new boiler

A back boiler has broken down in my aged parents home. I have been looking for a new one ( urgent) .
I have a limited choice since they have solid fuel and need a room heater. There are only two it seems a Charnwood or a Parkray. They have a five bed bungalow, reasonable insulation and double glazing. The boiler has to supply hot water and eight radiators ( sizes variable and not known).
The room heater was a Rayburn 80.
Problem , I have been given two estimates for the room heater . One company tells me I need 35,000btu , the other say 41,000 btu. Anyone know how I can work out which is correct? ( OK I know its an art form not a science but help would be appreciated. It seems the art has been lost to modern heating specialists where solid fuel is concerned!) There is only 70 difference in the cost by the way, so this isn't an entirely economic decision. Its about getting the right thing.
Or better still, since the Rayburn 80 was sufficient to supply the needs can someone yell me what capacity boiler that would have been?
PS. No , they are not switching to LPG or oil, so such irrelevant suggestions are not welcome. Thanks.
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It is certainly a science, not an art form. You should use a heatloss calculator program from a radiator manufacturer and carefully enter all the details of the house. This will then give you the steady state loss of the house. Bump it up 25% or so and add a few kW for the hot water cylinder and you have your figure.
However, 41,000BTu is not a huge boiler by any stretch of the imagination. I don't know too much about solid fuel burners, but have heard that ones with modulating fan draught are a good idea.
Christian.
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Thanks for the information. I have now ordered and paid for the Charnwood SLX 45i LA room heater and back boiler and it will arrive tomorrow. I expect to be fitting it this weekend.
Anyone any experience of this room heater/boiler? Am I likely to hit any problems?
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