Air Source Heat Pump

[Welderpaul]

We have a 9.1kW air source heat pump. House is 6 years old, well insulated, current regs etc. Its a wet system so air to water.
UFH downstairs, rads upstairs.

It manages fine all year until the weather gets to circa 0 degrees when the DHW starts to go luke warm and the heating struggles.

Its been serviced etc, and no known faults. We've been told that the heat pump is undersized. A quick wizz round with the laser shows the house is 200m2. The internet tells me that a 9kW heat pump is correct for up to 200m2, 4 bed, well insulated house.

I'll upgrade the heat pump if I have to, but i'm not sure it needs upgrading?

Any ideas of what to do/what the problem might be?

 

[voipio]

Is the air side heat exchanger icing up?

 

[Welderpaul]

Is the air side heat exchanger icing up?

No

 

[More Amps]

I think @Kayos works a lot with heat pumps and might be able to advise

 

[sparkysy]

They do struggle in low temperatures. We have two that are actually covered by the through barn next to us and they work ok to about -1 any lower and they can't cope. The one next door is outside wall mounted and year before last was a solid block of ice for a couple of days when it was really cold. He said it had packed up so we wnt to look at it and found the ice block.
I don't think yours is undersize they just seem to not be very good with prolonged low temperatures.
Sy

 

[Kram]

Is that 9.1kw as motor input power, or, the rated pumping heat?

 

[Welderpaul]

Is that 9.1kw as motor input power, or, the rated pumping heat?

 

[Welderpaul]

They do struggle in low temperatures. We have two that are actually covered by the through barn next to us and they work ok to about -1 any lower and they can't cope. The one next door is outside wall mounted and year before last was a solid block of ice for a couple of days when it was really cold. He said it had packed up so we wnt to look at it and found the ice block.
I don't think yours is undersize they just seem to not be very good with prolonged low temperatures.
Sy

Would a bigger heat pump help do you think?

 

[RaceDiagnostics]

The asterisks says it all, rated output is for outside temp of 7 degC and above. Surely an installation should be spec'ed for the lowest typical outside temp.

 

[Bornfree]

Would a bigger heat pump help do you think?

Not as well as a woodburner

 

[Wildefalcon]

So, as someone who was playing with the idea of a heat pump, let me get this straight: They don't heat the house when it's freezing?

 

[RaceDiagnostics]

 

[Welderpaul]

The asterisks says it all, rated output is for outside temp of 7 degC and above.

From what I can see its a way of standardising the rating. Extract from manual

 

[JimmyS]

It sounds like it's maybe undersized. I'd think you will have a heat loss of about 9kw at 200m2 floor area. Do you know what your flow temperature is to the heating system?

The COP on any heat pump is very dependent on source and sink temperatures. On the source side the hp has to extract heat out of the air. If the air is at - 1 then the refrigerant has to be at a low enough pressure to evaporate at say - 3 deg C. The lower the evaporating pressure the more work the compressor has to do. Plus ice is a good insulator and when the unit needs to go on to defrost it either needs to do a reverse cycle defrost or use electricity as its too cold for the ice to melt on its own.

Same sort of thing with sink temperatures as the higher the temp you need then generally the higher the discharge pressure needs to be so compressor has to do more work.

I'd give grant technical a call and ask them the output of. Your heat pump at - 1deg c outside temp and whatever your flow temp is and see what they say.

 

[Onoff]

Unless your house is super insulated and has exceptional airtightness both beyond what Building Regs call for then heat pumps aren't great given the comparatively low temperatures they offer and will struggle.

 

[JimmyS]

Nowt wrong with a heat pump system providing it's designed properly.

 

[Welderpaul]

Got fed up of listening to a YT rant but we don't have gas here so that's not an option. System was installed by the builder so I need to make what i have work.

 

[Gareth J]

No more than a suspicion, but I suspect that if you up the size, you might reduce efficiency in milder temperatures?

Might be more cost effective, especially as you have yours there already, to run the one you have flat out for more of the year and supplement it with something else during peak cold. A second heat pump, off peak immersions, oil filled rads, small oil boiler etc

 

[Welderpaul]

It sounds like it's maybe undersized. I'd think you will have a heat loss of about 9kw at 200m2 floor area. Do you know what your flow temperature is to the heating system?

The COP on any heat pump is very dependent on source and sink temperatures. On the source side the hp has to extract heat out of the air. If the air is at - 1 then the refrigerant has to be at a low enough pressure to evaporate at say - 3 deg C. The lower the evaporating pressure the more work the compressor has to do. Plus ice is a good insulator and when the unit needs to go on to defrost it either needs to do a reverse cycle defrost or use electricity as its too cold for the ice to melt on its own.

Same sort of thing with sink temperatures as the higher the temp you need then generally the higher the discharge pressure needs to be so compressor has to do more work.

I'd give grant technical a call and ask them the output of. Your heat pump at - 1deg c outside temp and whatever your flow temp is and see what they say.

I've had Grant out to it and both Grant and the other Engineer who has been out both tested the temp and said it was ok.

 

[sparkysy]

Was the Engineers test done under the current temperature conditions?
Under normal temperatures then the tests should have been fine.
With it being colder the efficiency is going to drop as someone pointed out earlier the external units need time to defrost.
Sy