I am in the process of replacing 2 sink valves in the downstairs bathroom, one cold and one hot. First I turned off the water at the main water shutoff, then I opened up some cold and hot water taps in several (not all) locations on both the first and second floors until no water was coming out. Then I went back to the bathroom sink valves (the ones I am replacing) and opened the hot water valve. I let it drain until empty, which took only a few seconds. The weird thing is that when I then opened the COLD water valve, a strong stream of water started coming out. At first it was cold, but then became very, very hot. I let it drain about 2 gallons without showing any signs of slowing down before I decided to give up. I assumed I was actually draining my hot water heater, and I have heard it can be damaged if drained while left on.

I guess my question is, what is going on and what do I do? In order to replace the valves, the water obviously needs to be shut off, but what is going on with the cold water drain? Do I need to completely (and safely) drain the hot water heater first?

 

Turn off the cold water supply at the water heater. That should keep the tank from emptying. To be on the safe side.... turn off the water heater if it is electric or put in "pilot" mode if gas.

You may have been siphoning water out of the tank if you are working at a point that is lower then it is.

 

Thank you! That was exactly the problem and the fix.

 

After opening the valve to the water heater, it's not a bad idea to purge the air from the tank (open a hot faucet) before turning it back on.

 

See water gushing from a hot faucet for a whole minute before turning the water heater heat back on.