Gary Swart
In the Trades
This is a frequent topic of discussion here. When a pressure regulator valve is installed, it creates a "closed system". When water is heated, it expands. In an "open system", one without a PRV, the expansion is absorbed by the city main. In a closed system, it has no place to go, so when the pressure builds too high, the T/P opens to prevent the tank from exploding. An expansion tank is placed in the system after it the PRV to hold the expanded water until the heating cycles is completed and the pressure drops to normal. Why your T/P is not tripping, I can not say. It may be that the T/P is not functioning as it should, but that is just guess on my part. Here's what happened in my own home. I installed a PRV and the next thing I knew, the T/P was opening. I replaced the T/P, same results. I replaced the replacement T/P, still the same results. I brought the problem here and was told about expansion tanks. Before I installed it, I did an experiment. I put a pressure gauge on the hot water faucet of my utility sink. When the water heater was not heating, the pressure was a normal 60 psi just where I had the PRV set. Then I opened another hot water faucet and watched the gauge. When the water heater kicked on, the temperature immediately began to rise. It quickly rose to 150 psi and the T/P valve opened. The temperature actually continued to rise several more degrees. I immediately installed the expansion tank and that ended the problem. So, why isn't your T/P opening? I can't say, but if I was there, I'd do the experiment as I described above.