Underfloor heating: Are the pipes tight enough?

Feelingly too far apart to be able to heat the room with 28 degrees flow temperature (compared to our new building, where it is laid much closer together).

Thank you very much for your answers. Tomorrow morning the heating technician is coming again. I will ask him if anything has been calculated. But I don’t really believe it either.

: The two smaller rooms have been made narrower.. :-)

: The maximum 28° flow temperature comes from the specifications of the cement tile manufacturer. Otherwise, it could have been significantly warmer. What do you mean by: "It is the flow temperature at laying temperature (so it only becomes relevant every 10 years)"? A ground probe heat pump, whose performance was calculated, is installed. Of course, I expect some savings compared to before due to the additional insulation. I would be happy about a calculation: Floor: 9.35 x 6.85m = 64m2, walls: 54cm rubble stone + 4cm Haga insulating plaster (not yet determined, U-value unknown), height 2.85m, of which towards the outside (minus windows): 48.3m2... as well as towards the adjacent room (10°C): 19.5m2. Windows and doors single-glazed about 3mm with storm windows (performed surprisingly well in thermal imaging): 14.5m2 Ceiling "reed gypsum" 64m2. Installation spacing is 15-25cm, on average probably 20cm. I’m curious and thankful for something "calculated" :-) Are more data needed?

: No, but in this building it’s simply not possible..

: Quite possible that the windows are from 1944.

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In advance: I am an amateur, not an expert.
You carry out the heating calculation based on the design temperature - that is, which heat loss must be compensated per unit of time when it is really cold outside. Your exterior wall has - assuming it is broken stone granite - a U-value of 0.93 W/m²K.
Your floor loses 0.22 W/m²K (assuming that the installed EPS has a conductivity of 0.035 W/mK).
For the windows, I am simply optimistically assuming 1.3 W/mK.
So you lose in total - assuming your design temperature is -14° and the room temperature is 20°:
0.93 W/m²K * 34 K * 48.3 m² = 1527 W (towards outside air)
0.93 W/m²K * 10 K * 19.5 m² = 181 W (towards adjacent room)
1.3 W/m²K * 34 K * 14.5 m² = 641 W (windows)
0.22 W/m²K * 20 K * 64 m² = 282 W (floor)
In total: 2631 W... now I don’t know what is above the room... you write it is a reeds gypsum ceiling? Is it insulated, or are there normal rooms above?

Excluding the ceiling, you need 2631 W heating output, which you have to compensate through the underfloor heating. Are you laying parquet? Then you would have 19 W/m² at 28°... so a maximum of 1235 W. If you lay tiles, it is 23 W/m², i.e. 1472 W.
If you increase the flow temperature to 35°, it is 37 W or 44 W, that is 2368 W or 2816 W.
Smaller installation distances could possibly provide another 10% more power per m² (at 35° 40 W or 50 W - so not a huge difference).
However, this is all without ventilation losses and without thermal bridges... here you can add another 30% heat loss power (if not more).

Conclusion? If it really gets cold, you will have big problems if you don’t heat additionally or insulate your walls - I would possibly consider 10 cm ETICS instead of 4 cm thermal plaster here. That way you could save about 1000 W of heat loss.