Boiler sizing..

OP just a rough guide that we use as a ball park calculation to do it quickly NOT an accurate figure just an alternative way of double checking the calculation are accurate:

5,000 Btu per radiator (whatever size unless all are really big)
15,000 Btu for hot water (if using a cylinder)
+ 10%

simple but effective way of assessing you calculations are in the right area

KW = Btu divided by 3412
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I have a question re. sizing.

I'm putting in a boiler into a large "games room". There is minimal hot water demand (1 kitchen sink, 1 WC basin).

I quoted for a Worcester 24 junior or a Glow Worm Flexicom 24cx.

However, having another look today, I wondered whether these were big enough as there are 5 x old cast iron school rads (9 columns, 300mm high x 1400mm), plus there will be 5 new rads (3 x 600x1200 dbl, 1 600x800 dbl, 1 600x600 sgl).
I've approximated the size of the rads at 21kW.

I think these are OK as the HW is minimal, but when I put into Worcester's website "find a boiler" 4-5 beds and 1 bath as a comparison, it comes up with a 30 or 37CDi.

Any opinions welcome


If the rads total 21 kW then a 24 kW boiler will have 3 kW spare capacity.

If you are a boiler installer then you should know that oversizing reduces efficiency.

You could also do a whole house heat loss calculation to check the existing rads are adequate.

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But surely the best way is to ask some randoms on an internet forum to do your job for you. It's a lot cheaper than paying out for proper training.

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