Quote Originally Posted by ****hiller View Post
Yeah, ........but.....(there's always a "yeah,but"...right?)

In theory, an out-of-control heating element could definitely be the cause, but....is everyone in the household asleep at the wheel as regards that temp thingie?
Dang ... it was early in the morning when I came up with that .... hadn't had my coffee yet .... does that count?

I'm wondering ... if you put a pressure gauge on the cold water supply to the tank to check the water supply pressure to see if the water pressue is spiking .... in other words maybe the PRV is starting off at 40 psi and spiking at a higher pressure which might cause an imbalance to the air charge pressure to the expansion tank. So by shutting off the power and water to the tank ..... running water from a tap somewhere and see if the water pressure rises.

This would fill the expansion tank with cold water and not allowing any room for the hot water to expand to .... increasing the pressure and not necessarily the temperature is tripping the T&P valve.

Just a guess.