Heat pump on the flat roof is buzzing loudly

When it comes to sound, you will hardly get anywhere without an expert report. In addition, there is now your practical problem that the heating season is over and the humming will hardly occur for the next 8 months. Even if you turn on the heat pumps specifically for a sound expert report, they will not have the same load now at 20 degrees outside temperature as in winter. I would not only go to the expert here but also to a specialist lawyer to make sure you don't miss any deadlines.

If they were arranged like that in the end, then there is another problem. Each pump always gets "cold" air "forced" on it, which results in a significant efficiency loss. How **** stupid do you have to be...

Oh yes, I hadn’t noticed that either (thanks also for Tolentino’s comment). They must have had some idea in mind, I just don’t know what. It makes sense (or rather doesn’t) that each unit has to deliver higher output because it receives the cooled air from the other. If you realign them now: should two opposite units "blow air at each other" or "blow air away from each other"?

These are split units, not monoblock design. Therefore, these are copper pipes for the refrigerant with insulation. They are also bendable. Flexible pipes, look like the connecting parts under the sink from angle valve to faucet and are insulated separately, not delivered finished. But those are only used with water.
For refrigerant (split unit) a coupling for vibration decoupling/damping must be used at the connection point and additionally the copper pipe can be routed in a loop.

The heating engineer said he couldn’t decouple the refrigerant connection pipes, a coupling for vibration decoupling/damping would inevitably be made of plastic, and that would not withstand the refrigerant temperatures. He says something like that doesn’t exist, it’s not possible. What do you think about this statement?

When it comes to noise, you can hardly get anywhere without an expert report. Added to that is your practical problem that the heating season is over and the humming will hardly occur for the next 8 months. Even if you run the heat pumps especially for a noise expert report, they won’t have the same load at 20 degrees outside temperature as in winter.
I would not only go to an expert here, but also to a specialized lawyer, so you don’t miss any deadlines.

Thanks for the tip! At least I have experienced that it hums properly when I turn the room thermostats of the underfloor heating all the way up, drain the hot water tank and then set it to high-performance mode. But if it’s going to be over 20 degrees in the next few days, probably nothing much will happen regarding heating, that’s true ... About the lawyer: We reported the defect to the developer immediately after becoming aware of it, the acceptance was only last week and we now have 5 years warranty. I’d also be in favor of a lawyer to put pressure on, but of course that’s also a cost issue and shakes the business relationship with the developer. The latter, after everything they have put us through in recent months, doesn’t really matter anymore anyway ...

One more question: I happened to discover in a photo (see attached) that the small, round, black feet directly under the unit were mounted differently (once with a gap to the beam, once without). Can that be a problem?

Thanks for your committed comments! Regards, Nik

They must have had some reason for it, just don’t know what.

Looks nicer. Otherwise, I wouldn’t assume anyone gave it much thought here. In my opinion, it would be correct if the air is sucked in the middle and both heat pumps blow away from each other. Ideally, also so that the adjacent heat pump is not directly in the lee.

I would also like the idea of a lawyer to put pressure on, but of course that is also a cost question and shakes the business relationship with the developer.

An initial consultation isn’t particularly expensive, but helps you assess the legal situation. Whether you then have to put pressure through a lawyer is part of the discussion, but not necessarily the goal. I also wouldn’t drag it out just because there is a warranty. At some point, you want to enjoy your time in the new house instead of just arguing. By the way, it’s not you shaking the business relationship, but the developer through botched work.

Oh yes, that hadn’t occurred to me either (thanks also for the comment from Tolentino). They must have had some idea in mind, I just don’t know what.

They didn’t think anything at all. They aren’t paid to think along. If (as said: if) there was a plan, then the planner is responsible and the pipe layer carries it out. But actually he should at least have raised concerns. One should send the picture to Daikin as a deterrent example of the nonsense their specialist companies sometimes come up with. Either they all stand in a row, which of course looks bad and may cause other problems, but at least staggered and arranged so that none gets the direct cold air of the others. As it is, it definitely cannot stay. Turning two around I also consider suboptimal. If one runs at low speed and the other suddenly goes full throttle for some reason, then they hinder each other... But since Mr. Daikin cares about the quality of his installers, he should make a constructive suggestion...

The heating engineer said he couldn’t decouple the connection lines for the coolant, an intermediate piece for vibration decoupling/damping would necessarily be made of plastic, and it wouldn’t withstand the temperatures of the coolant. Such a thing doesn’t exist, he says, it’s not possible. What do you think of this statement?

I think compensators also exist for refrigerants. Maybe inquire directly with Daikin. Otherwise, lay the refrigerant copper lines at the connection to the outdoor unit in a loop and don’t tighten the clamps fixing the line to the walls too much, and mount them above the insulation. So that the copper and clamp are separated – if clamps are installed at all.

At the indoor unit to the heating circuit, compensators or flexible hoses can definitely be installed. There you can check on the line itself whether vibrations are transmitted.

One more question: I happened to notice in a photo (see attached) that the small, round, black feet directly under the unit were mounted differently (once with a gap to the beam, once without). Could that be a problem?

No, they were simply leveled so that they stand straight. Just like with your washing machine.