On Mon, 07 Mar 2005 19:57:26 GMT, Bedouin
wrote:

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?



You'd have to do the sums to know.

There are two major elements.

- Heatloss through surfaces. You can get a reasonable number for
that if you are methodical about materials used etc.

- Heatloss through air changes. This assumes that you either use
standard numbers suggested by the Buikding Research Establishment or
make a reasoned estimate. You can't really know if your dining room
has 1.5 or 2.5 air changes an hour.


You can arrive at reasonable numbers for radiator sizing this way.
i.e. you can out in enough capacity to be sure of good results. it's
hard to do that fo the whole house and sizing a boiler though.

If the house was OK consistently on 22kW (meaning burner on the
Netaheat at max) then you whould be comfortable at 28kW and have a lot
of margin at 35kW.

A house needing 22kW suggests to me that it's either large or has poor
insulation because it's older and you can't put insulation in enough
places to make a big difference. If that's the situation, then it's
more important to look at heat loss because you will have little to
address the variations during a day. The temperature will change
quite quickly with the outside temperature. If you have a situation
where the insulation is better, there will be less intra-day
temperature change and so you don't have as much sensitivity to sudden
weather changes.

There's no magic way, unfortunately. If you are concerned, and the
house is not well insulated, then I would go for higher power. This
will cost more, but not as much as getting it wrong and having to
replace the boiler with a larger one......






--

..andy

To email, substitute .nospam with .gl