Relief valves are used to protect against the build up of excessive pressure in a closed system. The liquid in a closed system changes volume when the temperature of the liquid changes.
In heating and cooling systems, this additional volume is stored in an expansion tank. A weak link, in the form of a relief valve, is built into the system to protect against failure of this function.
The relief valve is fitted with a pre-set release pressure, equivalent to the highest operating pressure within a system plus an additional amount for safety purposes. In heating and cooling systems, blowdown from the relief valve will therefore only occur when a failure in the expansion tank system occurs.
A relief valve is also used in the production of domestic hot water. When water is drained from a hot-water tank, the water temperature will fall. When water is no longer being drained, the temperature in the tank will rise again as it is heated. It is not possible to have expansion in this system, as the hot-water tank intake is protected against backflow by a non-return valve. The rising pressure will therefore result in blowdown from the relief valve. Thus, relief valve blowdown will occur regularly in hot-water production.
Isolating valves must not be located between the heat source in the system and the relief valve.