Thank you very much for the valuable tips.
Not every tip is worth its weight in gold, even if it is expressed with confidence and without any bad intentions.
Example supply temperature maximum 25-28
A supply temperature of 25°C is not practical and I have not yet seen it implemented. Whether the supply temperature at design temperature (NAT, approx. -10 to -12°C) is 30° or 35°C does not play a major role in electricity consumption; these are single-digit percentage values you can save by the smaller temperature differential. However, lowering the supply temperature below a certain value brings other advantages.
With 35°C supply temperature, you have a floor surface temperature of around 27°C and, for example, a room temperature of 22°C. If the sun shines and your room warms up to 24°C, you still have significant heat output to the room and you need a single room controller that shuts off the respective heating circuits. Classic underfloor heating with classic individual room control.
If the supply temperature is lowered to 30°C, you have a floor surface temperature of about 24°C. If you now have a higher heat input into the room through the sun or other sources, you automatically have almost no or no heat output to the room from 24°C. You no longer need individual room control; instead, the temperature in the rooms automatically adjusts based on the installed pipe length and the flow rate.
If you want to reach 23°C or 24°C in the bathroom, you need this 30°C in the supply; otherwise, you cannot warm the room. Especially in the bathroom, where you have higher air exchange. You won’t get far with 25°C. Maybe 28°C works, but 30°C works just as well and 28°C brings no added value.
At 30°C, of course, underused rooms or bedrooms can also be equipped with individual room controllers, but at least half should be "unregulated". The desired temperatures are then set once centrally via hydraulic and thermal balancing and adjusted in adjacent rooms on request.
Pipe spacing nowhere larger than 10cm; in the bathroom the walls are mandatory; maximum pipe length 80m
A smaller pipe spacing is necessary so that the heating works with a lower supply temperature. Did I understand that correctly?
The pipe spacing can also be larger than 10 cm and the walls are not mandatory in the bathroom either. The important thing is the result of the heat load calculation. First calculate, then plan pipe lengths; from this results the pipe spacing and whether walls should also be covered. The goal should be a low supply temperature at design temperature. Ideally below 30°C to be able to do without individual room control without problems.
Regarding pipe length, I would not go over 80 m with regard to flow and pressure loss, but especially I would not vary it greatly. Balancing becomes more difficult the larger the differences between heating circuits are.
And with 7.5 kW you can heat a 300m² house on the Swabian Alb to 25 degrees. So way too big.
For a 150m2 house you end up at about 4 kW.
That contradicts itself. Building Energy Act 2023 with 150 m² results in about 35-50W/m², depending on volume, location, and window area. So 5-7.5 kW heat load. The 7.5 is rather the upper limit, but the questioner did not say he is building to KfW40 standard. More precise information comes from the heat load calculation and room-specific heat load calculation, which should absolutely be carried out. Otherwise, the underfloor heating cannot be designed and the size of the heat pump should not be determined based on square meters.
Regarding the buffer, I would depend on what kind of bathtub and how long you shower. We have 300 liters, 4 people and mother-in-law in the annex; water has never run out unless I played around with the buffer :)
To avoid misunderstandings: BUFFER tanks serve for room heat supply, a DOMESTIC HOT WATER tank to fill the bathtub. The former, as already mentioned, should ideally be completely avoided; the latter should be chosen with a large heat exchanger surface (>3m²). 200-300 l depending on the number of bathtubs.
He just wants it cozy and quick; you will pay the bill in the future with your electricity and it will not get cheaper.
Harsh accusation, I cannot confirm that. There are uninformed, committed, and those you mentioned. In my experience, they each make up about a third.