Air Source Heat Pump Freezing

curlylegend

I can't see any mention of heat pumps freezing over at low temperatures. Ours freezes over at anything lower than 3C. The defrost system activates but the effect is pretty feeble and all it seems to do is consume more electricity without giving us any more heat. Leaving the thing on set to 18C inside temperature with sub zero outside temperatures, results in a 45KWh consumption in 24 hours. And struggling to reach 18C inside.
So when it's really cold outside it just doesn't do what a gas boiler does...and costs more !

MultiFuelBurner

@curlylegend

How long have you had the heat pump?

What's the make and model and kwh rating?

What is the EPC of your property and what does it say your property will need each year to heat it (this will be in kwh)?

Do you know if when installed your system has anti freeze mixed with the water to stop freezing?

Have you check your heat pump controller for any freeze settings to stop it freezing up?

Ours is going to use the most energy for the year in the next 5 days as temperatures plummet.

tacpot12

The defrost cycle might not be working because the system doesn't have enough volume of water. (Heat pumps do freeze up in the cold weather, but this should be very temporary until the defrost cycle clears it)  You should be able to find the manufacturer's recommendation for the minimum volume of water needed in the system for the defrost cycle to work correctly either from the manual or directly from the manufacturer's help line. You then need to check how much water there is in the system. If you have a maintenance company for the system call them for advice, or pass the whole problem to them. 

QrizB

curlylegend said:

I can't see any mention of heat pumps freezing over at low temperatures. Ours freezes over at anything lower than 3C.

You'll only get ice forming on the evaporator at low ambient temps and high humidities. I don't know where in the UK you are but we've had those conditions here in the central South recently.

If the humidity is lower, there won't be enough water vapour in the air for it to condense. Similarly, if the weather gets really cold (not 3C but -3C, say) the absolute humidity will fall further and you might come "out the other side" of the frosting window.

As others have said, this is a known issue with heat pumps and they usually have a mechanism for dealing with it. If you have a maintenance contract, ask your maintainer for advice.

Scot_39

How much does the COP drop with external temperature - at your operating setpoints ?

Can you remember what a similar days GCH or other heating was using before the ASHP ?

Suspect 45 kWh at a low COP - say of 2->2.5 - so 90->110 kWh heat energy - may not be a silly number - in an older generation semi or detached / bungalow though given temp forecasts for the next few days across many parts of UK.

And is your flow temperature even set high enough for your radiator sizing - if struggling to keep rooms at 18C ?

Problem is higher flow temps and lower outside temperatures both influence COP efficiency.  Some posters on another forum saying going from 50C flow to 45C at -2 outside can increase their ASHP COP 20% or so = and so try to keep sub 40 even in winter - by running lower (one quotes c2.5 at 50 c 3 at 45 and c3.2 at 35).

Are you trying to run your heating in short sharp blasts or continuously - if former - it might be better to drop flow temps and run for much longer - and benefit from an increased COP.  Fans on radiators to boost air flow might allow lower flow temperatures too - or simply allow you to reach your 18C setpoint at current flow temperatures.

Your ASHP temperature compensation may be increasing flow temps more or less than you are aware of. But the hotter the house - the higher the costs to get it and keep it their.  As losses too are a function of internal vs external temp / wind / sun etc

matelodave

Our heatpump also freezes and defrost itself.

On a cold damp day there are great clouds of vapour coming off the evaporator when it does it. We haven't noticed whether the unit uses more energy when its doing it, however ours is a split system, the heating circuit doesn't go outside to the external unit, so I guess its the refrigerant that gets reversed to defrost rather than the heating fluid.

