Design of underfloor heating for KFW 55 house with ventilation system

First of all, nice that it was a good conversation and that you have a good basis for discussion. That the planner uses studded mats and the heating engineer staples reinforces my belief that in this area of planning (almost) only idiots are at work. However, my sample size is still too small to put it on a scientifically sound footing. ;)

However, I am of the opinion that compromises won’t really get you anywhere in this area. It is a house that you will probably want to live in for at least the next 20-30 years and hopefully still use for a long time afterward. Just as a comparison: We even have 15m² less on the upper floor than you and 8 heating circuits. Unfortunately, for us it’s not according to KfW55 but rather the Building Energy Act, so somewhat worse, but the, at least calculatively, better construction method you have should not lead to a reduction in heating circuits but rather to a reduction of the supply temperature, meaning more heating circuits, smaller distances, and shorter heating loops. I would ask him to use a larger HKV and to keep the lengths preferably under 100, rather around 80m. You also still have the storage room on the upper floor. Does it need to be heated? Isn’t it enough if it is just a bit warmed on the way to a room? This way you save a connection again and it is also possible to align the lengths better. Of course, you have to pay extra, but as long as the heating engineer benefits from it, that’s well-invested money.

As far as KfW is concerned, in my opinion you don’t have to worry. They receive the calculation and everything you plan with the heating engineer is an improvement anyway. They certainly won’t notice anything about it.

For heat pumps, I would not accept a supply temperature of 35°C in 2023. That is why I would always plan a wall heating system within the ranges. The 15°C and 24°C have already been mentioned. More than a few degrees difference between the rooms is not acceptable. Large differences in heating circuit lengths are also hydraulically problematic. Deviations of more than 1/4 to 1/3 in length between the shortest and longest circuit must be strictly avoided.

If the heating specialist is "only" the executing SUB of your contracting partner, the GU, then the GU is your negotiating partner. And I would not accept the compromise either. Quickly increased the recommended maximum of 100 m per heating circuit by 20% to 120 m. And as always, a low flow temperature should be your goal.

What I mean is, even if you calculate with 35, you can definitely go lower and it will still stay warm

For me, the calculation is done with 40 degrees, the basement also has underfloor heating. Do you think it would be possible for me to also "definitely go lower"? It is a KfW55 house.

There are only 2 reasons to calculate with 40 degrees: incompetence or cost savings. It definitely has to be less.

My calculation is based on 40 degrees,

I seriously wonder how a heating system in a new building can even be designed for 40 degrees. That only works if a) it was designed for -60 degrees outside temperature or b) an indoor temperature of 28-30 degrees is desired or c) a geriatric nurse or butcher planned it or c) pipe spacing of 30 cm was assumed to offer the cheapest price possible.
In summary: This is the biggest nonsense we have read here for years regarding heating design in new buildings.