Replacing radiators

20 Mar 2014
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United Kingdom
Hi there
I'm replacing all the old radiators in a house I'm renovating.
My question is should I replace with type 11 radiators or type 21 ? I'd like to replace with type 11s because they are cheaper ! But I want them to run efficiant. Cheers
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You have to start by finding the output needed for the rad(s) in each room. Best to oversize slightly, by 10 to 15 %.

Then it's up to you what type you fit. If the calculator says you need a rad of x BTUs, then you will need a much longer type11 than a type 21, because 21s have double panels, so give out a lot more heat than a same sized type 11.

EG. 600 x 1000mm size rad -

Type 11 (single panel) = 3100BTU approx

Type 21 (double panel) = 4600BTU approx

So, big difference in rads heat output of same size but different types.
If you have a condensing boiler it's always better to oversize than undersize (more efficient), especially if you have TRV's to reduce the room temperature eventually anyway.
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