Supply temperature from 40 degrees to 35 degrees "pattern" or not

Because with such a high flow temperature your heating costs increase enormously.

The plumber will definitely be very sorry about that! ;-) So - I think it became clear that basically everyone here, including me, actually finds 40° flow temperature outrageous. BUT, for the sake of completeness, I wanted to mention that just last year a study was conducted. It was about the heat pump suitability of old buildings and up to which flow temperature you can reasonably use a heat pump. And to my surprise, it turned out that with modern air-to-water heat pumps the efficiency difference between 35° and 55° flow temperature is only 13%. So it is very likely that 40° flow temperature will not ruin you. I am not allowed to link, but the entire study can be found online. Maybe this helps.

Calculated at a maximum of 20° does not mean that you can't manage more later.

It's then just nicely calculated.

It’s just nicely calculated then.

Seen that way, yes, but I can still raise the supply temperature later if I want or need to.

If I design the system for 35°C and a 15/20 cm pipe spacing and later want to lower it to 30°C, it only works up to the point that the pipe spacing, etc., allows. Once the screed is in, I can’t change the pipe spacing anymore, but I can still adjust the supply temperature.

You can always raise it, but not lower it...

Especially when building with a general contractor or a prefab house manufacturer, you often meet little sympathy for such special requests. They then say we have always done it this way, it was fine for 20 years, so it will be fine for the next twenty years running at 35°C or, as in this example, 40°C supply temperature.

But as you already said, you can calculate it to fit any way you want.

It would take about 24 years for the investment to break even.

Unlike solar systems, etc., I would assume a significantly longer service life >30 years for underfloor heating; therefore, I would still consider a payback after 24 years a good investment.

Especially if you can then switch to a smaller heating system, it might become even cheaper. Try discussing the entire topic again with your general contractor.

If I design the thing for 35°C and a 15/20 installation spacing and later want to reduce it to 30°C, it only works up to the point where the installation spacing, etc., allows it.

Why would one want to reduce the target temperature (flow temperature) later? That makes no sense! The design is done now.

Maybe because one lets oneself be pressured by the [GU], doesn’t know better during the planning, or doesn’t care since one is installing a gas boiler first.