Thanks for all that.
Yes, there are two zones but they run under auto-adaptation i.e. Mitsubishi’s own system which apparently uses some sort of smart learning type stuff. I really need indoor temperature ‘knowledge’ as I can get a lot of solar gain etc.
The house is entirely (all new) radiator based, around 25 in total and roughly split evenly between the two floors. All the emitters have TRVs other than those where the Mitsubishi remote wireless sensors live.
The plumbing/wiring is the ‘simple’ two zone system as described in the FTC6 manual i.e. same flow temperature to both rather than the more complicated version that uses more temp and flow sensors plus a mixer valve.
The programming is basically 20C in both zones from 6am to 8pm (up)/11pm(down) with the setback to 18C/19C. Weekends similar but with 21C downstairs and 20C upstairs.
The system volume is quite large and has a buffer tank/two pumps. The drop in flow temp you see is when the ‘cold’ water in the just opened zone hits the system. I monitor the CH flows and common return temperatures and they correspond.
TBH I find it keeps the house perfectly warm even when the outside temperature is below zero. Just not that efficient in terms of COP compared to others! The delta T in the ASHP flow and return temps seems to be smaller than optimal but now I’m unsure. Both pumps are Wilo Para SC - the CH one has a 9m head, the HP one a 6m head. All the HP plumbing is 28mm, CH a combination of 28/22/15.