A properly installed relief valve on a hot water heater will allow excessively pressurized and heated water to be released from the tank. The safety relief valve can prevent excessive temperature, pressure or both. Under certain conditions, the tank in a hot water heater can explode.
Water heater tank explosions can be violent enough to cause severe property damage, injury and even loss of life. Such explosions are usually caused by either mechanical defects in the tank such as corrosion or age-related deterioration, or water in the tank being too high of a temperature.
Pressure of the water in a hot water tank rises as it is heated due to thermal expansion. Water under pressures greater than atmospheric pressure can be heated above its usual boiling point (212 degrees F) and still not boil. In a multiple failure situation, for example if the heating thermostat malfunctions and heats water above 212 degrees at the same time that a check valve in the cold water supply main gets blocked shut, the superheated water will eventually rupture the tank, instantaneously converting each cubic inch of water into a cubic foot of steam with an explosively expansive force.
Pressure relief valves are designed to circumvent excess pressure in a tank. They are not intended to act as control devices for regulating water flow pressure; they are set to open at a predetermined overpressure. Temperature relief valves, designed to prevent excessively high water temperatures in the hot water tank, can be a separate valve or incorporated into the pressure relief valve unit, called a temperature and pressure relief valve (TPR).
Safety Valve Standards
Water heater safety relief valve installation is governed in the U.S by American Gas Association (AGA), Underwriter’s Laboratories (UL) or Federal Housing Authority (FHA) safety requirements. Valves themselves must comply with the requirements set forth in ANSI Z21.22.
The relief valve’s rated capacity in BTU/hour is to equal or exceed the hot water heater’s input capacity in BTU/hour.
Always follow the valve manufacturer’s recommendations when installing the valve on top of a water heater. You should never re-use a relief valve when replacing a water heater, as it will almost certainly have different pressure and temperature requirements. Not only that, but the old valve may be worn, in need of maintenance, outdated, or all three.
TPR valves have a temperature sensing valve incorporated into the pressure relief valve. The entire unit is located on the top of the hot water heater, and the temperature sensing element extends down into the top 6 inches of the water tank, since the top area of the tank will contain the hottest water. As the water in that area becomes heated past the 210 degrees F danger point, the thermostat within the valve expands, forcing a valve disc out of it’s seat, opening the valve to allow hot water to flow out of the tank.
The hot water is replaced with incoming cold water from the supply line, thus cooling the water in the tank. When the water cools below a certain point, usually 10 degrees or so, the thermostat will contract, forcing a loading spring to push the valve disc back down into its seat, shutting the valve. Essentially, a relief valve functions as the path of least resistance in the system.
The pressure relief portion of a TPR valve works in much the same way, albeit there is no separate pressure sensing element. Rather, the valve is designed to be forced open when it’s internal spring loading pressure is exceeded by the pressure of the water in the tank.