"Thermal expansion tanks do not have a great history of longevity either, and create a 'dead end' in the potable water system". There are far more posts on this forum regarding failed PRVs than failed expansion tanks. Expansion tanks are easily checked for correct operation and easily replaced if necessary and without shutting off the water to the house IF they were installed properly with a shut-off valve. Expansion tanks are no more of a dead end in the potable water system than an unused hose bib. The expansion tank in my home was installed in 1995 and is still operating while in that same time period my PRV has been replaced three times. With respect, I agree to disagree and as I said... "you use the PRV with bypass in your house and I'll stick with the tried and true and simpler solution".

Isn't the PRV spring loaded? Once the pressure is released, it should close and seal. In this case, when I flipped the test valve, the "plunger" pulled out but was not pulled back in. I think the spring is shot. I doubt its actualy due to high pressure, more likley the PRV has failed on its own. The only other thing could be the supply side pressure reducer failed and is allowing to much pressure into the system. Unfortunatley, there is no pressure gauge on the system to check. I may place a pressure gauge over a boiler drain and open the drain just to check the pressure is OK. I can isolate the boiler from the zones so repair is not an issue for me. Just a threaded 3/4 connection, so thats easy.

22 pounds is to high. 12 is more like it. They probably increased the system pressure to purge the system and forgot to reset the pressure reducing valve. The relief valve as it is called probably lifted off its seat because you were close to or at its release setting which is usually 30 pounds. Use the drain on the boiler to reduce the pressure to 12 pounds and make sure the reducing valve isn't feeding anymore water into the system after that. If it is the valve has to be adjusted. And no you cannot put anything at all, including another relief valve, anywhere on the first valve, that could be very dangerous. To change the relief valve all you have to do is isolate the boiler from the piping on the supply and return, then drain the boiler down to below the point where the valve is installed and screw in a new one. Hopefully whoever installed the system originally was nice and put valves in for you.

@ Vith... "If the pressure of the water in the water heater (and the water heater cold inlet and expansion tank) is higher than that of the city then you will get flow from the water heater into the cold distribution till the pressure equals out". Not really... in a closed plumbing system with a thermal expansion tank the tank will accommodate the water volume increase when the pressure rises just a couple PSI over the static pressure. In a closed plumbing system the increased volume of heated water has only two places to go as long as no one opens a faucet. One place is the T&P valve and the other is to force open a toilet ballcock valve. Every other valve in the plumbing system is a positive off valve. "I would rather ensure no thermally expanded heated water can ever be capable of backing up into the cold line even though I have an expansion tank. That is just my opinion". You're welcome to your opinion but you're worrying about something that doesn't occur. We're not looking at gallons of water moving in the system when the water is heated. When thermal expansion burps a T&P valve you usually get an ounce of water while the pressure is relieved. Much more than an ounce moves into the thermal expansion tank as the water volume rises when the water is heated but if the tank is correctly sized and the air charge correctly set you will never get anything but cold water in the expansion tank. If it makes you feel better you can install a dozen check valves and all that accomplishes are more connections for a leak to occur and more moving parts to fail.