On Fri, 23 Jun 2017 18:07:17 +0100, Dave Plowman (News) wrote:

A heat pump normally relies on a heat exchanger kept at a (reasonably)
constant temperature. Normally pipes buried underground.

Air source is an alternative, they don't like air temps between 0 and
about 5C, condensation forms and freezes requiring regular, energy
consuming, defrost cycles. Below freezing the air is dry and IIRC
they'll still be effective down to about -10 C.

One on the side of a house would be hot on a hot day and cold on a cold
one.

It'll be cold as the heat is being pumped out of it, it's not a
passive heat exchange system a solar thermal setup.

--
Cheers
Dave.