TBH I haven't really taken a lot of notice, the unit just  gets on with it and still keeps the place warm even when the temp drops below zero - however we sometimes tweak the flow temp um a degree or two when it gets really cold (possibly tonight as its down to -1 already and the flow temp is up to 39.8 degrees)

BobT36

curlylegend said:

I can't see any mention of heat pumps freezing over at low temperatures. Ours freezes over at anything lower than 3C. The defrost system activates but the effect is pretty feeble and all it seems to do is consume more electricity without giving us any more heat. Leaving the thing on set to 18C inside temperature with sub zero outside temperatures, results in a 45KWh consumption in 24 hours. And struggling to reach 18C inside.
So when it's really cold outside it just doesn't do what a gas boiler does...and costs more !

LOL. Are these things a scam, or what? 
I remember the air con at my work, always too hot or too cold. We complained and got told "it's based on the outside temperature", well er, isn't that the point, that when it's hot out you want it cool in, and vice versa? 

Reed_Richards

If you have a monobloc ASHP unit, defrosting is done by reversing the flow direction so hot water is taken from inside your house.  A lot of ASHP systems have a buffer tank that ensures you have reservoir of warm/hot water for this purpose.  However a buffer tank sub-optimally configured can also lead to a reduction in efficiency so some installers will avoid using one.  

QrizB

I saw this on social media and immediately thought of this thread:

Is -25⁰C low enough for us to all agree that heat pumps do work in the cold?

Gerry1

QrizB said:

I saw this on social media and immediately thought of this thread:

Is -25⁰C low enough for us to all agree that heat pumps do work in the cold?

But are they readily available on the right of the pond, together with knowledgeable installers and all at a realistic price?

curlylegend

Thank you so much for all your replies.  I have to let you know that we are a pair of 75 year old retirees living in France, near Nantes. Neither of us are in the least bit technical I'm afraid.
Our house, four bedrooms, salon and kitchen was built in 2005 and apparently complied with all regulations then. We had a gas powered Combi boiler supplying hot water and central heating. For the last six or seven years it had started to become unreliable and was costing us money. The final thing that made us look for alternative heating was the fact that the gas supply pipe was fractured and this would have been a major upheaval as it was set into the concrete foundation of the house.
This was in September 2021 and we made enquiries about having a heat pump installation. We got two local estimates and one online estimate.  None of them could guarantee delivery and installation before February 2022. The prices ranged from €15000 to €22000 but we could get a government grant !  How much of a grant ?  No idea !
To be honest, meaningful grants are only available to people who are so poor they're just one step away from being homeless.  And to qualify you can only use government approved fitters.  Racket ?
We got over the hot water situation almost immediately by buying an immersion heater from a local DIY store and a helpful neighbour plumbed it in for us.  He put us in touch with a central heating firm who were not government registered and they sourced a heat pump for us which arrived in January 2022 and was installed in February 2022. But we couldn't use it until it had been checked and commissioned by the sellers. This took another seven weeks !
Phones not answered and emails ignored.  Now I wish we'd never dealt with them.
It was put into service for only a few weeks, it was late April now and we didn't need heating.
So it was winter 2022/2023 before we used it regularly. It was a sore disappointment.  The control was indecipherable
And the best we can do is use the three fixed settings i.e. 20C, 18C or 15C.
When the weather wasn't too cold it was OK but slow to reach a comfortable temperature in the house. Then the Control itself started to go haywire. It wouldn't turn off when it reached the setting engaged and all manner of icons and warnings were being flashed on the screen. We asked for a visit from the engineer and were told it wouldn't be for another two months.  We took to cutting the power to the unit for a few minutes and letting it reset.  We've had to do this a few times.
The radiators were filled with a 50/50 water antifreeze mixture. Then the outside air is below 3C to 4C the outside of the air pump can freeze over and ice builds up at the bottom. As far as I can make out there is a sort of defrost system which gets triggered and you can see unfrosted lines going across the heat pump. But inside the temperature starts falling and really struggles to get back up. Meanwhile the electricity consumption is going up and doing us no good whatsoever.

I'm sorry this is a bit of a rant, but it might give somebody a laugh. Even if it's at the expense of a pair of old pensioners without a clue.
I would have considerable difficulty supplying answers to most of the technical enquiries but here are some photos that might help.