This is a new rental, and there was no hot water for our first evening in the flat. The host's first response when we said we had no hot water was to suggest that we "let it run for a while." Of course, we had already tried that with each tap in the flat.
So the next morning we had to start trying to actually solve the problem. We guests, rather than the host, figured out how to temporarily restart the water heater and get some hot water the next morning.
Host was then very responsive and booked a plumber to come over. We were out by the time the plumber came, so he phoned us (the guests) to get into the flat. Then the hot water issue was solved, and no other problems with our stay. Very nice flat and location for us.