Heated water expands and this can result in a potentially dangerous build-up of pressure in a tank-type water heater. Hot water heater design solutions include temperature and pressure relief valves as well as expansion tanks (right).
Relief valves are used for storage water heaters that operate above atmospheric pressure. That said, they should not be used for controlling thermal expansion. Pressure relief valves are designed to automatically relieve pressure when it reaches the pressure at which the valve is set. They are held closed by a spring or something similar. Temperature relief valves are designed to automatically discharge at the temperature at which the valve is set. Combination temperature and pressure relief (T&P) valves are designed to relieve both temperature and pressure.
All relief valves are installed in the shell of the water heater tank. Temperature settings should not exceed 210 °F (99 °C). Pressure should not exceed 150 psi (1035 kPa) or the manufacturer’s rated working pressure if it is less than this. Additionally, the relieving capacity of both temperature and pressure relief valves needs to be equal to, or more than, the heat input to the storage tank or water heater.
Pressure relief, temperature relief, and T&P valves require discharge piping, which is part of the hot water heater design. This piping must not be directly connected to the drainage system and each device must have its own piping. Our MEP engineers ensure that the pipe is not smaller than the diameter of the valve’s outlet and they design the system so that it discharges through an air gap in the same room as the tank or water heater. Discharge pipes are installed so that they flow by gravity to a visible termination point. They don’t have valves, tee fittings, or threaded connections at the end of the pipe.
Antisiphon devices prevent siphoning of storage water heaters and tanks. This might be in the form of a cold water “dip” tube with a hole in the top. Alternatively, a vacuum relief valve may be installed in the cold water supply line above the top of the tank or heater.
Vacuum relief valves are used with bottom-fed tanks and water heaters.
Expansion tanks are included in hot water heater design if a plumbing system is “closed” and the back-flow that would normally run between the building and the main water supply is blocked so it can’t flow back into the city water supply. In this situation, when the water pressure rises, there is nowhere for the back-flow to go. Expansion tanks provide relief because the excess volume of water flows into the expansion tank and lowers the pressure.
Valves must also be incorporated in the hot water heater design to allow for shutdown either by disconnecting an electric hot water supply system from its energy supply or, in the case of other systems, energy fuel (gas or oil) supply.