Got a phone call in the morning, from my father, his central heating system not working. Never mind in summer, but no hot water too. In Germany the usual way is a burner heating both the central heating and the hot water tank through a shared water circulation system.
Anyway, when I arrived, I found the controller dead, no display, no power on LED. It's a quite modern one, so no way to run it in emergency mode (the older ones can work on an electromechanical thermostat and near to none electronics involved).
So we call the service, because my father would like to have hot water asap again. The guy arrived later, checked the system, phoned the manufacturers hotline (they're quite good at that, one can reach them 24/7 with the correct SLA). Diagnose: Controller defective, have to order a new one. Will arrive on monday (for more money involved, one can get spares within hours). Agreed on that, but I mentioned, I might be able to repair on component level, it's just electronics. The repair guy agreed, if I can't fix it, we should call him on monday, and he'll have the spare part on tuesday.
So I took the controller home, and found this cause for the malfunction:
and this poor resistor.
Lucky enough, nothing else was damaged, the thing works again and my father has hot water again since an hour ago.
It was an easy fix, but I'm a bit concerned about the quality of stuff thats supposed to last 10 to 20 years (this unit is around 4 years old). Carbonization shouldn't happen here, since creeping clearance was fine for the applied voltage.