Is it worth paying the extra cost for KFW55 or not?

Our neighbor has such a system. Air-to-water heat pump installed inside. I can hear the humming very clearly at night. We sleep about 10-15m away.. I hear the noise clearly. Not for sensitive people. I am already looking forward to our new home... no air-to-water heat pump all around.

Then the thing wasn’t installed properly. You usually don’t hear it. Only when a mechanism switches does it click, and you can hear the "rush" of the water. But close the utility room door and you don’t hear anything anymore.

The neighbor's air-to-water heat pump is an indoor variant of the air-to-water heat pump (no outdoor unit) and the thing is, as said, housed in the neighbor's house. I only hear the humming noises at night when it is really quiet. I think it is low-frequency structure-borne sound that is transmitted to us through the roof structure. I am very sensitive to it. The thing obviously starts preparing hot water at the same time every day, then that's it for restful sleep.

When I read web articles "... if you set them up like this and that with appropriate spacing, then they don't disturb the neighbors" But what about yourself!? It also has to be okay for you!

One more question: Would you also use a gas heating system with a gas tank? That is, if you don't have a gas connection where you live? Because that's also one of the reasons for the air-to-water heat pump for us.

I would avoid using a gas boiler if possible. However, I did use one because I was not willing to install wall/ceiling/floor heating in the renovation project.

But if you do, please no tank. Either you have to buy it, then you regularly pay for the inspection (I don't know the intervals), or you rent it and are tied to the landlord as the supplier. The prices are not very appealing either.

But yes, you absolutely have to think about the installation location and decoupling. Otherwise, you'll have trouble or cause some.

Thanks again for the answers!

I discovered something new while reading into the topic: brine heat pump with ground collectors. Sounds very promising. No disturbing noise. Better performance factor. Only a small additional cost. That would give me a good feeling even if we only have the energy-saving ordinance standard. Where is the catch? Why is it used so rarely?

The ground collectors require relatively large areas and considerable earthworks, both of which often lead to choosing the air-water heat pump instead. We have a brine-water heat pump with deep drilling and a photovoltaic system and wanted this out of conviction. KFW55 was then possible with relatively little effort for the windows, insulation, and roof. Of course, the repayment grant is not sufficient for this. But the photovoltaic system probably breaks even in the end, the heating costs +9k, and the additional cost for the windows, insulation, and roof was less than 5k in total.

Whether it was economical... idk. But I like the feeling that I can say that on balance I produced all of my heating electricity and half of my household electricity myself. However, it is definitely not cheaper.