Meibes Prescor B Safety Relief Valve 1/2", 6 bar, Brass, Art. 27100

Its set pressure is 6 bar, at which it starts to vent. For the system temperature the maker gives a range of 0 to 95 °C, while 140 °C is a peak load rather than a continuous working temperature. Compare both values with your water heater before fitting.

For hot water, yes: Prescor B is a boiler valve that protects heated drinking water in closed systems. For gas, no, and not for central-heating circuits either: the maker states that the valve has to be used in a closed drinking-water system.

Fit it in a dry, frost-free place on the closed drinking-water system of the water heater it protects, and check that the capacity and opening pressure marked on the valve match that system. Remember that the hot water released when it opens can cause burns.

Water expands as it heats, so in a closed system the pressure rises until it reaches 6 bar, and at that point the valve releases water. According to the maker, this can be prevented with a suitably sized expansion vessel designed for drinking-water systems.