I woke up today to a full drip pan under the gas water heater and my utility room slowly flooding with water. Of course the drip pan was never connected to a drain and overflowing.
After shutting off the water to the water heater the flooding stopped completely. I did some investigation and noticed whenever I manually open the top mounted pressure valve I can tell that some water is discharged through the connected pipe, but I can also hear an amount of water flowing down somewhere inside the water heater. I cannot see any water or wetness when looking closely at the valve connection at the top. I am not sure where the water could be leaking inside the unit. The relief valve is connected to pipe that does not drain into the pan btw. It leaves the room through the wall behind the water heater and does not show any signs of leakage.
Could it help to replace the pressure valve? Is it likely there is a hole in the tank somewhere, but if so why doesn't it leak all the time but only when I open the pressure valve?
I am planning to buy a pressure meter today and check the house water pressure as well as test the pressure on the heater when I reopen the water supply. Will update later with readings.
*Update: I did test the pressure and the house pressure was a little high (75psi, which I now lowered to 65). I also found where the water comes into the drip pan. It is right from the burning chamber, but again, it only flows when I open the pressure relief valve on top of the unit. I can also hear air bubbles moving up while having the relief valve open, so the outflow is replaced by air. (See new photo with markings)