Pressure Release Valve on a Water Heater

[donradcliffe]

The common reason for lifting the relief valve on a boat water heater is the thermal expansion of the water in the tank which is contrained by a check valve. In other words the tank starts cold at the pressure of your water pump. Then you run your propulsion engine which raises the water temperature to 160-180 F (quite a bit higher than if you turn on the electric heater). Unless you crack open a hot water tap, that expanding water has no where to go, and the pressure will build until something gives and about 1/4 cup of water leaks out. Most boat piping is going to leak before 150 psi, so your relief valve should lift by 100 psi. Make sure to put a hose on the relief valve exhaust to direct that 1/4 cup to the bilge.

[a64pilot]

Since the hot and cold sides are interconnected, it raises the pressure of the entire system. On our boat the lines have enough flex so that the pressure increase is moderated well below the T&P valve opening point so ours never opens.
If your plumbed with Pex piping, then maybe the pipes don’t have enough flex to keep the pressure increase below the opening point?