Underfloor heating air-water heat pump. House too warm when the sun shines

Then the differential between the supply and return flow becomes too small and the heat pump does not operate as efficiently.

I have to interject here. Isn't it actually the other way around? With the small area, where presumably the shower is also excluded, you have to heat the entire room. Now you’re also turning the flow down. That means you have a lower average temperature. With that, you probably won’t get the room to a comfortable temperature and have to raise the temperature curve. Now the other rooms get too warm again and you turn down the flow there. Consequence: different return temperatures again. Do you have a temperature sensor on each return line? Otherwise, the water in the heating circuit would mix anyway. In my case, the rooms where it should be warmest (bathrooms and the living room) are fully turned up. Presumably, this also depends on the heating system. In my heating system, the return flow doesn’t matter at all. My heating is only controlled by the supply line.

Do you now have significantly more flow in the hallway than in the large bathroom? Then you can reduce the flow in the hallway, but not to zero. Was the heating circuit in the large bathroom actually completely open before yesterday’s changes?

This is nicely warm, so turn down the heating curve!
Important, what are the other room temperatures and the flow rates now?
You need them now to balance the heating circuits with each other.
Please set them and then we will continue. What is the current heating curve setting? Total flow rate?
Be patient. We don’t know your underfloor heating yet due to lack of documentation. But it will work out.