Don't worry, the general contractor also had enough objections regarding the windows and the staircase. However, all of these could be resolved. The windows will be made with waterproof basement windows, of course not in the size as drawn (I don't have the revised plan yet). And a waterproof roof will be installed over the basement stairs.
How do you explain or how does it come across to you that the general contractor views this as uncritical, but criticizes the heat pump? The problem with the basement stairs is not the roof but the sealing.
I do assume that it will work out. In this region, all basements have to be waterproof against water pressure, and it is a local provider. From previous projects, I am not aware of any problems.
I also fell for that misconception "local provider, flawless reputation, almost only buildings in groundwater so far" ... and got burned. I don't want to cause panic, but where do you get the certainty from the start that it will be the same for you later? Hopefully, you are not leaving this to chance, see my previous post.
Excerpts from the construction description:
Base slab made of waterproof concrete 25 cm thick
Basement exterior walls made of waterproof concrete 25 cm and receive a 12 cm thick perimeter insulation
At the transition from the base slab to the rising basement exterior wall, a joint sheet is installed.
No more detailed description of the concrete is given
Okay, at first, no exposure classes are specified and no consistency class either. Although the latter is only of secondary importance. Has the groundwater been classified as "concrete aggressive" according to the soil report? The thickness of the base slab and the walls are ultimately also part of the structural calculation and without detailed data hardly assessable. The joint sheet can be Pentaflex or equivalent. This is a specially coated "tape" which swells after concreting and ultimately provides the sealing.
I suggest you type into the well-known search engine "WU concrete crack width limitation" and read the PDF from the first search result. From this, you can obtain very well, a lot of important information around the topic of waterproof concrete (basement) and what to pay attention to.
To come back to your problem with the heat pump. If you insist on the installation inside the house, you will have to have a solution worked out by a specialist and sit down together with the general contractor to discuss it. What surprises me here is that the general contractor remains calm about the basement stairs but has concerns about the ventilation breakthrough which is about 1.5 m higher anyway.