On 17 Feb 2006 08:15:39 -0800, "Lunny" wrote:

I've "Mysoned" the house and it says the boiler size required is 19kW.

That sounds more reasonable.


Allowing about 4kw for hot water would mean I'd need 23Kw.

This is where some interpretation of the results is needed.

The 4kW figure stems from when it was common practice to heat the
water in the cylinder using convection circulation from the boiler
(aka gravity). This would be about the typical trasnfer rate and
would be happening all the time that the heating is running as well as
when HW only. Therefore the figure was added.

Nowadays on new installations one can't fit gravity systems (building
regulations) and the typical arrangement is to use a diverter valve
and fully pump the circuit to the coil of the cylinder. Especially
if the cylinder is a Part L type or better still a fast recovery, the
heat trasnfer rate can be 20kW or possibly more. The idea is that
when HW reheat is needed, the boiler is switched over from heating and
all of its output goes to the cylinder. Since this is more, then
reheat is fast and then the boiler is switched back. Ergo, you don't
have to allow for water heating.

Would
getting a 24kw boiler be a good idea? Would it mean the boiler would be
overworked at all or does it just mean that the house/HW will not heat
up as fast? What would be the advantages/disadvantages of oversizing
the boiler?

I think that something in the 24-25 or even 28kW range is fine.

If you go for a modulating boiler, and typically will with a
condensing model that is normally required nowadays, it will run at
lower output when running the heating. In any case, for much of the
year you won't need anything like 19kW anyway.


--

..andy