Annual performance factor calculation for funding (parameters and calculation tool)

If the heating load is correct, a 12kW unit would be more sensible, especially if the lower modulation limit is reduced as a result. I was able to have my underfloor heating planned verifiably at 28/24 with -12° standard outdoor temperature.

Regarding "room-by-room heating load calculation": no, it does not exist. Neither of the two heating installers does it without an advance payment. Regarding 12KW machine: I want to receive the subsidies. For this, an annual performance factor of 4.5 must be achieved with an air-to-water heat pump, or a soil-to-water or water-to-water heat pump must be used. We have a standard outside temperature of -13.7. So I don't find an air-to-water heat pump. Really a shame and annoying.

How can the heating engineer make a statement about possible/impossible flow/return temperature without an existing room-specific heating load calculation? Considering the size of the house, I do not assume that it is built on a towel-sized plot. Is a ring trench collector not possible?

The property around is somewhat smaller due to the size of the house. We have a garden of about 240 sqm in front of us. But with a brine heat pump, the storage tank must fit the 10,000 houses subsidy. Which the heating installer does not have. Now I have found a Nibe 2120-12. At -14 degrees and a flow/return temperature of 35/27, it would achieve an annual performance factor of exactly 4.5. Is the flow/return temperature valid?

What kind of storage is then required? DHW or heating buffer tank?

35/27 should be possible, but it does not exempt you and your planner from a calculation tailored to your object. Otherwise, it will go wrong despite obtained funding. Estimates, assumptions, gut feeling, and rough approximations often lead to problems with heat pumps, as can be read here and elsewhere repeatedly.