New construction a heat pump two opinions

Hello, we are currently building a Kfw 60 house, 133 sqm living space, 16 cm concrete wall with 12 cm Styrofoam and underfloor heating downstairs. Upstairs drywall, 160 mm insulated glass wool and radiators. The builder wants to install a 6 kW air-water heat pump because the plot is too small for geothermal energy. A heating company working for us would definitely install a 9 kW inverter heat pump to cover cold winters. The builder thinks that 9 kW is too big, even as an inverter device. However, we have to make a quick decision now and need some advice on what to install. There is apparently a heating calculation, but it is with the builder in their desk. Thanks Baupfusch

Hello,

...and underfloor heating downstairs. Upstairs drywall with 160 mm Iso glass wool and radiators...

The combination of underfloor heating and radiators is usually not very sensible. Especially when a heat pump is considered as the heat generator (HG).

... The developer wants to install a 6 kW air-water heat pump. Since the plot is too small for geothermal energy...

There is actually almost always room for a vertical probe! ;-)

... There is apparently a heating calculation, but it is lying in the developer’s desk.

That already rules out a candidate ;-) A heating load calculation is hardly helpful if it’s just sitting in a desk, especially since they are often wrong.
Moreover, performance specifications for air heat pumps, unlike other heat generators, are of limited use because heating output as well as COP can vary significantly with outside temperature.
Example:

Thanks, Baupfusch

Best regards.