 An Inspection Question:
What is a TPR Valve?
Q: What is the brass valve on the
side of my water heater for?
A: It’s called a TPR valve. All water
heaters have a relief valve that is sensitive to both temperature and pressure. They are
designed so that if the pressure inside the tank exceeds 150 lbs./sq. in., or the
temperature exceeds 210 degrees, the valve will open and let some of the overheated water
out. This lets cold water into the tank and decreases temperature and pressure. The valve
stays open as long as the temperature or pressure exceeds those limits. While a tank
failure from overpressure alone will cause water damage, a rupture due to the combination
of temperature and pressure can be very dangerous.
Q: What is the danger?
A: It’s physics. Water in the heater is
always pressurized by the force in the supply line, so it can be heated to well above the
212 degrees that it takes to boil water at sea level and still be liquid. At 50 psi,
the temperature of the water could reach 300 degrees or more. If a faulty thermostat fails
to turn off the power when the tank is hot, the resulting overpressure could rupture the
tank. The sudden decrease in air pressure to atmospheric conditions would cause the
superheated water in the tank to explosively flash into a huge volume of steam. In
fact, 50 gallons of water could create 80,000 gallons of steam, or about five times the
total volume of a typical 2000 sq. ft home.
Q: How should the valve be
mounted?
A: To work properly, the element of the valve
should be immersed in the top 6" of water in the tank, (from the side or top) and not
in the hot water outlet pipe. The relief valve should have an extension or discharge tube
that directs the overheated water down to the floor and away from anyone who might be in
the area to prevent scalding or burning. The size of the pipe must match the opening size
of the TPR valve discharge (usually ¾ inch), and should not be capped, threaded, or
obstructed. The tube should terminate between 6 and 12 inches from the floor or be
directed to the exterior of the home.
Q: Are they all the same?
A: No, they are available in different heat
ratings. Be sure that the heat rating of the TPR valve (expressed in BTU/Hr.) exceeds the
BTU rating of the water heater. If the capacity is less, the relief valve will not be able
to discharge the overheated water at the same rate that it is produced, creating an unsafe
condition. Look for BTU ratings on the nameplate mounted on the valve (the AGA number),
and on the data plate mounted on the side or top of the water heater casing.
Q: Will my Home Inspector test the TPR valve?
 A: Your inspector should
examine the TPR to determine that it is correctly installed and has a drain tube that is
unobstructed and set up to discharge appropriately. He will also note any visible defect
or corrosion. He will not actuate the valve unless a technically exhaustive inspection is
called for. The problem is that TPRs cannot be tested under high pressure or temperature
in a home setting. They can be tested to see if the valve will release water by lifting
the lever on top of the valve, but frequently the valve will not reseat and turn off when
the lever is released. Therefore it is not typically done.
Q: If excess pressure can create a hazard and
the valve can’t be tested, what do you recommend?
A: Don’t panic! A water heater that is
equipped with a properly installed TPR valve and discharge tube is quite safe. One
reason is that the TPR has a built-in failsafe: it contains two independent mechanisms to
limit tank pressure to safe levels.
Q: What is involved in replacing the TPR Valve?
A: They are inexpensive (about $15) and can be
replaced by a plumbing contractor in less than an hour.
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