Hi Jonathan,
I apologise in advance as I am going to be the bearer of doom and gloom, I have the same heat pump in a smaller house than yours and I am finding it a nightmare to be honest.
I have to ask, how big are your radiators?
Your radiator schedule would be good to see.
This heat pump has a minimum electrical input of about 900w, it doesn’t matter how low you go on the flow temperature, that is as low as it goes.
That is because it is really a 16kW heat pump.
Mine doesn’t like going below about 35c flow temperature and it often uses no less electricity running at 38c.
I have found that running as radiators works best for me, my COP is worse and my heat output lower if I try to run at dT 5c on fan coils.
I get to a dT of about 6.3c on radiators at a flow of 36c
i cannot get to the radiator dT of 8c as the flow rate is too high, it only goes as low as 10lpm.
We all see a difference between the tow temperature sensors R1T and R2T, there is a loss through the back up heater.
I am MID monitored and R2T is what comes into my house even though R1T is higher on my heat pump as well.
I would try radiators with the fixed dT of 8c, you probably won’t get 8c but it may be better.
I gave up on the Madoka fttb
I just run a fixed flow of 36c and either turn the heat pump off or open windows to control room temperature.
I find the initial heating phase to be drawn out and poor in efficiency terms caused by at least 30 minutes of high flow rate.
My one is difficult on occasions after DHW or periods of being off, sort of like a failed start, never really gets going and shuts down after an hour or so and has another go.
I find as it gets colder my heat pump cannot maintain 36c and the flow temperature slowly decays to around 34c, I have no idea why as I have an overshoot of 4c set. When it is warmer, no problem and the overshoot works as the house temperature rises.
I also find my heat output drops at a constant flow temperature when it gets colder outside, the opposite of what i would expect. I would have thought the radiator performance would improve as the heat loss increased, I see the opposite.
If I leave it running it always shuts down and restarts 6 hours later, you can set you watch by it. It must be a thing for these heat pumps. It is annoying as it sucks all the heat that I have paid for out of the house and starts again, for no apparent reason.
The 8kW version does not do this, it will run continuously.
I have tried different dT and none work for me, radiators has always been best in terms of efficiency.
My honest opinion is that this is a dog of a heat pump and it is all because it is really a 16kW heat pump with a lowest output commensurate with a home that needs a 16kw heat pump.
As you may be able to tell, I hate it!