TPR Valve

Introduction The temperature pressure relief (TPR) valve is a safety device that is designed to: prevent the water in the tank from exceeding 212° F, and prevent the water pressure in the tank from exceeding 150 pounds/square inch (psi).

Basic Physics (Temperature and Pressure of Water)
Water boils at 212° F when at sea level or under 14.7 psi of pressure, however, the boiling point increases when under pressure, which allows the water to superheat beyond 212° F.  Typical residential water pressure ranges between 40 and 80 psi and at 50 psi, the water can reach 300° F.

Potential Problem
If the water heater's thermostat, which controls the heat source, malfunctions, the pressurized water in the tank could continue to heat and superheat (beyond 212°F).  This will cause two problems:

• First, since water expands when heated, the water pressure in the tank will increase as the water is superheated.  If the pressure exceeds the maximum pressure threshold (approximately 300 psi) the tank could rupture or even explode.
• Second, the release of superheated water (now significantly above 212°F) would immediately return to atmospheric pressure (the boiling point of water is 212°F) and flash into steam, causing a sudden increase in volume and release of energy.  The force of the flash steam would be tremendous.

How it Works If  the water reaches a temperature of 210°F (before superheating occurs) or the water pressure exceeds 150 psi, the valve will open and release the overheated water.  This will allow cold water to enter, lowering the temperature of the water in the tank and consequently the water pressure.  The valve will remain open as long as the temperature or pressure exceeds the preset limit.  The valve must be located on the tank at the top or on the side near the top, where the water is generally the hottest.  TPR valves are not included with the water heater, so be sure that the TPR valve matches the BTU (heat rating) rating of the water heater.  A Discharge Tube must be attached to all TPR valves.