I have just had my visit from Daikin.
It was just a chat today, they need to come back to run some diagnostics for a day.
It was all very positive and she was impressed at how well the heat pump was running in a house this size and understood my issues.
We were talking about the inability of the heat pump to hold the requested flow temperature or increase it and I think we now know why.
The flow temperature decays quite often. This is because the compressor is at its minimum level and it cannot get rid of the heat produced. It cannot reduce compressor power any further so has to reduce the flow temperature to reduce the heat being produced. I don’t know how it can do this or if it is even possible, it’s a theory that needs to be proven.
The house and the heat pump are fighting each other, the heat pump wants to produce more heat than the house can take and something has to give.
Even though I have overshoot set at 4c this rarely has any effect, the heat pump generally just gradually reduces the flow temperature until the house can take more heat and then it increases again.
It works but the probable reality is the heat pump is so oversized that it is a real issue.
Hopefully their diagnostics will show in detail what is actually happening and why but this is what we think is happening.