# Calculating heat output for a replacement boiler

Hi. Our old Rayburn multifuel cooker/boiler just died, and I need to replace it with something else. The rayburn used to run a central heating system of 13 radiators with a total area of approx 12 square yds; 9.6 square metres.
Can anyone tell me what Btu output figure the replacement stove should have to run the existing radiators comfortably? Or where on the web I could find the formula to work it out for myself?!
Many thanks.
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In the 1970's, when radiators didn't have fins - like they do today - a rough figure was 180 BThU/Hr per square foot of surface area.
I'm not sure whether your 12 sq yds is just one side of each rad, or both sides. If it is just one side, you need to double it to get surface area - and the above formula then gives a heat output of just under 40,000 BThU/Hr. If any of the radiators are doubled, or finned, they will have a higher output and you will need to take this into account when sizing a replacement boiler. Does your boiler also need to provide domestic hot water? If so, add a further 15,000-20,000 BThU/Hr.
Roger
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Roger Mills wrote:

NOTE: This does not compute if you have a hot water tank. You need very little to heat a tank - maybe 500-600W. You DO need a lot of its a comnbi system trho, since it has to het the water onthe fly,..but most of these seem to chop the heating whilst the bath is running anyway...

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Does your boiler also need to provide domestic hot water? If so, add

I'm sure that 15,000 BThU/Hr (about 4.4 KW) had used to be the recommendation for DHW. This would heat a 30 gallon tank from cold to 140 deg F (60 deg C) in about 2 hours. Your 500-600 watts (< 2000 BThU/Hr) would take 15 hours or so - no use if several people want baths!
Roger
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On Mon, 10 Nov 2003 12:12:56 -0000, "Roger Mills"

That rings a bell with me too. You can get fast recovery cylinders however, which will accept a much greater heat input. Although the less time required to heat, the more you can probably afford to discount the loss to the CH. I'd stick with the 15000btu.
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Roger Mills wrote:

No, but they don't want baths every minute of every day :)
Its usually acceptable to rob some power from the radiators while heating water - its generally done at different times of the day anyway. I owuld expect e.g. to put the heating on at dusk, and water either earlier or later, then run it very early in the morning - 3am say - whereas heating maybe only comes on at 6-7 a.m. Some houses I have been in, we used to use the immersion anyway in very cold weather.
What is important is to cover the worst case - where you have full heating needed on a cold christmas day. Then maybe add a kilowatt on top of that. In practice you just pick teh 'next size up' boiler. Its not a hige efficiency saving to have a boiler sized on the limit anyway.

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140
would
Not many programmers cater for your suggested arrangement of having the water on at totally different times from the heating.
To cater for a "worst case" of the heating full on on Christmas Day AND several people wanting baths without waiting for the next "night shift" for the water to get hot, you need the boiler capacity to be considerably more than that needed to service the radiators. Your single kilowatt is totally inadequate - 4 or 5 being needed.
Roger
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On 9 Nov 2003 11:06:34 -0800, snipped-for-privacy@beeb.net (bodger) wrote:

The first thing is to dump the idea of Btus. They are a deprecated unit and make calculation harder. Working in metric units and watts is very much easier.
Take a look at the web sites of the radiator manufacturers such as Myson, Stelrad and Barlo and find radiators of similar type and size to those that you have. Take care to look at whether they are double or single panel and with or without fins. This makes a big difference to output. Once you have the sizes and types, look up the output in Watts from the data sheet. For a standard convention al boiler operating with 82 degree flow temperature, multiply the figure in the table by 0.9 because the output and heat requirement will be lower than the test temperature.
Add these and add about 20% for margin. If you are replacing with another Rayburn, then you can find the specifications on Aga Rayburn's web site. If you are considering a wall or floor mount boiler then a condensing model is a good plan because of increased efficiency. Many of these types as well as conventional boilers modulate the burner and so will match output to the requirement, so getting the sizin g perfect becomes less of an issue anyway
.andy
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