The water is so hot that the first time I turn on the water heater, I got scalded.
This could happen from 2 reasons:
1. Problem: There was hot water in the boiler before turning on the water.
Cause: The interval is too small.
Solution: Turn down water volume
2. Problem: Heater is always on.
Cause: Broken PCB, Broken Heater, or Ground.
Solution: contact service center