find out what you need.
Balance the system, and short out the thermostat, and leave the system running flat out for a few hours. Measure the internal and external temperature and if the difference is less than 30 degrees celsius, you need more insulation or a bigger boiler :-)

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PoP wrote:

It depends ENTIRELY on the standard of insulation. My plumber (the one who thought pressure came not from teh header tank, but teh hot water tank) almost refused to install my boiler as he considered it only big enougjh for a three bed semi. I have (estate agent spik) a 6 bed cottage.
Its been fine. The house is built to current regs insulation wise.
The actual calcs to size a boiler consist in doing an elemenatl loss calculation of teh whoole outside of the house, adding in ventilation losses, gains from south facing windows and walls, working out the heat you want it and the worst temp the outdie will get to, and coming up with the number of kilowatts (or BTU/H) needed to heat it.
If its any help, fully insulated house, about 400 sq meter total floor area, and I am using a 65kBTU/H boiler.
Poorly insulated house - treble that at least.

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