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how many btu's do I need?

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Princess Sue | 22:43 Tue 11th Jul 2006 | Home & Garden
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Help - I need a fairly quick answer to this! I have a room 8ft x 16ft by 8ft high - 2 windows (double glazed) - it is a new building - two outside walls, but plenty of insulation (ceiling/floor/walls) - what I need to know is what size radiator should I have - this will be a living room and I am a cold person. I have to have an idea tonight because builder needs t know by tomorrow morning. thanks

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The size will depend on the heat required for the room. This can be established by your plumber/installer.
However, as a guide:

Calculate the volume of the room by multiplying the height, width and length to get the volume in cubic metres.
For bedrooms, hallways and kitchens allow 40watts per cubic metre, and multiplying the total by 40, and for bathrooms, living and dining rooms, multiply the total by 50. This will give the output required in watts.
Example: A bedroom 3.6m long x 3m wide x 2.4m high has a volume of 25.92cubic metres. Multiply this by 40 to get 1036 watts.
If the room has large windows or exposed walls, increase the total by 10%.
Select the appropriate sized radiator for the output required.
Always choose the next size up if the exact output is not available
Allow more length and less height for radiators under a window
To convert watts to Btu's, multiply by 3.412.

Using the above calculation I reckon yours works out as 2.5m x 5m x 2.5m = 31.25m x 50 = 1562.5 Watts plus 10% = 1718.75 Watts. Or 5864Btu�s I would check these
figures on your calculator or with your builder. Hope it helps.
Question Author
Thank you both - both answers were brilliant as I couldn't actually work it out myself - I kept getting in a muddle. I am sure now that the bigger radiator of about 7000 btu's is going to be ok and I will be warm enough to sit all night and send questions to answer bank. Thanks again, it has been a real help
I would suggest the target maximum temperature should be considered the same regardless of which room it is - unless you deliberately intend to have somewhere permanently less comfortable than elsewhere (storage cellar perhaps ?). Oversizing a radiator simply means the target temperature will be reached more quickly before a thermostatic radiator valve (you are surely planning one) shuts it off. The minimum size you require depends on the heat loss from that room. Knowing this, if you know nothing about the numbers go for a high-output radiator as large as the space under your windows will take (two if the windows are far apart). With that you will not want for heat or suffer draughts to any noticeable degree. Put it/them away from the window(s) and you will actually amplify all draughts that creep about.
Question Author
Karl, thanks but are you saying they should be positioned under the window or not? The windows are double glazed. The builder has swapped to a bigger radiator, but two would have been better. I think it will be fine, but looks like we will have to wait for the winter to find out!

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