Heat pumps

[cryptogram]

Has anyone any experience of these? I'm talking about those which heat a house by compressing air, the heat from which then enters the house - a fridge in reverse.

[Vir Campestris]

Cross-posted to uk.d-i-y for better information.

I gather the weak point is in cool damp weather. The outside "cold" end condenses water out of the air - this is the same thing that makes air conditioners need a drain. When it's cool enough and damp enough it isn't water condensing out but ice - and that doesn't trickle away.

Luckily cool damp weather is a rarity in the UK :)

Andy

[dennis]

I have been running an air to air heat pump for heating for a few days this winter..

It likes cool damp weather as it can take the latent heat out of the water and pump it in.

It only drops the air temp by a degree or two when it working like that.

You get puddles of water around the unit but no ice so far.

[Jim K]

don;t they have a heater like "normal" A/C and frost free fridges? to melt any ice so formed...

Course using elec to heat it to melt ice rather messes with alleged efficiencies of the whole thing in the first place...

[harryagain]

I'm told this defect (iceing up) is exacerbated if the evaporator (outside bit) is undersized and hence runs colder than ideal.

[Dave Liquorice]

Well it gets the latent heat from the phase change vapour to liquid which is OK. You don't want to take the latent heat of the next phase change, liquid to solid, if you can help it. Ice is a good thermal insulator and you'll have to use energy to melt it to regain a decent (short term) COP.

It's not trying... I've seen aircon units frozen solid with air temps in the mid 20's and only reasonable humidity.

[Nightjar]

When I had a/c installed in one of my factories, it was a lot more expensive to buy one that was designed for the British climate than it was to buy one that was designed for use in Japan.

Colin Bignell

[dennis]

I assume you mean the internal units, we haven't had enough warm weather to see if the internal unit ices up, but if it does then you can just keep blowing the warm air you are cooling over it until it melts. No effect on bills unless its still frozen when switch it off.

[gremlin_95]

The units I am familiar with simply switch to 'defrost' which basically reverses the cycle to cooling mode which melts the ice.

[Andrew Gabriel]

That's what mine does. If the weather is just right, it sends a giant plume of fog across the back garden when it's finished the defrost cycle.

I tend not to run it for heating when the outside temperature is just a little above freezing, as I doubt it's any more efficient than an electric heater when it's having to keep switching over to defrost cycles. It works fine when outside is below freezing, as there's no water to condense out and freeze.