I disagree with you here JK. PRV for a water heater is set to 210 degrees F and a pressure of 150 PSI. The valve will blow if either condition is met. Obviously, 210 degrees is below the boiling point of water at sea level and 150 PSI can be met with a water hammer effect on a water system. So it entirely possible for a PRV to operate periodically with nothing wrong with the system. And let's not forget thermal expansion of the water in the heater form cold to hot. A 40 gallon heater will expand the volume by at least 1/2 gallon. It has to go somewhere.
These periodic releases of pressure do corrode the spring over time and they do break allowing the 'gush' of water out of the system.
Did you ever hear of a competent plumber installing a boiler PRV (30 PSI) on a water heater?It does happen.

