Using BT12 (or BT2 in my case) was my understanding, but I’ve been watching my system closely this morning and now I’ve swapped to thinking it’s BT25 instead, as you suggested. It is BT25 dropping below S1 that appears to prompt the DMs to start decreasing. I guess it’s BT25 that provides the most accurate indication of what temperature the emitters are seeing.
My system is unusual in being plumbed much more robustly than most seem to be:
- I have a 100 litre Buffer Tank just to add Volume, to guard against short-cycling
- I have a Low-Loss Header to provide Hydraulic Separation - given I have 3 pumps on different circuits for UFH, Radiators, and MVHR Duct Heater
With your set-up, when the Compressor (and Charge Pump) are Off, your other pumps will be pulling ‘Return’ water up through the Buffer Tank - as well as pulling a bit through the Outdoor Unit.
I know my LLH does an excellent job of Hydraulic Separation; when the Charge Pump is Off, even with all three of the Emitter Circuit Pumps running, the Heat Meter only registers a tiny flow through the Heat Pump (but I do run the Charge Pump almost all the time, since I need to empty the Buffer Tank). Your Buffer Tank is probably rather less effective at Hydraulic Separation.
My system’s schematic for comparison (not to scale!):
