On Dec 24, 9:15*pm, "
wrote:
I am heating with oil so air source is a good lot cheaper than oil
1kw in = 3-4kw out.

Assume 2.5kW out for 1kW in (CoP = 2.5).
Perhaps Economy 7 backup for when CoP tends to 1.

Plus very little servicing and no oil tank required.
An average size house requires 12kw of heat just
to maintain temperature in cold spells.- Hide quoted text -

12kW heat at CoP 2.5 means ASHP of 4.8kW :-)



I thought domestic ASHP were 9kW and 14kW.

Basically...
- Single 9kW unit would be ok (or 12kW in this instance)
- Thermal Store suitably sized (acts like a buffer / battery)
- Backup by a) E7 Storage Heaters, Automatic (plain reliability)
- OR b) 5-9kW run electric inline boiler
- OR c) 2x 3kW thermal store elements

The backup are for if it is serviced, any failure, so cold that ASHP
ends up dropping to CoP 1 (ie, parity 12kW in = 12kW out as opposed to
12kW in and 24kW out).

No idea what prices you have been quoted, but also look at the Sanyo
CO2 heat pumps - they are very powerful and CO2 based so they really
DO produce very high CoP at very low temperature (night time operation
in deep UK winter is no problem, they are rated to CoP 1.0 down to
-25oC and optimised for cold weather).


Also consider insulation.
If you are solid wall, at least consider Celotexing the main living
area - disruptive but the difference between say solid-wall and
65-90mm of Celotex/Kingspan is enormous. A hamster running around will
make you reach for a cooling fan.