New Radiator - BTU value

7 Jun 2011
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East Lothian
United Kingdom
Parents wanting new radiator for their bathroom. The B&Q brochure they have is listing each radiator by BTU.

Their room is 3.04 length x 1.6 width x 2.65 height (all measurements in metres). They have one external wall, with a standard sized double glazed window. House is 1930's bungalow with double skin brick walls internally and externally.

What BTU do they need for the radiator for the room? And what does the BTU actually mean?

From the calculator on the B&Q website, this gave a figure of 1,566 BTUs - is this the MINIMUM value of BTU their new radiayor should have? Ie they could get a radiator with a BTU of, say, 1,600 or 1,700 BTUs but not 1,300 or 1,400? IF so, what is the tolerance either side of the recommended BTU value - ie is it generally recommended for the new radiator to be e.g.+/-10% of the recommended BTU or should the new radiator BTU never be below the recommended level.
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I have no idea how big a "standard size" window is...indeed, I wasn't aware that there was a standard size for a window. I'd also be asking what the window is made from, as wood, glass, and metal frames all have different insulation values. Without this info, along with roof construction, insulation levels, target temperature and geographical orientation it's impossible to give a more accurate answer than the B&Q online estimator.

BTUs are British Thermal Units. They are to kW what pounds are to kilograms. There are roughly 3412 BTUs in one kW

If you're using the B&Q calculator as your guide, you can go over as much as you like but shouldn't go under. A higher value will provide faster heat up times, so theoretically you can go as high as you like although if you're very much oversize you'll find that it's difficult to control the temperature.
Since about 1980 the heat output of radiators has been measured in kW.

I really don't understand why so many DIYers seem to want to use outdated figures!

With kW you can directly compare rads with fan heaters and electric fires.

Also a power output specified in TWO digits is far easier to handle!

Most rooms need between 1 and 2 kW with large through lounges about 3-4 kW.

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People use BTUs because DIY sheds like B&Q still use BTUs Tony

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