John Wood Electric Water Heaters - Manual (Page 9)

pressure build-up. Causes of discharge can be thermal
expansion, excess system pressure, too high a temperature
setting on the thermostat or something in the water heater
causing excess temperatures in the heater.
Thermal Expansion
When water is heated it expands. For example, in a 40-gal-
lon water heater, water being heated to its thermostat set-
ting will end up expanding by approximately 1/2 gallon. This
extra volume created by the expansion has to go some-
where or pressure will dramatically increase, such as when
water is heated in a closed system. A good indication of
thermal expansion is when the T&P valve releases about
one cup of water for every 10 gallons of heater capacity with
each heating cycle. The T&P valve is functioning properly
when it relieves pressure caused by thermal expansion, but
frequent relief can result in a build up of natural mineral
deposits on the valve seat, rendering the valve inoperative.
Should this happen, the T&P valve needs to be replaced. To
prevent this 'T&P VALVE' from discharging hot water, the
loss of energy and reduce the possible build-up of lime in
the 'T&P VALVE' there are two (2) recommendations:
OPTION 1: Install a 125 psi Pressure Relief (only) valve in
the cold water supply line. Make sure that the discharge of
this valve is directed to a drain to prevent water damage and
it is protected from freezing,
OR
OPTION 2: Install an expansion tank on the cold water sup-
ply line. For every 50 U.S. gallons of stored water, the
expansion tank must have a minimum capacity of 1.5 U.S.
gallons.
Temperature Relief
The T&P valve will discharge varying amounts of water, but
typically more than you would experience from thermal
expansion. Check the temperature in relation to the setting
on the thermostat dial. A malfunctioning thermostat could
cause the water to get too hot.
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