Heat pump for KfW55 house 148 sqm

Is there a generally valid minimum duration that can be considered material-friendly?

that varies from manufacturer to manufacturer and from design to design. The regulations are often set at the factory so that the minimum running time is always ensured. Buffers and hydrl. switches make this absolutely certain. Often this is between 10-15 minutes.

Whereas in a new building with underfloor heating a hydraulic separator and actually also a heating buffer can be avoided if the underfloor heating is well planned and executed. Especially a hydraulic separator can become an efficiency killer (just like a bypass valve). However, this requires care in planning, execution, and operation. Search the web for flow30 and read into the topic.

It will now only give you unnecessary gray hairs with the developer project. You already have fundamental problems regarding the sizing. There won't be much going on with "flow30".

Buffer, switch, ÜV and ERR are the compensators for a botched hydraulics. I can therefore understand every manufacturer who requires such components. Unfortunately, the installer is often just as much a layman as the client :rolleyes:

Thank you for the many responses.

Attached are some photos. In the bathrooms, the laying distance is rather around 10cm, in the living rooms around 15cm.


It is a property developer, even though the boundaries to the general contractor are often interpreted fluidly by the company. It was a two-part notary contract, land from the project GmbH, construction contract from the construction GmbH, all subsidiaries. We knew that and still decided on it. I do not regret it either, I would choose the land again. But if I had the choice, I would have chosen the general contractor myself. Especially regarding the contract design, there was zero room for maneuver for us, and we definitely tried.
Construction time 52 weeks
no contractual penalties
at least it is a consumer construction contract


We received the heating load calculation, as it was suggested to us that we could quite possibly take care of a pump ourselves now.
Standard heating load 5115 W
Norm heating load (with heating-up surcharge) 6233 W

Talking to the trades on site and arranging small matters works quite well depending on the trade, but especially with the sanitary… not at all. Absolutely overwhelmed one-man company with his wife as secretary.

Getting in touch with the developer also only works insufficiently. On the phone only snappish or decision-free secretaries, answers to emails barely at all.
Instead, letters come with a slightly threatening tone:
Since they cannot procure a heat pump for us in time, the house will be completed without a heat pump.
To dry the screed, they are renting us a Hotboy for €275 per week. (3 weeks rental period)
Afterwards they provide a fan heater for 8 weeks (free of charge, no less) so that trades like the tilers can continue working.
Then we are supposed to accept the house without a heat pump. It would then be installed later.

And that’s where it gets too much.
In our opinion, the Hotboy should be provided without rent; we are already squeezed enough with the electricity costs.
We do not want to accept the house without a heat pump, and we are under no obligation to do so according to the building code.


Thanks for the encouragement. It will probably be the 8kW at some point. Then we have to make the best of it.


Thanks for the tip with the planning tool.
I entered the data. As it seems to me, it results in a 6kW system (blue line), but the 8kW is close, screenshots attached.