@cathoderay I have a suspicion that the minimum output of your heat pump may be a factor in the surprising result shown in your latest graph, regarding the y-axis intercept. Looking at the graph, there is a floor at 4kW below which there are no points, and the random scatter of points doesn't look quite the same between 9C and 12C and 4kW and 5kW, its denser there. That's a tell tail that something different is going on in that region.

I asked Google for "Midea 14kW heat pump minimum output" and the AI response was "The nominal 14kW Midea M-Thermal air-to-water heat pump typically has a minimum modulated heating output of roughly 3.5 kW to 4.5 kW depending on ambient and water flow temperatures".

It will be interesting to see what a graph of the daily averaged data gives.

To clarify, for the daily average power, just add all the hourly averages together and divide the total by 24. (The y-axis of these graphs is really just power output as it is energy delivered in an hour per hour, or energy delivered in a day per day).

If your heat pump stops running for a while because it hits it's minimum output, how does that appear in your raw data? (I would have expected to see some values below 4kW for hours where lower output is required, and the heat pump ran for some number of minutes at 4kW and spent the rest of the time off). 

Posted by: @rob-of-york

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The outdoor temperature can change much faster than the indoor temperature. This is because the air outside has very little thermal inertia (heat capacity) compared to the house. This is easily observed on summer days when the heating is off. The swings in outdoor temperature between day and night can be very large, often 15 degrees or more, whereas a well-insulated house will only change in temperature by a few degrees. Further, there is a lag between the two cycles as the house take time to warm up and to cool down. Thermal inertia can be described in terms of a time constant, which technically is defined as the time for the temperature of an object move about 63% of the way towards thermal equilibrium with its environment, when that environment undergoes an instant change in temperature. This time constant can be quite long, e.g. a few days for a well-insulated house, more for one of stone construction.

This matters because it tells us something about the intervals of time that it would be appropriate to average over to get each data point for a graph.

Absolutely agree.

When I did my heat loss determination I averaged over a day and found that the fit was better, albeit only marginally, if the consumption was delayed by 12 hrs relative to the OAT (I plotted against OAT because I didn't have IAT but knew it was fairly constant). 

Posted by: @jamespa

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You appear to have a fixed loss (ie when DT=0 you still have to supply the house with energy to keep it at a constant temperature)

I agree, the plots suggest this, but it can't be what happens on the ground, or can it? My OAT plots show data points in the high OAT range (when present eg the Oct to mid-Dec 2025 one) where the heat pump should not be delivering any energy. I need to go back and look at the raw data, perhaps even wait until I put the heating back on, and see what is actually happening.

Posted by: @rob-of-york

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If your heat pump stops running for a while because it hits it's minimum output, how does that appear in your raw data? (I would have expected to see some values below 4kW for hours where lower output is required, and the heat pump ran for some number of minutes at 4kW and spent the rest of the time off). 

The minimum output is probably around 4kW (from the Engineering Data manual). One of the things about Midea heat pumps is they do a lot of cycling. You can just about see it, in very compressed form, in the LWT/RWT, on the right hand side of the January 2026 Heating Parameters chart I posted earlier. It is slow frequency cycling, so we tend to just live with it.

I will as I say go back and have a look at the raw data to see if there are any clues there as to what is going on, and will also do the daily plots.    

Posted by: @jamespa

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Posted by: @jamespa

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You appear to have a fixed loss (ie when DT=0 you still have to supply the house with energy to keep it at a constant temperature)

 

 

I agree, the plots suggest this, but it can't be what happens on the ground, or can it?

No it cant, but it could be a (material) fixed or quasi-fixed consumption by the heat pump, or a variable loss elsewhere in the pipework (eg the PHE) that appears as quasi-fixed over the relatively small range of the plot.  As @rob-of-york says it could be linked to your min output.  It could also either be real or an artefact of the measurement technique.  Points closer to the origin would help distinguish between these cases and IMHO its worth investigating.

I think there may be some clues here that point to the weather compensation curve being at least in part responsible for what we see happening. This is early Oct 2025, when the OAT was high enough at times to mean no energy need to be added, but it was. The upper chart is minute by minute data, the lower one hour by hour:

The OAT reached 18-19°C on the afternoon but the heat pump kept on running. I think it does this because it is on, and that is what the weather compensation curve tells it to do: when the OAT is 15° or more, set the LWT (can just be seen in the upper chart as the red line) to 30°C. The only way to stop it putting out heat at higher OATs is to turn it off!

Note the peaks and troughs of the LWT straddle the set LWT, ie the mean LWT is reasonably close to the set LWT.

This is the Midea weather compensation curve from the Freedom Installation manual:

Any OAT at or above T4H2 will give rise to a LWT at T1SETH2. If the OAT goes into the 20s, the heat pump will still pump out water at 30°C.