ᐅ Heat pump KNV or IDM air source heat pump

All right, thanks.
I will check with the manufacturers.

How can I improve this bivalence temperature or the bivalence point?

Selecting the right heat pump. It must have enough capacity at low outdoor temperatures to operate without an electric backup heater. And/or improved thermal insulation.
The bivalence point depends on the heating load/insulation of the house, the desired temperatures, the design of the underfloor heating (flow temperature), and the characteristic curve (compressor performance at outdoor temperature) of the heat pump.

There is hardly any better thermal insulation than what is already used; the construction is done with 38cm (15 inches) thick insulated bricks.
Now every heat pump salesperson will tell me theirs is the best and can do everything.
My installer lays pipes with 10cm (4 inches) spacing in every room and a small section of underfloor heating in the bathroom. The room temperature is then controlled via the flow temperature, as I understand it.

Most heat pumps regulate based on the return temperature. The supply temperature is then determined by the temperature difference between the supply and return. This depends on the energy provided and the volume flow. The optimal temperature difference depends on the refrigerant. It’s quite a complex science...

Make sure that the heat pump is not undersized for very cold days. However, it doesn’t necessarily need to provide all the required heat through the compressor at the design outdoor temperature (NAT), which occurs very rarely. Equally important is that the heat pump can modulate its output down to very low levels. Otherwise, you might end up with a heat pump that cycles frequently at milder temperatures.

I haven't found any 7-8 kW pump that operates below 3 kW.