LW Heat Pump Alpha Innotec LWDV with 12KW

Daniel-Sp, I can only tell you what is stated in the offer. It shows two storage tanks (300l domestic hot water and 200l buffer tank for heating). The calculation was done by an external engineering office, according to their statement. With the 0.16, about 20W/m2 are supposed to be necessary, and in my opinion, that is not realistic. Below 30W/m2, it will not be warm enough to feel comfortable.

With 0.16, about 20W/m2 are supposed to be necessary, and in my opinion that is not realistic. Below 30W/m2 it does not get warm enough for one to feel comfortable.

How do you come to this statement? Have you calculated it? If calculated, based on what? Internal gains considered? Calculation according to PHPP? Or gut feeling of the heating installer?
As an example. We have an HT of 0.201, no! controlled residential ventilation, NAT -12. Outside temperature this year so far not below -4°C.
Calculated heating load (not according to PHPP!) at 4400W at 22°C room temperature. That is 22°C room temperature. Heat pump idles 4-8 hours/day with minimal load of 2 kW and supply temperature around 24-26°C. The calculations include a lot of reserves.
If the calculation of HT 0.16 is correct, then the heating load is just that low, be happy. But if you buy the heat pump too big because you are afraid of freezing, you will only get problems during operation.
Critically check the heating load calculations and buy a heat pump of appropriate size.
The same applies to the design of the underfloor heating, do not calculate uniformly with watt/m2, but consider the heating load of the room. An interior hallway has a significantly lower heating load per square meter than a bathroom with a window and exterior walls and a higher desired temperature! When designing with xW/m2 over the entire house, it will get too cold in the bathroom and too warm in the hallway. Unfortunately, this type of design is the norm.
Regarding the buffer tank, if the heating installer really wants to sell a buffer tank, then take the smallest one available and connect it in the return line, not a separation buffer!
I believe you need to study the topic even more deeply. You will not be able to tear out the underfloor heating anymore. Tiles and washbasin can be replaced without problems. Most builders spend much more time and brainpower on the design of bathrooms and kitchen and do not properly take care of the heating. Do not make the same mistake. Unfortunately, you will have to check the calculation yourself; do not rely on someone who does not live in the house later and has to bear the energy costs themselves.

Daniel-Sp: Don't get me wrong. This is not based on my claims, but I can only relay what the heating technician says. I am a layperson, read up and look for ways to make the right decision. It is really difficult right now to find a competent heating engineer who does not just stick to their "old" way. Then I will delve deeper into the [floor heating layout]. But it will only work if I find a heating technician who has the same understanding.