TPR valve leaking - closed system? Hidden check valve? Pressure mystery
Hello all! I've been a long time lurker but have encountered a problem that's puzzling me and would love some input...
I recently installed a point of use hot water heater at my kitchen sink to provide faster hot water as mine can take a bit to reach my kitchen sink. The TPR valve periodically leaks (out of the actual discharge area as if it is actually working at intended; not around the threads or anything), I am assuming due to excess pressure in the tank. It's non-violent, just opens up enough to let some water out during heating. It dispenses hot water around ~120 degrees at the current setting which I like at that sink.
During diagnosing I discovered my street pressure would range from 78-90 psi so I installed a pressure regulator, dropping the pressure to 55-60psi to help protect my plumbing. I got a Wilkins with a built in backflow valve to allow any increases in pressure above street level to flow back (assuming nothing else stopped it; I specifically got this regulator as it has a proper backflow and not the cheaper bypass that only allows backflow at ~120+psi like some of the cheaper pressure regs). There was no change in the leaking from the point of use kitchen hot water heater TPR valve from adding this pressure regulator. It leaked both before and after its installation. I am waiting to see if the main house water heater still leaks as it also did before the installation of the regulator.
Here's an mspaint drawing of my setup:
I do not have a check valve in any visible locations. If I have one it is either built into the water meter near the curb or between the main hot water tank and kitchen in the buried line. They re-ran the line from the meter to my house last year in one continuous length of pex so I know there's nothing there.
Now my symptoms lead me to believe I have a closed system for whatever reason so adding an expansion tank before the cold inlet of my main hot water heater should allow pressure relief for the entire system.
BUT, I've been watching the pressure gauge located after the pressure regulator and it never showed an increase of pressure while the kitchen water heater was heating up and subsequently discharging from the TPR. If the pressure in the kitchen water heater was building, wouldn't it show up in this guage unless there's a check valve blocking it from reaching the guage?
I'm a little puzzled and don't want to go to the trouble of adding an expansion tank until I know that there isn't something else that is going to prevent it from solving the issue properly.
I've also replaced the TPR valve with another one to verify that it was not just a bad TPR valve. Same symptoms.
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