ᐅ Sizing of Our Air-to-Water Heat Pump for a KfW55 New Build Semi-Detached House, 120 sqm

Hello,

We are building a KFW 55 semi-detached house with a living area of 120 sqm (1292 sq ft) plus an attic of about 15 sqm (161 sq ft) with heating, working with a developer. We are currently reviewing the offer from our developer for the planned air-to-water heat pump.

To receive the BAFA subsidy of 35% for new buildings, our developer planned the Nibe F2120-8 with VVMS320. We applied for the BAFA subsidy back in 2020; the semi-detached house is currently under construction.

The heating load calculation by the energy consultant resulted in 3 kW (temperature -7°C (19°F)). Since up to 4 people may eventually live in the house, we are also accounting for about 0.8 kW for domestic hot water preparation. The semi-detached house will have underfloor heating.

Now we are worried that the Nibe F2120-8 air-to-water heat pump might be oversized and possibly not operate efficiently (issues such as short cycling, switching on and off, etc.).

We would appreciate your opinions on our construction 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 forgo the BAFA subsidy.

Thank you!

The heat pump can only modulate down to 3kW, so the idea of "one winter, one heating cycle" won’t work.
Are the -7°C (19°F) the design outdoor temperature? How was the heating load calculated? Using PHPP?
According to your information, the heat pump is oversized. Whether it cycles frequently depends mainly on the hydraulics. "Cycling" usually refers to very frequent compressor starts with short run times of just a few minutes and premature termination of the heating cycle due to poor hydraulics. For example, 3-4 starts per day with run times of 3-4 hours each would not be considered cycling.
What options do you have regarding the choice of heat pump? Are you building with a general contractor (GC) or have you assigned the heating system trade separately? Is it possible to use other heat pump manufacturers?
Also, pay attention to the hydraulics. No heating buffer tanks, no separation tanks, sufficient flow rate in the heating circuit. Design of the underfloor heating based on a room-by-room heating load calculation according to the desired temperatures for each room. Has the delivery of all calculations been contractually agreed with you?

Daniel-Sp schrieb:

The heat pump can only modulate down to 3kW. So, "one heating cycle per winter" won’t work.
Are the -7°C (19°F) values the design outdoor temperature? How was the heating load calculated? Using PHPP?
Based on your information, the heat pump is oversized. Whether it cycles frequently mainly depends on the hydraulic setup. Cycling usually refers to very frequent compressor starts with short run times and prematurely stopping the heating cycle due to poor hydraulics. For example, 3-4 starts per day with run times of 3-4 hours each I would not call cycling.
What options do you have regarding the choice of heat pump? Are you building with a general contractor (GC) or did you contract the heating system separately? Are other heat pump manufacturers an option?
Also, pay attention to the hydraulics. No heating buffers, no indirect storage tanks, sufficient flow in the heating circuit. Design the underfloor heating based on room-by-room heating load calculations according to the desired temperature for each room. Has the contractually agreed handover of all calculations to you been ensured?

According to DIN EN 12831-1
with a general contractor (GC) and actually no choice...

Maybe you can at least push through the 2040 models, which can regulate down to about 2.3 kW.

Stammheimer schrieb:

According to DIN EN 12831-1

The actual heating load will be even lower due to internal and solar gains.