Installing the water heater.
6
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 water heater 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 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 (refer to the illustration on page 9). 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.
Inspect Water Heater
Inspect the water heater for possible damage. Check the markings on the rating plate of the water heater to
be certain the power supply corresponds to the water heater requirements.
