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Is My Midea Heat Pump Inherently Defective?

cathodeRay · 2025-01-03T11:29:33Z

There is no doubt that my heat pump fails to provide adequate heating at lower outside air temperatures. This is an 'MCS fail' - it can't keep the room temperatures at design temperatures when it is colder outside. This is despite the fact that on paper it should be able to do so. Given an empirical (measured not calculated) heat loss of around 9 to 10kW at -2 degrees outside, and a claimed (by Midea) output at that OAT (and LWT I have) of around 11.3kW, it should be able to cope, but it can't. It is a CQC heat pump - a Can't Quite Cope heat pump. It has been sub zero outside overnight, and this morning the room temp is 18 degrees where it should be 21 degrees. The last 48 hours has provided a very useful range of outside air temperatures (OATs), meaning I can plot my heat pumps behaviour over a wide range of temperatures over a short time frame, which makes it easier (but no necessarily easy, the plots are inevitably cluttered!) to see and compare what is going on. This is what I have (note the flow rate is plotted as true value x 10, to make it easier to see what is going on): And there is indeed a lot going on here. Starting on the left, we have a 3 hour period of moderate OAT (just over 10 degrees). Here the heat pump does what it does most of the time: it runs with a fixed flow rate and appears to modulate its output by cycling, which in turn means much of the time the leaving water temperature (LWT) / returning water temperature (RWT) delta t is too low. This is not how a heat pump is supposed to run. That said, the trailing 60min COP of just over 4 is OK. At around 1300 on 1st Jan, a day of storms, the OAT suddenly drops (cold front passing?). This happens to coincide with a timed DHW heating cycle (the high LWT spike), but once that is over the Set LWT increases, as it should, but the flow rate remain the same, and the heat pump continues to cycle, albeit with a marginally higher average LWT/RWT delta t, and the compressor (dotted line) can be seen to work a bit harder, and the trailing 60min COP drops to about 3. But it is still cycling, and this is not how a heat pump is supposed to work. In the early hours of 2nd Jan, the OAT continues to fall, and the heat pump does something unusual for it, it starts running as it should. It modulates (in this case drops) the flow rate, the LWT/RWT delta t rises, and it stops cycling. The COP however continues to fall, as the heat pump is working harder. Note how relatively stable everything is between 0600 and 1200. This 'normal' steady running state is extremely rare on my heat pump, and only seems to occur at around 5 degrees OAT. Above that OAT, it cycles, below that OAT it starts robbing Peter to pay Paul (ie defrosting) as we can see on the right hand side of the chart. As the OAT drops to zero and then below on the evening of 2nd Jan, the heat pump starts using defrost cycles (the LWT drops below the RWT), and the heat pump sucks heat out of the building and dumps it in the garden (albeit doing a spot of defrosting on the way), as can be seen in the larger OAT upticks that coincide with the defrost cycles. All the while the compressor is working harder and harder. These defrost cycles are extremely damaging to performance. The energy out (red bars in lower chart) hardly rises from what it was in the previous 12 hours, despite the fall in OAT, the energy in rises, and the COP drops below two, and worst of all, because this is what really matters, the indoor air temp starts to fall. The heat pump is now firmly established in CQC mode, and it will stay in that mode until the OAT starts to rise. Until that happens, the indoor air temp will continue to remain cooler than it should be, and may well fall yet further. I should perhaps add, because the flow rate is so central to what appears to happen, that the Midea reported flow rate is credible. Although it is calculated from unspecified 'electronic parameters', rather than directly measured, the reported values are very close to those seen on an independent analogue flow meter in the pipework. As I said at the outset, this is an MCS fail. The question is why, and what can be done about it. We can effectively rule out heat pump under-sizing, because if we use real world heat loss and heat pump capacity values, the heat pump should, at least on paper, be able to meet the heat demand at low OATs (11kW supply to meet a 9-10kW demand). Instead the answer must lie in the way the heat pump is functioning, or rather not functioning. It seems to me there are two problems. The first is that it fails to make use of its theoretical ability to modulate (vary) the flow rate. The almost always 1.4m^3/h flow rate is a striking feature of my heat pump (that drop seen in the middle of the chart is a very rare occurrence). Compare the extreme left and right hand ends of the plot. In both cases, the flow rate is effectively the same, despite the fact the OAT has gone from above 10 degrees to below zero degrees. Accordin...

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Air Source Heat Pumps (ASHPs)

Last Post by cathodeRay 2 years ago

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cathodeRay

(@cathoderay)

Famed Member Moderator

Joined: 5 years ago

Posts: 3093

Topic starter 09/02/2025 11:54 am

Posted by: @bobtskutter

↑

Swapping the radiator for a fan coil might not give you additional heat output because it's not the radiator that's causing the "restriction".

Absolutely, I am very aware of that! Moving to a fan coil is an (major) aesthetic improvement, but only once I know it will have adequate flow.

