Just an update to say @jamespa and I are still working on this, waiting for data to accumulate. In particular, we are waiting for data from an independent OAT (outdoor air temperature) sensor, to see how it compares to the Midea OAT, which is really the AIT (air intake temperature), and as such is affected by the heat pump's own acivity. 

@cathoderay But surely, when the heat pump fan has been running for a few seconds, it should be reflecting the true OAT? I suppose the ambient temperature during times when the pump is not running may be slightly higher due to residual heat but proper siting of the sensor should be keeping this error to the minimum. Regards, Toodles.

@toodles - I am inclined to agree, but the plot of what Midea call the ambient temperature does show changes in temperature as the heat pump goes on and off, typically by one or two degrees, and the heat pump does after all exhaust cooler air which might well be sucked round to the rear of the unit. We also don't know where in the outside unit box Midea site the 'ambient' sensor. What @jamespa and I are interested in is the OAT the house sits in, since that is a major determinant of heat loss. There are anomalies in the data that suggest the heat loss in significantly higher when running without overnight setbacks, more than can reasonably be explained by the marginally higher average IAT that typically happens without setback compared to with setback. One possible explanation is that the no setback periods just happened to have a lower OAT, but somehow we are not picking up that lower OAT with the Midea ambient (IAT) sensor.

@toodles, @cathoderay 

There is clearly a difference is air intake sensor reading between when the heat pump is on and when its off, with the value being typically 2C higher when its off.  Following @toodles comment it occurs to me that there are (at least) two possible interpretations

1.  The heat pump is in a bit of a cold well so its action cools the air around it when its on

2.  The air intake sensor is warmed, when the heat pump is off, by the waste energy from the heat pump itself

Un until now we have been implicitly assuming (1), but (2) may be more likely (although if it is (2) then the effect would 'wear off' during a prolonged off period eg a setback, which Im not sure we are seeing.

As @cathoderay says there are some anomalies and, rather than speculate further, @cathoderay and I are waiting for the data from the independent OAT sensor.

@jamespa Out of curiosity, I asked Homely for a graph of outside temperature and heat pump on/off times this morning but I see no particular consistency of temperature drops and rises with on/offs.😳

As an aside, I think Homely has to please different nomenclatures for, as you will see, when I ask for compressor on/off, I only see a straight line. I see another straight line if I select HP Outdoor temperature; this suggests to me that the names apply to different makes - or maybe there is more information but Daikin don’t provide it!

Regards, Toodles.

@toodles - that plot looks like it is a plotly plot (from the icons visible in the top right corner), which is what I use, but the legend is rather busy, and it is not always clear which trace is which! Some of the Midea nomenclature isn't crystal clear either, and as we know Samsung call weather compensation water law, which is as clear as mud.

Looking at your chart, I think it probably only updates the OAT every 30 minutes, and overall it seems to show a steady rise, apart from 0900, when it falls, and that does coincide with a heat pump start up. The OAT on my charts is potted at one minute intervals, and looks something like this:

In this chart most of the OAT variations are one degree, but sometimes it is two degrees. Interestingly, the big spike in LWT at 1300 (DHW heating) has no more effect on the OAT than the other cycles.

Posted by: @jamespa

↑

1.  The heat pump is in a bit of a cold well so its action cools the air around it when its on

2.  The air intake sensor is warmed, when the heat pump is off, by the waste energy from the heat pump itself

1. I don't think my heat pump is in a true cold well, I have certainly seen far worse. The back of the heat pump is in front of a wall (as most are) and that will mean it sucks in air from the sides, and the air from the sides might well be replaced by cooler air from the front. But that's about the extent of it. There is vegetation to one side and in front of the heat pump, but it is not that close (2 metres or so) and it is rather porous vegetation, not a dense hedge. The heat pump is very slightly raised on the actual ground it sits on.

2. Possible, but I think unlikely, the heat pump is after all off, and I think I am right in saying there is a bit of over-run on the fan when it starts up and shots off. I might be able to confirm this from the minute data. 

@cathoderay Yes, it is a plotly plot and I suppose the 30 minute intervals are to reduce data load on their server or maybe it is also down to Daikin, Our pump is raised a few inches err… 10 cm off the ground and though next to a brick wall, it has space to exhaust without undue problems. Regards, Toodles.