Turn on the underfloor heating at the controller

Oh dear, I also said stick with it.. but wipe away the water and that's fine.

Which of the two levers was it?

If the heating engineer refills it, then please with the correct water.

Yes, afterwards you are always wiser. It was the cold water lever. The water was pumped through the pipe and thus through the heater. Only that there was no heater there and everything just ran from the pipe in the wall onto the floor instead of through the heater. Well, I made a mistake there.

The plumber was here (from the company that installed everything). He refilled water (was very easy, everything is already connected to a crane and just needs to be turned on), showed me how to control the underfloor heating, and reset everything. It only took about 10 minutes in total and hopefully everything is fine again now. Just to be safe, I hadn’t showered in the past two days because I wasn’t sure if using hot water was a good idea when the heat pump wasn’t working properly. Now I can shower again. :D

No one has ever faked being sick, but quite a few have frozen to death.