Radiator size

15 Jan 2007
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United Kingdom
We moved in to the house in December and one of the things we knew we needed to do is replace the living room radiator as it has had a leak in the past and is quite rusty.

We have found over this cold spell that the room doesn't ever get really comfortably warm and so we want to make sure that the new radiator is correct for the room size.

I found a guide in B & Q that advised room volume in cubic feet x5 would give the correct radiator output in BTU, this gives us a fairly massive 7700 BTU requirement which I am struggling to find a suitable radiator for that will fit in the available space.

Another online calculator from a heating supplier comes out at 4400 BTU so I wondered if there is a definitive calculation that we should be using to find the correct radiator please?

Also, are there any recommendations regarding manufacturers/ suppliers please?

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1. The detailed (and hence accurate) calculation for room heat loss is quite involved. It means you need to know the thermal conductivity of the walls, windows, ceiling, floor and doors.
2. Why not:
2a. Turn the radiator off.
2b. See whether a 2 kW electric fire warms the room sufficiently. Use a fire without a fan, as the fan can give a false impression.
2c. If it doesn't heat enough, try a 3 kW (or add a further 1 kW). Or better still one which can be switched to give different wattages.
3. Use the results of 2 to determine how many kilowatts you need.
4. Buy a radiator which gives, say, 20% more heat. You can always turn it down.
5. As you will appreciate, the above will give you a result for the outside temperature on the day(s) you try it, so a cold snap would be a good time to do it.

Before you do any of the above, see if the existing radiator gets evenly hot all over. If its hot at the top and cooler at the bottom, it may have sludge in it (rusting?), and a straight replacement, or type 22 if the existing is type 21, might do the trick. If you do find sludge is the issue, consider installing a magnetic filter on the return pipe near the boiler. In fact, consider that anyway if you haven't already got one.

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