Wsm239 wrote:
Dutch_12078 wrote:
After seeing the sample photo, as Doug said, it's the Safety Relief Valve. Below is a picture of what one typically looks like before it has tripped.

Thanks, this is very helpful. Mine does not have the red insert that is shown on yours.
It is possible the tank was overfilled. But would it empty completely if that was the case? Or would it just empty just enough to bring it to "normal"?
Doug might have the answer to that. I haven't experienced one tripping yet. It shouldn't be possible to overfill the tank if everything is working right, but I suppose it's possible the overfill prevention device valve has failed open. A properly trained refiller should be using the Fixed Liquid Level Gauge, sometimes referred as the "vent" or "spitter valve", to determine when the tank is filled to the proper 80% of the rated water capacity regardless of whether the OPD works or not.