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help sizing rads based on room by room heat loss

AdamK · 2025-03-28T11:14:17Z

scratching my head a bit as to what size rad i need for a given room based on its heat loss. the system is designed to 40c flow @-3.1c outside. lots of sites list rad outputs based on delta t50, is this the flow temp? so if i can find out the output at dt40 for instance is that the output i need to match to this chart?

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JamesPa

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16/04/2025 10:41 pm

Posted by: @jamespa

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Posted by: @adamk

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@jamespa thanks for the info but I think I must have something wrong. The heat loss for the whole house is about 7.5kw. The room is only 3.756m x 3.686m. Current rad is a P+ 1450mm 380mm.

Have you got the loss calculations, if so perhaps you could post?

My house loss is also about 7kW and I had one room well over 1kW, went for a Mitsubishi fancoil for that, very nice piece of kit

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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AdamK

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Topic starter 17/04/2025 10:16 am

@jamespa heat loss list on first post.

it’s the one that says nearly 2kw, I’ve just split it into 2 rooms as we’re going to put a sliding wall between the dining and living room, plus one rad is in one area and another in the other.

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JamesPa

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17/04/2025 2:11 pm

Posted by: @adamk

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@jamespa heat loss list on first post.

it’s the one that says nearly 2kw, I’ve just split it into 2 rooms as we’re going to put a sliding wall between the dining and living room, plus one rad is in one area and another in the other.

The living/lounge and kitchen in your list aren't dissimilar to mine and were, for me, by far the most difficult rooms to find radiators for. I ended up with four rads in total covering a kitchen/living room and a separate long living room. The balance between two halves of each room wasn't perfect, but it hasn't mattered in practice. One of the four rads was a mitsubishi fancoil and another was the existing rad with a bunch of computer fans added (not sure how much difference that has actually made)..

I avoided type 33 as I had already fitted one in the bedroom (the other difficult room) and the wife hates it. Personally I quite like it as it looks 'serious - but I would advise to look at an example of a type 33 before settling on one, they are quite noticeably deeper.

Having got a fancoil I wish we had got a couple more. They are nice and neat, quite small, not particularly noisy and with three of them I might have been able to do some meaningful cooling in summer. Jaga Strada are also popular fan radiators and those who have them seem to rate them highly..

I was designing for 45C where the oversize factor (ie the factor by which you have to divide the DT50 output to get the output at the chosen FT) is 2.8. At 40C its 3.9 which makes quite a difference. In practice Im running at 42 so half way in between, presumably because the loss was a bit overestimated.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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AdamK

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Topic starter 18/04/2025 12:48 am

@jamespa so if i have a 1988w rad at 40c @-3.1 it will only be 509w?

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JamesPa

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18/04/2025 8:51 am

Posted by: @adamk

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@jamespa so if i have a 1988w rad at 40c @-3.1 it will only be 509w?

If its 1988W at DT 50, which is how most radiators are specified (but note not all, one brand is specified at DT70!) then yes that's right. At FT40 and RT 20 your average DT is 17.5. So your rad output is 1988*(17.5/50)^1.3 = 509W.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

👍

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