Pressure increase in cold water pipe during hot water heating

In theory simple, in practice a bit complicated to implement. I then have to unscrew / seal the bathroom faucet. And even more complicated: the bathtub rim faucet. I can't reach the angle valves there... In the end, if it wasn't any of the other faucets, the only option left is to replace the cartridge... Thanks so far for the support!

Yes exactly, you can at least rule out the 08/15 faucets. Please keep us updated.

Now lick me fat. Although I have felt like I have tried 100 times to shut off the cold water supply to the boiler, I just tried it again today, but with a difference:

AFTER shutting it off, I checked whether the KFR valve was closing properly by running warm water somewhere until no more warm water came out (2-3 seconds).

During the heating process, NOW NO pressure drop at the pressure reducer!

Can it be automatically concluded from this that the check valve in the supply line is defective? I would be very surprised. The valve insert of the KFR was just replaced...

Or is it possible that the warm water can no longer push through to the fitting because there is effectively no pressure left in the warm water line?

Or can the hot water no longer reach the fixture at all because there is effectively no pressure left in the hot water pipe?

I would guess, yes. Three more words.

Thank you for the assessment! But still three words?

So that I could submit the post :D