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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).
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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.
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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.
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