Fresh air heating vs. air-to-water heat pump indoor installation, alternatives??

Why is the ventilation for a KFW55 house a no-go? Is the thermal insulation not sufficient?

Because the air-to-air heat pump takes its heat from the exhaust air to heat. With the heating demand that a KFW55 has, you will consume a lot of electricity in winter – actually like with a hairdryer. The electricity is converted 1:1 into heat. With a normal heat pump, it is up to 5:1 (1 part electricity = 5 parts heat). Therefore, this type of heating is not suitable for "poorly" insulated houses.

What, besides electricity consumption, realistically speaks against an air-to-air heat pump with heat recovery?

Nothing. Only that it may be 2,000€ heating costs in the new building (and you don’t have a floor that feels warm like with underfloor heating). In addition, buy a geothermal exchanger and a domestic hot water heat pump.

If I already have a solar system on the roof, the heating can well use electricity

Unfortunately, you have to heat in winter when the sun does not shine.

and it’s not even that cheap if you have to rent the excavator.

200€ for a whole weekend is too expensive for you?

The heat pump in the attic would be an option I only considered briefly... but there is still that clattering thing in the house, above the bedrooms...

It doesn’t clatter. That’s just a large fan that moves a lot of air and therefore makes wind noise.

Alright, now some sensible (go) arguments have actually come together that I haven't found on the internet before... I really didn't expect such high power consumption and poor efficiency, especially since everything was explained so nicely during the appointment with the claim that it would be so great and everyone who has it installed would save so much :-( Learned something again, and this time quite punctually. I guess the topic of ventilation is dead now. Actually a pity.

€200 for the excavator wouldn't be a problem, more that I can't operate one ;-) and don't know anyone who can. But I think I'll put the pressure on the providers to make them think about it (I think they don't know about it, that's why they don't want to) --> the heating installer has to cooperate as well.

Thank you very much.

Can or should I close the thread or will someone else do it?

The topic of ventilation seems to have just died. A pity, really.

Why? I would still always install a ventilation system. But that has nothing to do with heating, rather with airtight houses.

Nothing gets closed here that easily! And if so, only when nobody wants it... muahaha...

And separate ventilation and heating. They are not just indirectly related in your example. Otherwise, they are two systems.

An air-to-air heat pump is a heat pump like an air-to-water heat pump, only the recovered thermal energy is released into the air instead of water. However, on the market, I only know one system for an exhaust-air air-to-air heat pump. This only has a low heating capacity and the efficiency is really not that high, with just under 2. Claims of an efficiency of 1:1 are completely wrong.

I always find it astonishing who constantly speaks as an expert here. I am an energy graduate engineer and conduct consultations on the side as a kind of hobby. I am absolutely independent of manufacturers and types.
Exhaust air systems, i.e., exhaust air heat pumps, are very charming because the house does not lose heat through ventilation. There are two good systems on the market here, each being an air-to-water heat pump and achieving an annual performance factor of 3-4 with proper design (Vaillant versoTherm and Nibe F750). The weak point of these exhaust air pumps is also their low heating capacity of realistically about 5-6 kW thermal. Both systems are efficient without auxiliary heating up to about -15°C outside temperature.
For 150m^2 with KFW55, the heating capacity will certainly be tight. For you, I would rather go for 8 kW heating capacity... inverter controlled.
With proper design, the heat pump runs 24 hours a day in winter, but at low power. This is how the heat pump achieves its efficiency.
An indoor installation is not audible if the heat pump is set up with insulation. The humming should not be underestimated. But with insulated installation plus soundproof door, the heat pump sounds like a refrigerator at most in the living area.
Personally, I do not like air-to-air heat pumps, because the comfortable climate of a warm floor is different from warm blown air. Additionally, air is a poor heat transfer medium. For the air-to-air heat pump to run efficiently, it has to push a lot of air at low heating temperatures (~26-30°C) into the living space -> risk of drafts. Additionally, underfloor heating can also serve as a heat store.