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Page 8 of 55
Install the T/P valve into the threaded opening at the top of the water heater and
orient the discharge tubing so that any discharge from the valve will exit within 6
inches above, or at any distance below the structural floor, and cannot contact
any live electrical part.
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 buildup 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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