water heater venting water outside from what I think is mixer valve
I have a gas powered water heater tank that is continually leaking hot water outside via what I think is an overfill pipe. Having looked at diagrams online, I think it is coming from what is called the mixer valve (please see photos below). I originally thought it was coming from the pressure relief valve (PRV), but that is a separate plastic pipe and is not leaking. Note that I have two vent pipes to outside, one from the PRV and a second from this "mixer valve" thing.
I did test the pressure relief valve and it seems to be working fine. Flows when I open it and stops when closed. When I do operate the PRV, the venting of water from the "mixer valve" stops for a minute then starts dripping again. It is a vigorous drip, maybe a few gallons per day. We have no complaints about the volume or temperature of our hot water.
The water heater is 13 years old and despite having the manual, I do not know what manufacturer it is. There is no ID on the manual, not even a name. We live in a soft water area and this heater is the original install on 1999 house. Note that there is a continual ssshhhh sound like a faucet is running and it is coming from this valve.
I was hoping it was an issue with the PRV and maybe it is - I know little about water heaters.
Thanks for any help.
Valve in question: