Over time hard water sediment and mineral deposits can rust and cause your water heater pressure relief valve to freeze up. This is one reason to maintain your water heater by flushing and testing operation once a year. Opening the relief valve will help flush away any debris in the valve.
Thermal Expansion: T&P relief valves are typically designed and manufactured to relieve on pressure at 150 psig and on temperature at 210 degrees F.
DO NOT open the valve if it has not been plumbed outside or into a sealed drain. It will release lots of HOT water fast and could burn you. If you need help with water heater maintenance, call an expert at PDM Plumbing, Heating, Cooling.