Boiler Sizing/Futureproofing

[jb3009]

Hello all, I am planning to have my gravity fed system converted to an unvented system with a system boiler. The house is 4 bed 2 bathroom (1 bath 2 showers). 15 radiators at present. I have been offered either a 26kW or 32kW system boiler, the difference in price being about £100. I have been having a look on the web about boiler sizing and dont seem to find any definitive answers as to which would be the better option for this property? I plan on extending in a couple of years and possibly adding 4-5 more rads and approx. 35sqm of water fed underfloor heating. In the event of me extending, i dont want to be in a position where a 26 needs replacing to facilitate the extended system. Likewise i dont want to get the 32 and find it costs a fortune if i dont extend. The 32 modulates to 6kW and the 26 to about 5 kW.

Thanks,

 

[Ben-gee]

It’s nothing to do with number of radiators. Do a heat loss calculation for the property.

 

[John.g]

And try and install a boiler with a minimum output of ~ 2 kw, they are available even from 30kw I think.

 

[snowhead]

To comply with building regs at the time the extension would have a substantially lower heat loss than the equivalent area in the current house and will reduce the heat loss from the house by eliminating all or part of an external less well insulated wall.
So you don't need to increase boiler size purely on the additional sq footage, ideally you need the calculation for the extension doing before picking the boiler.

 

[Ben-gee]

If you time the hot water to reheat when no space heating is timed, then you only need boiler rated to the space heating requirements and don’t have to add on hot water requirement.