Heat pump weather compensation

[Colin Green]

I had a heat pump installed late last year into a house we're doing up to move into. The installer can't get it to work properly. The problem was that it wouldn't heat the house more than 12° above the outside temperature. They seem to think that the flow temperature is not what is being asked for. They fiddled with it a bit and it will now reach 21° inside but it can take hours to get there, even with the stats set to 18° overnight. 0.2° or 0.3°C per hour temperature rise is about what it manages. We have UFH downstairs and radiators upstairs

I've been fiddling with the temperature compensation curve. It turns the flow temperature down as the outside temperature goes up. What kind of range would you expect the compensation curve to be set to? It's in the format of 50° flow below 5° outside, sloping down to 40° flow from 15° and above.

 

[MNW67]

I can't answer that question but here are a few things to consider.

What sort of temperatures was it designed to run at.

Is it a low temperature heat pump or a high temperature heat pump.

Do you know what the output of the heat pump is and what is the calculated heat loss for the house.

Is there a display showing the actual flow temperature. What sort of figures have you been getting.

Are the radiators heating up uniformly.

Are you confident the installer fitted big enough UFH and radiators.

 

[Colin Green]

The heat pump is a Daikin Altherma 3m high temperature heat pump. It is rated at 10kW. The house has an ECP rating of A99 but the 6.5kWp solar array is most of that. The loft is insulated to 0.13U. The walls have filled 60mm cavities. The windows are modern but average. The floor is solid concrete.

We asked them for a system that would hit 21°C when it is -17° outside. The design says -14 but it isn't even doing that. I believe the design said the heat loss would be around 6kW with a 30° temperature differential. ISTR they said the dT needed to be 23° for the UFH to emit enough heat.

The rads they brought were too small so I sent them packing. They're now much bigger and get reasonably warm. I think they're 1200 x 600 doubles and are pretty thick.

The installer came round last week with a clamp temperature meter. He said he was getting I think 47° flow inside the heat pump and 1.5° less going in to the buffer tank, but the return to the heat pump was >5° less so the heat pump was continually working hard.

I wonder if the heat pump is undersized or whether it is just set up badly.

 

[MNW67]

Based on those figures, on a day like today your heat loss would be more like 3KW, so 10KW should be plenty. AFAIK, heat pumps are usually rated at 45C flow and 7C outdoors, so based on that you will be getting the full 10KW today. I would say that the heat pump has the capacity to easily be heating up your house. So something else must be wrong! Could the emitters still be too small? Maybe pipes are too small so not enough flow? Those are just guesses!

As an aside, there are other forums with dedicated ASHP sections:

Air Source Heat Pumps (ASHP)

UK Self Build forum and web community with advice about building, extending or renovating your home along with general DIY & energy efficiency discussion

forum.buildhub.org.uk

Air Source Heat Pumps (ASHPs)

renewableheatinghub.co.uk

 

[Notch7]

Have you looked at heat geeks YouTube channel or maybe considered getting one of their engineers to check your system.

 

[Colin Green]

So I've read the manual and looked through the settings. One setting is called kW limit. It was set to 2kW. I guess this is the max electrical power it will use. On a cold day with a CoP of 3, that limits the heat output to 6kW. Even on a mild day where you'd expect a CoP of 4, that limits the heat to 8kW. I've set it to 5kW. We'll see in the morning how much difference it makes.

In the meantime, I'm doing the commissioning as set out in the heat geeks video. Ideally I need colder weather. I shouldn't have to wait for long.

 

[MNW67]

So I've read the manual and looked through the settings. One setting is called kW limit. It was set to 2kW. I guess this is the max electrical power it will use.

This setting?

Sounds complicated. This is from the manual for a different model, but gives a bit more detail. Might be helpful?

 

[MNW67]

So I've read the manual and looked through the settings. One setting is called kW limit. It was set to 2kW. I guess this is the max electrical power it will use.

I've found the setting you meant now! Please ignore the info in my post just above.

 

[Ltnman]

Shouldn’t the installer be tasked with getting it to work properly. Was it their first install?

 

[Colin Green]

Shouldn’t the installer be tasked with getting it to work properly. Was it their first install?

Yes and yes. We got a fair discount for it being their first installation. Plus the MCS inspector will come to assess their work at our house to certify them as installers. The implication being that they'd do it by the book. They had a heating engineer design (and warrant) the system so it should be up to the job. I'm getting on and doing the setting up because its been going on too long now. They're not off the hook. If I don't get it done, they're getting someone in to do it.

 

[Colin Green]

It's inconclusive but promising. Changing the kW Limit setting may have improved the heating. For the last few days I've been setting the compensation curve and leaving the heating over night. All the stats are set to 18° from 9pm and 21° from 7am. I check the temperatures are 9am and 10am to see what's what. Most days, the room stats show temperatures between 19.5° and 20.5°, and an average of 0.2° or 0.3° more at 10am.

Last night, the rooms were a bit warmer at 22° when I left at 9:30. The outside temperature was 10° at 9pm and 10° at 9am when I checked them again. But all rooms had hit 20° by 9am, some had hit the 21° set point, had switched off and were cooling down. Like I said, it is inconclusive because the rooms wouldn't have been cold at 7am, but it is promising.