Construction costs are currently skyrocketing

Well, in the narrower sense (so when we talk about a Feist passive house), I believe the idea was to insulate thickly and do heat recovery with the ventilation system. And then the remaining heat demand (after calculation and simulation of passive heat gains from people, the sun, and household appliances) should only be very small (please look up the exact numbers yourself). And because that is so little, heating is done through the ventilation system, which you have anyway. The heat absorption capacity of the air is limited by the temperature at which dust begins to smolder, and the airflow should not be increased for heating purposes. So really only a little heating will be added to the airflow that is already necessary for hygiene reasons (of course depending on the number of people in the house).

Then you can save on other heating systems (except maybe electric underfloor heating in tiled areas, especially the bathroom for comfort). I haven’t heard of anyone building an EH40 with such minimal heating. Is a fully comprehensive air heat pump with water underfloor heating so affordable that people prefer to forgo the extra energy consultant costs for the PHPP and a few extra measures for insulation for a higher building standard (serious question)? Or is the additional cost simply too high? Or is there so little trust in science and energy consultants (maybe with good reason) that this can work? I mean, these things have been around for a while...

It seems more sensible to me to invest in a higher quality building envelope than in a higher quality heating system that will eventually break down.

Solar gains are nice and all, but they can also fail for a week. The incandescent bulbs of my youth with their corresponding heating output have been replaced by energy-saving LEDs; unfortunately, now the 200W of waste heat in the living room is missing.

Yes, what really makes me suspicious is the tendency to heat directly electrically (infrared, electric underfloor heating, partly also water flow heaters; because: you need so little anyway and that comes from the photovoltaic system) and the devices for direct electric heating are so cheap anyway that you can heat a lot before a full heat pump would pay off.

But if I now only have to provide a little supplementary heating with a maximum efficiency of 1 direct electric heat, couldn't I also provide much more supplementary heating with an efficiency of 1:3 using an air-to-water heat pump and only consume the same amount of electricity?

Actually, a passive house is defined in such a way that it does not require a dedicated heating system and stays warm through the recovery of waste heat. Additional heating is used at most on the few coldest days of the year. In this respect, you might have a building envelope at passive house standard, but strictly speaking not a passive house.

A nice definition (regardless of the technical specifications):
“A passive house is a building in which heating and cooling have such a low energy demand that it can be provided with very little effort and at the same time the energy demand for all consumers in the building is so low that it can be sustainably covered by a renewable energy supply.”

Whereas a 150sqm house may still have a heat demand of 2,250 kWh per year. This is not negligible and is often (mostly) not manageable by controlled residential ventilation alone. So, a heat pump is added - preferably an air-to-air heat pump, which can then cost a lot of money in deep winter if planned incorrectly (see Proxon).

Then you can save on other heating systems (except maybe electric underfloor heating in tiled areas, especially the bathroom for comfort). I haven't heard of anyone building an EH40 with such minimal heating.

Sure, that is offered and installed very often. Bien-Zenker offers it as standard for their 40-series houses. Comfort climate heating. So air-to-air heat pump. Streif Haus as well. There are various providers that rely on this.

For the passive house, unlike the Gebäudeenergiegesetz standard or the KfW efficiency houses, there are no technical minimum requirements and currently no suitable funding framework. The former probably conditions the latter. There is only the German Passive House Institute, which specifies a target value that the annual heat demand must not exceed 15 kWh/m² and the heating load must be a maximum of 10 W/m², with airtightness of less than 0.6/h at 50 mbar pressure difference. The specifications are purely physically justified by the type of heating only through reheating the supply air. The 10 W/m² can still be achieved with the given minimum air exchange at just under 40°C. With a higher heating load, more air or a higher temperature would be required. Deviating from this, one can of course also simply use infrared heaters or other electric direct heating instead of reheating the air. I would recommend electric surface heating for maximum comfort.

Then you can save yourself other heating systems (except perhaps electric underfloor heating in tiled areas, especially the bathroom for comfort). I have not heard of anyone building an EH40 with such minimal heating. Is a fully comprehensive air heat pump with water underfloor heating so cheap that people prefer to forgo the extra energy consultant costs for PHPP and some extra insulation measures for a higher building standard (serious question)?

From a certain living space size, the additional central heating as an air-water heat pump together with electricity cost savings is simply cheaper again than the extra effort for PHPP planning and additional insulation. Probably the construction costs balance out and then in the end the higher electricity costs of direct heating remain against the additional maintenance costs of the heat pump.

Or is the surcharge really that high? Or is there so little trust in science and energy consultants (perhaps rightly so) that it can also work?

Most energy consultants will not recommend passive house planning, similar to how most heating installers prefer to plan an oversized heat pump. The main thing is not too cold and if it goes wrong, you’re the fool. Additionally, the user must also fit the house with appropriate behavior (ventilation) and the construction supervision must be carried out meticulously (keyword thermal bridges and especially airtightness at windows and connections).

In the normal case, with poor construction execution, there may be 3-5 W/m² more heating load and you do not notice because the heating system manages that as well. In a passive house, this leads to 18°C in the living space.

For the passive house, you should have confidence in your abilities and preferably also gain experience in this field as a planner with colleagues with corresponding knowledge before tackling it independently. The examples mentioned with passive house owners whose buildings do not get warm confirm this.

Overall, the passive house has not really established itself as a standard, but most of the knowledge in modern construction (airtightness, minimum air exchange, avoidance of thermal bridges) ultimately stems from earlier research and construction projects related to the passive house and has been adopted from there.

Friends of mine lived in an apartment (Miete) without heating. There was only an electric heater in the bathroom. The end of the story was electric fan heaters in every room. It was so semi-cozy…