Hey Harold please RE READ this question.
The person said the “pressure relief valve at the hot water heater.”
You Harold are thinking like a PROFESSIONAL who reads the ASME section IV regarding “Safety” & Pressure “Relief” valves.
We both know there is a big difference in not only operation BUT design and function.
This writer said “At the HOT WATER HEATER” and you kind sir took it as a boiler instead of a hot water tank.
If indeed this is a hot water tank (pressure vessel) with a proper T&P type of protection then this valve is most lightly “SET” at 150 PSI or 210 degree F
There are some valves that are rated for 125 PSI and in a closed system this kind of pressure is very possible.
Before the use of back flow preventers Or check valves hearing of a T&P operating was very rare.
What folks fail realize for ever action there is a reaction.
Installing a backflow for example without a properly designed expansion tank is one of the many dangerous conditions.
But the powers that be still allow anyone including stumble bum helpers to dabble in these installations and until a few folks get killed or maimed you will continue to read about valves seeping UNTIL metal fatigue causes the tank to rupture like we read about in engineering reports (NFPA ETC)