T/P Valve Discharge pipe:
1. Must NOT be smaller in diameter than the outlet diameter of the
valve, or have any reducing couplings.
2. Must NOT be plugged or blocked.
3. Must be made of suitable material for hot water.
4. Must not be over 15’ in length.
5. Must not have more than two elbows.
6. Must terminate at an adequate drain.
7. Must not have a shut off valve between relief valve and tank or
relief valve and termination of discharge.
CLOSED SYSTEM THERMAL EXPANSION: Periodic discharge of the T/P
relief valve or failure of the element gasket may be due to thermal
expansion in a closed water supply system. The water utility supply
meter may contain a check valve, backflow preventer or water pressure
reducing valve which will create a closed water system. During the
heating cycle of the water heater, the heated water expands causing
pressure inside the water heater to increase. The T/P relief valve may
discharge hot water under these conditions which results in a loss of
energy and a build up of lime on the relief valve seat.
To prevent this from happening, there are two recommendations:
1. Install a diaphragm-type domestic hot water expansion tank
(suitable for potable water) on the cold water supply line. The
expansion tank must have a minimum capacity of 1.5 U.S. gallons
for every 50 gallons of stored water.
2. Install a 125 PSI pressure relief valve in the cold water supply line.
Make sure the discharge of this valve is directed to an open drain
and protected from freezing. Contact your local water utility or
plumbing inspector for information on how to control this situation.
Never plug the outlet of the relief valve.
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