My water heater has a shutoff valve at the cold inlet. The shutoff valve was closed and cold water line was disconnected from the water heater. I wanted to restore water to the house while I work on the water heater. When I opened the main water shutoff valve, cold water entered the water heater through the hot outlet and overflowed out of the cold inlet.
Is this normal? How did cold water enter the hot line? Also what is the device with the knob at the top in the pic below?
Thanks for help.