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9
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 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 (see illustration below). 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.
Typical Installation
COLD
HOT
Heat trap 6” minimum
Heat trap
6” minimum
Union
To gas supply
Sediment trap
Cap
Union
Drain Pan Pipe
to suitable drain.
Drain
valve
Relief valve discharge line
to suitable open drain.
To cold water supply
6” Air gap
Blower assembly
1/4” per ft. max. slope up or
down for horizontal venting.
Auxiliary drain pan
Union
(Optional)
Anode
Vent connector
Manual gas shut-off valve
Gas Control (thermostat)
Jacket door
Temperature and
pressure relief valve
Water Supply Connections
Refer to the illustration below for suggested
typical installation. The installation of unions
or flexible copper connectors is recommended
on the hot and cold water connections so that
the water heater may be easily disconnected
for servicing if necessary. The HOT and COLD
water connections are clearly marked on all
water heaters. Install a shut-off valve in the cold
water line near the water heater.
IMPORTANT: DO
NOT apply heat to
the HOT or COLD
water connections. If
sweat connections are
used, sweat tubing to
adapter before fitting
adapter to the cold
water connections
on heater. Any heat
applied to the cold
water supply fittings
will permanently
damage the dip tube
and heat traps.
Shut-off valve
Shut-off
valve
Thermal expansion tank
(if required)
Combustion Air
Inlet Openings
Hot water outlet to
fixtures
Water Heater Jacket
NOTICE: The
National Fuel Gas
Code (NFGC)
mandates a manual
gas shut-off valve:
See (NFGC)
for complete
instructions.
Local codes or
plumbing authority
requirements
may vary from
the instructions
or diagrams
provided and take
precedent over these
instructions.
Flammable Vapor Sensor
75 Gallon Shown
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