Sizing of our air-to-water heat pump in KFW55 new build semi-detached house 120 sqm

Hello,

We are building a KFW 55 duplex house with a developer, 120 sqm + attic with about 15 sqm heated, and we are currently questioning the offer from our developer for the planned air-to-water heat pump.

To receive the BAFA subsidy of 35% for new construction, our developer planned the Nibe F2120-8 + VVMS320 for us. Of course, we applied for the BAFA subsidy back in 2020, the duplex is currently under construction.

The heating load calculation by the energy consultant resulted in 3 kW (temperature -7 degrees). Up to 4 people are supposed to live in the house sometime in the future, so we are also calculating about 0.8 kW for hot water preparation. The duplex will have underfloor heating.

Now we are afraid that the air-to-water heat pump Nibe F2120-8 is totally oversized and possibly does not operate efficiently (keyword cycling, switching on and off, etc.).

We would be very happy if you could share your opinion about our building project and the air-to-water heat pump. We might still have the option to choose a different air-to-water heat pump, but then we would have to forego the BAFA subsidy.

Thank you!

The heat pump can only modulate down to 3kW. So "one winter, one heating cycle" is not going to happen. Are the -7°C the NAT? How was the heating load calculated? According to PHPP? According to your information, the heat pump is oversized. Whether it cycles frequently because of this mainly depends on the hydraulics. Cycling usually refers to very frequent compressor starts with run times of just a few minutes and premature termination of the heating cycle due to poor hydraulics. I would not call 3-4 starts per day with run times of 3-4 hours cycling, for example. What options do you then have regarding the choice of heat pump? Are you building with a general contractor or have you awarded the heating trade yourself? Are other heat pump manufacturers also possible? Also pay attention to the hydraulics. No heating buffers, no separation tanks, sufficient flow in the heating circuit. Design of the underfloor heating according to room-specific heating load calculation based on the desired temperature for the individual rooms. Is the handover of all calculations to you contractually agreed upon?

The heat pump can only modulate down to 3kW. So it won’t work with "one winter, one heating cycle". Are the -7°C the NAT? How was the heating load calculated? According to PHPP? According to your information, the heat pump is oversized. Whether it cycles frequently because of that depends mainly on the hydraulics. Cycling usually means very frequent compressor starts with run times of a few minutes and premature termination of the heating cycle due to poor hydraulics. 3-4 starts per day with run times of 3-4 hours, for example, I would not call cycling. What options do you have regarding the selection of the heat pump? Are you building with a general contractor or did you assign the heating trade yourself? Are other heat pump manufacturers an option? Also pay attention to the hydraulics. No heating buffer tanks, no separation tanks, sufficient flow in the heating circuit. Design of the underfloor heating according to room-specific heating load calculation based on the desired temperature for the individual rooms. Is the handing over of all calculations contractually agreed with you?

According to DIN EN 12831-1 with a general contractor and basically no choice...

Maybe you can at least push through the 2040s, they can throttle down to about 2.3 kW.

According to the DIN EN 12831-1

Then the actual heating load will be even lower thanks to internal and solar gains.