I’ve spent the week experimenting with various things, and I can confirm that it is the room thermostat that is driving the behaviour I’m seeing. To summarise, the room stat calls for heat, which triggers the heat pump to start up. [The ‘OperationMode’ I was charting above correlates directly to the signal from the room stat]. If the room is close to the set point, then the call for heat will be short, but the heat pump will continue for the minimum of 20* minutes. [*Actually the minimum can be as short as 10 minutes when the flow temps are high enough]
I’ve experimented with Honeywell’s secret settings, but can only select 3, 6, 9 or 12 cycles per hour. Problem is, all of these divide into 20 minutes neatly.
From what I’ve read elsewhere, 20 minute cycles aren’t necessarily bad for heat pumps, and performance shouldn’t suffer too much. But I think I can perhaps get better CoP with longer runs. Maybe. I need to run some experiments for a few days before I’m sure.
As I’ve learnt from the other thread, the room stat will only modulate when the temperature is within 1.5° of the set point. For larger differences, it will send a persistent call for heat. This made me realise that I had my heating programs setup backwards: I was setting the early morning to be warm-ish, switching to a higher temperature by late morning (house is occupied all day). What I actually need is for a higher startup temperature to get things going, then drop down a degree for the rest of the day. With this setup, I got a solid 2 hour run this morning!
Will continue to monitor and fiddle with things to try and find the most optimal configuration…
Related discussion: Advice on air-source heatpump/thermostat settings?