Valve in inlet pipe? MYSTERY.

[jmulla]

Can someone help explain what this is?

Small apartment gas water heater. Only 3 years old, no previous issues.

On the cold water inlet side, between the heater and the shutoff valve, there is some kind of valve (a white plastic insert with a white corss-shaped knob) in the elbow where the pipe enters the tank. See pictures.

Water started spraying out after something heavy fell on the heater and probably hit the white knob. Water spraying out was hot. After shutting off the inlet valve, the leak stopped (presumably when the pressure subsided.) Then, after turning on the inlet valve, NO MORE LEAK!!

What is this thing and what happened? Can someone explain?

 

[phishfood]

I am not 100% certain on this, but I believe that to be a vacuum relief valve. I have always seen them with a black plastic cap on the top. It has a weak spring inside of it to keep the piping sealed, but with even a very slight negative pressure the spring will allow the valve to open and let air into the system. In your case, the item dropping onto the top of the valve pushed it open and it jammed partially open. Shutting the pressure supply off allowed it to unjam and reseal.

To be safe, call a plumber to come check it out.

 

[Beni]

Right, its a vacuum relief valve, required by some codes. Have not seen it used on residential heaters before...