A further update to this in case anyone has similar issues.
I increased our curve to shift the degmin lower and try to minimise the amount of time the system is switching off for. This is intended as a temporary measure to correct the effect of the BT25 sensor being incorrectly located. Once BT25 is in the right place we will adjust the curve back down.
This has been reasonably successful - we were seeing it switch off for about 30% of the time even though the UFH was calling for more heat and that’s now down to <10%. I’m also using the immersion (switched manually) to keep BT6 at 40-45C, firstly to minimize the hot water load on the heat pump and secondly to provide warm water for defrosts, without it getting up to 60C where it caused an alarm. We are still getting tank defrosts, which pull BT6 down but they are getting less frequent as the outdoor temperature is rising.
However, even with the system in ‘heating’ mode for most of the time, the house is getting colder faster than the heat pump can supply. I think this is because the buffer tank cooled down a lot when the system was off for the washer failure (BT71 is running at 20C) plus the house is now at 14-16C so the return flow is going to be correspondingly low. A quick estimate suggested the buffer tank would need about 2kWh input to get it warmed up and that’s without any load from the house. Given that all our thermostats are calling for heat and it is still regularly stopping for defrosts, that is obviously hard for it to achieve.
I suspect this low buffer tank temperature may be why the control system has switched to tank defrosts. Unfortunately in our case that combined with an unnaturally high HWC temperature as we had the immersion holding it at 60deg, and this was triggering an alarm which aborted the defrost cycles.
We didn’t experience this issue last year as the system wasn’t off for any length of time and the immersion was in auto, so the buffer tank wouldn’t have cooled down enough to switch it over to tank defrosts and the tank would have been at 40-45 apart from during legionella runs.
I’m hopeful that a bit of warmer weather will reduce the defrost frequency and allow the buffer tank to warm up and correcting the position of BT25 will give the system a better view of the heating load.