Rheem 50, 85, 105 gallon Use & Care Manual page 5

Thermal Expansion
Determine if a check valve exists in the inlet
water line. Check with your local water utility.
It may have been installed in the cold water
line as a separate back flow preventer, or it
may be part of a pressure reducing valve, water
meter or water softener. A check valve located
in the cold water inlet line can cause what is
referred to as a "closed water system". A cold
water inlet line with no check valve or back
flow prevention device is referred to as an
"open" water system.
As water is heated, it expands in volume and
creates an increase in the pressure within
the water system. This action is referred to
as "thermal expansion". In an "open" water
system, expanding water which exceeds the
capacity of the storage tank flows back into
the city main where the pressure is easily
dissipated.
A "closed water system", however, prevents
the expanding water from flowing back into
the main supply line, and the result of "thermal
expansion" can create a rapid and dangerous
Inspect Storage tank
Inspect the storage tank for possible damage.
pressure increase in the storage tank and
system piping. This rapid pressure increase can
quickly reach the safety setting of the relief
valve, causing it to operate during each heating
cycle. Thermal expansion, and the resulting
rapid and repeated expansion and contraction
of components in the storage tank and piping
system can cause premature failure of the relief
valve, and possibly the heater itself. Replacing
the relief valve will not correct the problem!
The suggested method of controlling thermal
expansion is to install an expansion tank in the
cold water line between the storage tank and
the check valve (refer to the illustration on the
page 8). The expansion tank is designed with
an air cushion built in that compresses as the
system pressure increases, thereby relieving
the over pressure condition and eliminating the
repeated operation of the relief valve. Other
methods of controlling thermal expansion
are also available. Contact your installing
contractor, water supplier or plumbing
inspector for additional information regarding
this subject.
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