TPR valves are safety devices installed on water heaters. A TPR valve is designed to automatically release water from the tank if the pressure or temperature in the tank reaches unsafe levels. If the water temperature exceeds 210 ̊F (which is just below boiling point), or if the tank's pressure exceeds 150 PSI (which is the maximum normal operating pressure for a water heater), then the relief valve will activate. Once activated, water discharges from the pipe to the termination point.
WHAT IS A TPR DISCHARGE PIPE?
The TPR valve should be connected to a discharge pipe (also called a drain line) that runs down the length of the water heater tank to an acceptable discharge location, such as a drain pan. The drain line is a safety feature, attached to the TPR valve, to prevent burns or injuries in the event that the valve is activated and extremely hot water is released from the line. To prevent injury or damage to property, the drain line should not terminate more than six inches above the floor or waste receptor.
While inspecting homes, CNHInspections has found missing discharge pipes, and pipes that terminate higher than six inches above the floor or waste receptor.
WHAT CAUSES THE WATER TO OVERHEAT?
If the limit switch fails, and the burner does not shut off when it is supposed to, it keeps heating water in the tank. When the water in the tank approaches boiling point it wants to turn to steam. Without being exposed to the atmosphere the water cannot turn into steam. This leads to the water expanding which creates outer pressure. The increased pressure and water temperature results in the TPR valve being activated, thus releasing hot water from the tank.
HOW DOES THE VALVE SENSE TEMPERATURE AND PRESSURE?
The TPR valve is purposely located in the upper six inches of the tank where the hottest water is. The valve location is more important for temperature than for pressure, as pressure is consistent throughout the tank. The valve extends three inches into the tank. The valve is filled with wax, and is connected with the temperature release. As the temperature increases, wax inside the tube expands and pushes on a spring which opens the valve. The pressure part of the TPR valve works similarly, except water pushes directly on a spring and operates the valve if the pressure exceeds 150 PSI.
During our home inspections, CNHInspections often observes various defects with TPR valves. Though these reported defects may seem minor to the untrained eye, they are an important safety feature which can prevent harm and bodily injury! Improperly installed TPR valves or discharge pipes should be evaluated and repaired or replaced by a qualified plumbing contractor.
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