Eternal GU125T, Installation And Operation Manual

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Typical Installation
10
Thermal Expansion
Determine if a check valve exists in the inlet water line. Check with your local water utility company. It may have
been installed in the cold water line as a separate back
 ow 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  ow 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 water heater  ows 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 pressure increase in the water heater
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 water heater 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 water heater and the check valve (see illustration above). 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.