On Thu, 24 Dec 2009 22:25:49 -0000, Roger Mills wrote:

I am heating with oil so air source is a good lot cheaper than oil
1kw in = 3-4kw out. Plus very little servicing and no oil tank
required. An average size house requires 12kw of heat just
to maintain temperature in cold spells.

So why do you need 9kW to drive the pumps - 3 or 4 should be more than
enough?!

I suspect he's still think in normal boiler all or nothing mode and
needs 25kW to heat water directly like a combi (spit).

Heat pumps doen't work like that you can't just switch it on and have
an instant 25kW of energy available like a gas or oil boilier. What
needs to be done is calculate the average daily energy demand during
the winter and have a heat bank/thermal store that you replenish
(heat wise) from the heat pump. The heat pump still needs to provide
a goodly proportion of the requirement but not all of it. The heating
will be off or set back at night and possibly during the day so the
average energy demand is lower than the peak. Size for the average
not the peaks.

--
Cheers
Dave.