Christian McArdle wrote:
I am intending to replace it shortly and will use it as a guide to the
correct size of the replacement (which will be bigger)


Modern boilers are modulating. In the past, you guessed the heat output and
bought a boiler that matched (i.e. 12kW or 14kW or 8kW etc). Now you just
buy a 28kW model (which is halfway to heating a palace) and it just turns
itself down when less is needed. It means that careful consideration of
boiler sizing is no longer required. The only issue is that instantaneous
combis need as much power as you can find for reasonable performance when
heating water. For heating, any 24kW or 28kW system boiler will heat almost
any house. If it doesn't, then you need more insulation, not a bigger
boiler.

This is certainly a comparatively expensive house to heat - every room
has 2 outside walls and the rooms we use most all have 3! Yes in theory
more insulation would be a good idea but that is not easy (solid walls
and all that); and I also have what is probably the best ventilated
under-floor void you've ever seen.

The problem with an over-specified boiler is cost and size so I'd rather
get the size right.

Going up to a 28-30kW model will give me modest increase over my current
22kW - but will that be enough?