The discharge piping serving a TPR valve should:
- Be constructed of an approved material such as CPVC, copper, polyethylene,
galvanized steel, polybutylene, polypropylene, or stainless steel.
- Not be reduced in size from the outlet of the valve it serves (usually no smaller
- Be as short and as straight as possible so as to avoid undue stress on the
- Be installed so as to drain by flow of gravity.
- Not be trapped.
- Discharge to the floor, to an indirect waste receptor, or to the outdoors.
- Terminate within 6 inches (152 mm) of the floor or waste receptor.
- Not discharge in a manner that could cause scalding.
- Not discharge in a manner that could cause structural or property damage.
- Discharge to a termination point that is readily observable by occupants
because discharge indicates that something is wrong.
- Be piped independent of other equipment drains, water heater pans, or relief
valve discharge piping to the point of discharge.
- Not have valves anywhere.
- Not have tee fittings.
- Not have a threaded connection at the end of the pipe so as to avoid capping.
FOR DIFFICULT TO DRAIN LOCATIONS
If the location of a gas fired water heater does not allow for a convenient gravity drain
(e.g. in a below grade installation, center of a building with a slab foundation) a
WATTS 210 automatic gas shut-off valve can be used to reduce the risk of damage.
The Watts valve senses an over-temperature condition and turns the gas supply to
the unit off. A pressure relief valve is still required, but can be installed at a different
Improperly installed TPR valves are one of the most common defects noted on inspections.
Domestic water heaters are required to have a temperature and pressure relief valve. This relief valve
provides protection in the event that the water heater thermostat or gas valve malfunctions, causing excess
pressure/temperature. Modern water heaters are manufactured with a valve fitted for the specific unit. To
minimize the risk of personal injury and/or property damage, the valve drain should be connected to a
properly sized discharge tube that is typically terminated at the building exterior. The termination point should
be in an area visible to the occupants to alert them of any problems. If not allowed to properly discharge, this
excess temperature and pressure will eventually cause the tank to explode. One such tragedy occurred in
Spencer, Oklahoma at the Starr Elementary School on January 9, 1982 where six children and a teacher were
killed when the school’s water heater exploded with the force and effect of two pounds of dynamite. This
occurred due to a malfunctioning TPR valve.
Click here to see a water heater explode!
Many water systems do not have back-flow prevention, which will allow the excess pressure to feed back into
the water supply. This may keep the water heater from exploding, however boiling water or steam could be
discharged through any fixtures that might be opened while the water heater is in an over-pressure, over-
temperature failure mode.
BRADY HOME INSPECTION