New radiator BTU calculation

1 Nov 2012
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United Kingdom
Hi all,

My house is a 1930s detached and the central heating system looks like it was fitted in the 70s or 80s. The rads get hot but the rooms never get very hot even with DG and insulation etc. Recently I fitted a new radiator in the downstairs toilet which never had one before and only has single glazing. This morning I noticed this room was nice and warm. I know the fins and thinner metal help heat rooms faster so I'm thinking about replacing some of the old rads with new ones (living room, bedroom, bathroom).

I have been looking at BTU calculators to help me decide which rads to buy but find they give very different results. For my living room which is 3.5 x 6.4 x 2.4 m I get values ranging from 7000 to 14000. The room has solid walls, concrete floor, large window and French doors (DG). The two shorter walls and one of the long walls are external and the other long wall which is north facing is internal. It currently has two rads, a single 740 x 1120 mm and a double 740 x 1280.

Which figures would you trust? I don't want to put rads in that are too small to heat it properly but don't want to put rads too big in either! I want to keep the rads roughly the same length so I don't have to make too many changes to the plumbing but I don't know whether to go for singles or doubles or a mixture.

My options seem to be:

Single 600 x 1100 3853 BTU
Single 600 x1200 4203 BTU
Double 600 x 1100 6714 BTU
Double 600 x 1200 7324 BTU

Should I go for a Double 600 x 1200 and a single 600 x 1100 (11177 BTU combined) or just go for the two doubles (14038 BTU combined)? Both rads will have TRVs. This will be a DIY job but I will be getting somebody in to do a new boiler in a few months and don't want to have to buy a super powerful boiler just because I put oversized radiators in!

Thanks for your help!
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It's an old boiler and doesn't allow the temperature to be set in degrees but it has a knob with four divisions between min and max. I had it set to half way but have recently put it up to 3/4. The house isn't cold and the rads and water get very hot. I'd just like to heat the house a little faster and new rads with fins seemed like the answer. They'd also look neater as the previous owners have painted the existing rads several times since the 1970s which probably doesn't help!

I only really started to think about it because the room I installed the new rad in felt warmer and I could actually feel warm air above the rad. Only a few cms of air around the old rads feels hot.

The BTU calcs just got me confused! Would new rads help the house heat faster? Should I just replace with new rads of almost the same length and forget about the BTU Calcs? They will have TRVs to control the room temp anyway.
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Just replace the old rads with new either the same size or as close as you can get but not smaller.The new convector fin rads will be more efficient and also give off a higher heat output than the old ones.Combined with the TRV's they will be fine,just remember not to put a TRV in the same room as the room stat :)

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