thecaptain: I’m assuming that they are like other ASHP’s and there’s an external unit that sits outside? That’s where you can regularly find temps of -15 deg C in Scotland. The ‘outside’ air is the heat source. On air-to-water they don’t work very well at those temperatures and have electric resistive heaters to make up the shortfall as the CoP drops away. Air-to-air I already admitted I didn’t know enough offhand and accepted wrecker’s correction.

Ok, I see where you’re coming from now. No, it’s not outside air being heated through the internal unit but where does the inside air come from in the first place? You need some fresh air inside. Relative humidity in summer is higher than in winter. You’re highly unlikely to have 100% RH at 10 deg C inside (unless you’re continually boiling the kettle) in winter either. I was coming at it from that angle. Your source air has low RH, unless you’re generating water vapour inside where’s it coming from?

I’ve come across at least two office blocks in Scotland with (electrically generated steam) humidification presumably to overcome static electricity issues (seems a bit of overkill to me personally considering the running costs)

Some light reading:

https://en.m.wikipedia.org/wiki/Air_source_heat_pumps

Seems you can get units to operate as low as -35 deg C (albeit at CoP’s considerably less than, er, ideal).

Guess I better apologise to the OP for derailing the thread. sorry!

metalheart, your argument on humidity applies equally to any more common soure of heating, be it resistive electric or water-filled central heating radiators. The only additional effect from the heat exchanger system is the fan blowing wamr air around which does probably add to the discomfort in dry conditions, but doesn’t actually dry the air per se. Don’t forget normal human life generates significant amounts of indoor humidity, which is often the cause of damp problems over and above the ambient climate. Maybe less so in an office with no cooking, bathing, laundry.

Don’t worry MH ….. it’s all good stuff!

cvilla: Yes the COP value can be a little misleading so a number of manufacturers are now quoting the SCOP which is the performance figure averaged across all the seasons.
As the [other] house is only occupied sporadically during winter I’m just looking to convert some spare PV output to heat in the house just to keep it above 11c!
I’d use the raspberry pi that monitors the PV production to switch the heat pump on for x amount of time if the power being generated was over a set threshold. Cloudy/dark = no heat pump. The place isn’t heated at all if we’re not there so anything would be a bonus.
During the shoulder months though (spring/autumn) the PV production could inject a reasonable amount of background heat.

Lots of info on GreenBuildingForum.
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11248&page=1

If you can pick one up as cheap as you say it sounds like it’s worth a try. I’d try to find some way of tracking the power consumption to see how much it’s actually costing you to run.

The other question is whether you can improve insulation and airtightness.

OP – we have them in the offices at work, which are basically extensions or partitioned off sections of an agricultural barn. They are many times more efficient than the storage heaters they replaced and can provide heat in the afternoon.

Basic observations are that the coldest place is next to one, which is also the noisiest place when a bit of casing isn’t sitting right. The opposite side of the room is far warmer and quieter. Mount the external unit high if you get snow, have had to dig ours out that are on the floor. Equally the one next to the outside toilet has made it a chilly place for a quiet sit down, but it’s off the ground, under a deck, so wins on that front.
The external units can drop a fair bit of water, which hasn’t been great for the ones over concrete.

There are days when we would far rather have a wood burner as the thing seems to have a mind of its own, a different temperature scale to us and a million running options on the controller. We just want heat NOW.