Hello,
When fitting a boiler, how do I work out what output the boiler should have? I can add up the rating of the radiators to give the power needed for the central heating but what do I add for the hot water? Also do I add a fraction to compensate for losses?
Thanks. Stephen.
The boilers in Noah's Ark were rated in btu's - *Kilowatts* is what you need!
Adding up the ratings of the radiators is putting the cart before the horse. You need to start by working out the heat losses for each room, and *then* choose suitable radiators for each room. The boiler needs to have sufficient capacity to meet the total heat loss, plus some for pipe losses, plus some for transient conditions (getting the house up to temperature from cold, fairly quickly) plus some for hot water.
It's best to use one of the free Heatloss Calculation programs from the likes of Barlo or Myson - they will add all the necessary 'bunce' factors for you.
Current practice for sizing boilers is the whole-house calculation method which you'll find on the diy wiki under central heating design.
Sorry, perhaps I should have made myself clearer. I used one of the online calculators which asks for room size, whether it's north facing, if it is double glazed, etc. and told me the radiator size that room requires. It was those figures I have added up. Whether the radiators that are in the rooms match those figures is another matter! They were here when I moved in.
I'll check out the web sites you suggested. Thank you.
Well, did the original boiler cope? If so just replace it with one of the same size. If it didn't - was it the boiler or wrong sized rads?
quick'n'dirty is between 50W/sq meter (well insulated house to modern standards) through 100W/sq meter (DG, cavity walls, but no cavity insulation, insulated roof, decent carpets on slab floors) through
200W/sq meter (single brick, but in good order otherwise) to 400W (old house, no insulation at all)boiler an rad sizing is not critical as long ass there is ENOUGH the thermostat will modulate down to what is required.
when in doubt go bigger.
The on-line calculators I've seen are pretty crude, and only give ball-park figures. You need one of the programs which you can download and run on your own computer. These take proper account of wall construction, glass areas, floor and ceiling losses, temperature in adjoining rooms, etc., air changes, etc. - and give a much more accurate heat-loss figure.
You can download the Barlo program from
Check out the Boiler Choice FAQ.
Thanks. I downloaded the Barlo program. Interestingly for my dining room it recommends just over 5000 BTU whereas the calculator I used on some web site (can't remember which, sorry) recommended just under
4000 BTU. Sorry to use such old units but Tool station's catalogue works in BTU hence I am calculating in BTU. I think Screwfix sell the same rads, only last time I looked, the figures were different because one used delta T = 50C and the other used 60C. I do have a patio door which perhaps explains why the program, which knew about this, gave me a higher value that the online calculator which did not.Andy, if you are listening, can you provide the other calculator?
Thanks.
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