Checking Boiler Capacity

[monday]

I am about to commission the build of a new single storey extension and have been checking the capability of the existing ch boiler to handle the new radiator(s). I'd be grateful for confirmation that my figures look about right.

I've used the boiler size calculator in the WIKI (also at www.idhee.org.uk) for a 1997-built 4 bed 2-storey detached house with floor area of 80m2 (each floor) in SE England. This gives a requirement of 43,200BTU(=12.7kW). I have also sized up the rated capacities of all existing radiators and towel rails etc and this comes to about 42,000BTU.

So the figures seem to tie up quite well in that sense.

The boiler is a 4 year old Glow Worm Flexicom 30hx (output 9-30kW) so that seems more than OK. But reading FAQ (19) in the WIKI, this seems to recommend a minimum of 20kW and for > 1 bathroom >35kW.

Are there some efficiency factors coming into play here that i am not aware of? I reckon I will need about another 6,000BTU for the extension.

 

[MJGas]

Your existing boiler is plenty big enough

 

[Whitespirit66]

But reading FAQ (19) in the WIKI, this seems to recommend a minimum of 20kW and for > 1 bathroom >35kW

Ignore the Wiki. The outputs referred to in it are for combis, which obviously doesn't apply to you.

General rule is sufficient boiler output to cover heating requirements, plus 2Kw for hw. As above, your boiler has capacity to spare.

 

[monday]

Many thanks, understand where the WIKI is coming from now.