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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
Water Supply Connections
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.
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.
Heat trap
6” minimum
Heat trap
6” minimum
Union
To gas supply
Sediment trap
Cap
Ground joint union
Drain Pan Pipe
to suitable drain.
Drain
valve
Relief valve discharge
line to suitable open
drain.
To cold water
supply
6” Air gap
Draft hood
Pitch up 1/4”
per foot
Auxiliary drain pan
Union
Anode
Manual gas shut-off
Thermostatic gas valve
Jacket door
Temperature and
pressure relief valve
Shut-off valve
Shut-off
valve
Thermal expansion
tank (if required)
Combustion Air
Inlet Openings
Hot water outlet to
fixtures
Water Heater Jacket
Vent connector to
chimney
Refer to the following illustration for suggested
typical installation. The installation of unions
or flexible connectors is recommended on the
hot and cold water connections so that the water
heater may be easily disconnected for servicing
if necessary. If flexible water connections are
used, the must comply with the standard for
flexible Water Connectors, ASME A112.18.6/
CSA-B125.6. The HOT and COLD water
connections are clearly marked on the water
heater. Install a shutoff valve in the cold
water line near the water heater. Refer to the
following illustration for suggested typical
installation.
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