Overpressure on hot water pipe

If that is the hot water line, yes. But it only makes sense if the problem occurs at all extraction points. Otherwise, it could also be regulated at the extraction point.

I assume the pressure doesn't just "increase again over time," but rather when the heating system actively produces hot water? Basically, that's somewhat normal. When water is heated, it expands. And when all the hot water taps are closed, the pressure rises. However, usually only minimally, because there is some valve on the heating system that releases the excess pressure. Maybe that valve is missing or incorrectly set – I would just have the installer check that.

It's the same with us. When the heating heats water, the pressure rises. The same happens when the solar system is working. The pressure sometimes rises with us similar to yours, but the amount is minimal. If you turn on any faucet now and let a few drops out, the pressure on the water meter goes back to normal. So there is pressure, but no volume.

How did it actually turn out here?

I have made the same observations. After heating water, the pressure is higher for a short moment when you turn on the hot water. Also, I seem to be losing pressure in the underfloor heating somehow.

I have thought of two things. It could be related to switching between hot water/underfloor heating or the safety valve. There is always moisture in the drain hose and I occasionally hear it dripping into our pump sump.

Ok, I looked at the installation and the hydraulic diagram of Vaillant again. The specified expansion vessel for hot water is not installed. Then everything runs smoothly through the valve during hot water preparation. I will take a closer look at it tomorrow and make a video of the manometer in the hydraulic station after hot water preparation and see if I notice any fluctuation when switching.

With us, it remains unchanged. I have set, due to our photovoltaic system, that the water is only heated at midday. This has already led to us having the problem at most once a day.

Today I noticed that the pressure at the safety group is very low. Isn't that actually contradictory?

Which manometer do you want to check?