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In the Bleak Midwinter... Midea heat pump performance, poor COP and windchill in action

cathodeRay · 2024-11-22T09:16:26Z

...Frosty Wind Made Moan. The last 24 hours have provided a challenge for my heating system, and as in past cold spells, it can't quite cope. Here is the chart for the last 24 hours, minute by minute data for the temperature variables, hourly data for the energy variables: The heating system has been in something approaching steady state conditions, which makes it a useful period to look at. Some summary stats for the 24 hour period: Mean outside air temperature (as measured at the heat pump, so not the 'true' OAT): 1.7 Mean set leaving water (flow) temperature: 49 Mean actual leaving water temperature: 46 Mean returning water temperature: 42.1 Mean LWT/RWT delta t: 3.9 Mean kW power used (which is also the kWh per hour): 3.025 Mean kW power output (which is also the kWh per hour): 6.068 Total energy used over 24 hours: 72.605 (ouch) Total energy output over 24 hours: 146.342 (cripes) 24 hour COP: 2.02 And the bottom line, actual mean indoor air temperature: 17.8 degrees, which is 1.2 degrees below the desired indoor air temp. The first thing to note is the mean kW power output, which, unless the numbers are wrong, is the heat loss at the current OAT. It has to be, because the building is in near steady state conditions (near constant OAT, near constant IAT), meaning the heat energy in equals the heat energy lost. Damn the MCS and Freedom spreadsheet calculators! Adjusting the heat loss at 1.7 OAT to the design OAT of -2 gives a loss of around 7.8kW, which is a lot less than the calculator based loss of 12.3kW. The second thing is: why can't the heating system cope? According the Midea's engineering data, my so-called 14kW heat pump should be capable of putting out just over 11kW of power in the current conditions, which over an hour is just over 11kWh of heat energy. Yet it is only putting out 6.1kWh. Why? The LWT/RWT delta t is on the low side (4 rather than ideally 5), but perhaps more importantly, it fails to reach the set LWT it has set itself (which in turn comes from the weather compensation curve): mean actual LWT is 46, compared to a mean set LWT of 49. Perhaps the poor performance is because it is both cycling (low frequency, but why cycle at all?) and defrosting. If you look at an individual cycle, eg the one starting just before 2330 last night, at the end of the defrost, the LWT is below the RWT, it then rises to 54 just after midnight (0015) at which point it cycles off, and then restarts at 0025, which then triggers a defrost at 0035. The net result is both the actual LWT and the LWT/RWT are compromised for much of the cycle, as can be seen in the zoomed in view of the cycle. Note for how much of the cycle the actual LWT is below, sometimes considerably below, the set LWT, eg between 0020 and 0050 the actual LWT is perhaps on average 10 whole degrees below the set LWT: What causes this operational failure? My initial thoughts are that it boils down to the control logic in the heat pump, in other words it is a design failure.

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Air Source Heat Pumps (ASHPs)

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JamesPa

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03/12/2024 10:34 am

Posted by: @cathoderay

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The heat pump logic says it is warm outside, so the house needs less energy (only enough to balance the heat loss at that OAT), but in fact the house needs more.

That's exactly what my analysis above would predict. After the rise in oat the bit of the house you live in continues, for a while, to lose energy to the middle of the walls at a higher rate than would be the case in the steady state at the new oat. This continues until a new thermal gradient is established which falls continuously from inside to outside, rather than having a minimum in-between. The heat pump supplies energy corresponding to the steady state at the higher OAT and thus there is a deficit. This is a direct consequence of the fabric thermal mass (heat capacity in scientific terms). I would expect the converse to happen if there is a sudden fall in oat.

My boiler, which had a control loop based on iat, exhibited different, but no less annoying, deviations from perfect control. The problem for the designers of heating control systems is, i imagine, that they don't know the characteristics of the house (which will vary widely) and can't rely on installers or homeowners to dial them in. That's where controllers which learn about the house over a period of time have the edge!

This post was modified 12 months ago by JamesPa

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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cathodeRay

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Topic starter 04/12/2024 7:50 pm

A similar pattern happened again last night and into this morning, low overnight OAT, followed by rapid rise in OAT fooling the heat pump into thinking the house was warm, IAT must be fine when in fact it was anything but, it was actually falling:

I have now written some extra code in my auto-adapt script to detect this and if detected, then add a boost to the WCC end points. I will report back once it has had a chance to run, and can be seen to be working.

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

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Judith

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16/12/2024 2:41 pm

I look forward to seeing the results of your script.

I fiddle with our WC curve increasing/decreasing between 0.55 and 0.75 according to the extent of wind chill. Strong winds and driving rain doesn’t get adequately compensated by WC alone, even on Vaillant’s active mode.

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

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cathodeRay

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Topic starter 16/12/2024 3:50 pm

Posted by: @judith

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I look forward to seeing the results of your script.

Thanks for your interest. It hasn't really been tested yet. The recent modification to the code was to deal with a rather specific circumstance, a rapid rise in OAT after a cold spell dialling down output prematurely, and that hasn't happened since adding the modification (I suppose a bit like it never rains when you take your umbrella!). What it doesn't (yet) do is deal with wind chill, which we have had rather a lot of recently. The script does do its thing, but it does it assuming no wind chill, meaning it doesn't (yet) do enough auto-adaption.

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

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