On 28/05/2012 22:48, Dave Liquorice wrote:
On Mon, 28 May 2012 20:30:47 +0100, Andy Champ wrote:

The limiting factor on air source systems is condensation - if the cold
end is below the dew point you get condensation.

Assuming there is any moisture in the air to condense. Condensation
isn't a great problem as it will drain away.

There usually is some moisture in the air and morning dew even in
deserts. It isn't a problem unless the heat exchanger is close to
freezing and the air is humid when ice builds up on the coldest part(s).

If it's also below freezing you get ice.

Air temp below freezing or evaporator below freezing? If the air temp
is below freezing there won't be any significant moisture present to
condense and form ice. So air source heat pumps work well with air
temps from 0C down to around -10C. They struggle, due to icing and
having to defrost, with air temps between 0 and about 5C.

They work great in Japan where winters are extremely dry - so dry that
it is unpleasant and static electricity is a real annoyance. And the
summers although very humid there are hot enough that the thing is in no
danger at all of getting below freezing.

But in the UK with our dank cold damp dreary winters for weeks on end
the thing will spend more than half its time defrosting. And aircon is
unnecessary in the summer save for half a dozen hot days and nights.

--
Regards,
Martin Brown