I am still trying to think of a way of making a diagnosis ie where is the obstruction, and what is it. I know how to drain the system and so also know it is not a trivial undertaking, and I would prefer to know if possible what I need to fix while it is drained down. I am also not inclined to do it during one of the coldest month's of the year, in case something goes wrong!

Midea 14kW (for now...) ASHP heating both building and DHW

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cathodeRay

(@cathoderay)

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Joined: 5 years ago

Posts: 3093

Topic starter 11/02/2025 8:15 pm

Naturally I was concerned that increasing the energy out (delivered to the house) by opening up the lock shield valves might increase the energy in (used). I now have enough data to be able to plot hourly energy in and mean room temp for hours at a particular OAT for before and after I opened up the valves. I have done this for year to date (note date time axis is categorical, they are just the hours that match the OAT criteria), for mean OATs of 5 (5.0-5.9, normal running) and 2 (2.0-2.9, defrost territory) , and marked when the valves were opened with a down pointing arrow:

Interestingly, no obvious penalty, in fact very possibly (need more data, and there are things I haven't controlled for...) an improvement! In both charts the mean room temp improves a bit (as expected) and in both charts the energy in appears to fall. It's more marked at 5 degrees than 2 degrees OAT, but still present on both charts. Truly heat pumps are a miracle, that surpasseth all understanding...

Midea 14kW (for now...) ASHP heating both building and DHW

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cathodeRay

(@cathoderay)

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Posts: 3093

Topic starter 11/02/2025 8:31 pm

Posted by: @cathoderay

↑

Truly heat pumps are a miracle, that surpasseth all understanding...

I've just remembered...two days after I opened the LSVs I dropped the weather compensation curve 2 degrees, and then 2 days later I dropped it by another 2 degreees.

Midea 14kW (for now...) ASHP heating both building and DHW

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BobTSkutter

(@bobtskutter)

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Joined: 2 years ago

Posts: 94

11/02/2025 9:26 pm

Posted by: @cathoderay

↑

I dropped the weather compensation curve

That'll do it 😉

Can you go lower?

Bob

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cathodeRay

(@cathoderay)

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Posts: 3093

Topic starter 11/02/2025 9:45 pm

Posted by: @bobtskutter

↑

Can you go lower?

Very possibly. The IAT is at the warm end of my preferred range, and it looks like we'll have fairly settled though cool weather here over the next few days so a good time to experiment. Watch this space...

Midea 14kW (for now...) ASHP heating both building and DHW

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JamesPa

(@jamespa)

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Joined: 4 years ago

Posts: 5559

12/02/2025 12:22 am

Posted by: @cathoderay

↑

Truly heat pumps are a miracle, that surpasseth all understanding...

Somehow that feels like the basis of a motto/strapline.

Without a doubt the combination of the nonlinear response to the main variables, the inherent complexity of the thermal dynamics of houses, contrasted with the extremely simplicity of the thermodynamics on which it is all based, really does make them fascinating beasts.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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Judith

(@judith)

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Joined: 3 years ago

Posts: 554

25/02/2025 11:47 pm

@cathoderay any updates in this mild spell are you boiling indoors yet?

2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof Solar thermal. 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (very pleased with SCOP >4) open system operating on WC

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cathodeRay

(@cathoderay)

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Joined: 5 years ago

Posts: 3093

Topic starter 26/02/2025 9:23 am

Posted by: @judith

↑

any updates in this mild spell are you boiling indoors yet?

Thanks for asking. Yes, the indoor air temperature has certainly been more consistent, a bit higher by a degree or two despite the fact I have lowered the leaving water temperature by a few degrees. The Mark 1 Human Sensor (me) also reports good comfort levels. All in all a success, and I don't think my costs have increased greatly, if at all, the lower LWT appears, as expected, to mean better performance, with COPs at times in the 4-5 range. Sometimes I think we forget how much difference a change in COP can make. For example, a seemingly not very large change, at least in absolute terms, from a COP of 3 to 4.5 means, all other things being equal, including having the same heating load, the unit costs part of my electricity bill falls by one third - where I would have paid £100, I now only pay £66.66, not a trivial saving.

Here's my chart for the last week. On Monday, I restarted the overnight setback (using room stat down to 16 degrees between 2100 and 0300), as I am interested to see how the now all valves open system copes with the recovery period after the setback. Previously it struggled, sometimes taking 12 hours or so to get back to temperature. On Tuesday morning, after a mild night, it recovered well, back to where it should be by 0700. Last night was significantly colder, and the recovery has been a bit slower, but by no means a disaster, despite being in defrost territory. Bear in mind this is partly managed by my auto-adapt script, which adjusts the LWT depending on how far the actual IAT deviates from the desired IAT. At the moment the script is set up to do one degree of LWT change for every degree of actual vs desired IAT deviation, up to a max of 3 degrees eg if desired IAT is 19, actual IAT is 17, deviation is 2 degrees and LWT increases by 2 degrees. These parameters are easily changed, by editing the script and restarting it. I am going to run the system as is for a week, and then see if it needs any tweaking.