Installation Considerations; Water Piping; Closed Water Systems; Thermal Expansion - A.O. Smith SERIES 120 Service Handbook

INSTALLATION CONSIDERATIONS

Installation information given in this service manual IS NOT a complete installation
instruction. Installation information covered in this service manual has a limited focus as
it applies to servicing. This service manual does not replace or supersede the Instruction
Manual that came with the water heater. Always refer to the instruction manual that came
with the water heater for complete installation instructions.
If the instruction manual that came with the water heater is not on hand, copies can be
obtained from the manufacturer's web site or by calling the toll free phone number shown
on the back cover of this service manual.

WATER PIPING

Closed Water Systems

Water supply systems may, because of code requirements or such conditions as high line
pressure, among others, have installed devices such as pressure reducing valves, check
valves, and back flow preventers. Devices such as these cause the water system to be a
closed system.

Thermal Expansion

As water is heated, it expands (thermal expansion). In a closed system, the volume
of water will grow when it is heated. As the volume of water grows, there will be a
corresponding increase in water pressure due to thermal expansion. Thermal expansion
can cause premature tank failure (leakage). This type of failure is not covered under the
limited warranty. Thermal expansion can also cause intermittent temperature-pressure relief
valve operation: water discharged from the valve due to excessive pressure build up. This
condition is not covered under the limited warranty. The temperature-pressure relief valve is
not intended for the constant relief of thermal expansion.
A properly sized thermal expansion tank should be installed on all closed systems to control
the harmful effects of thermal expansion. Contact a local Qualified Service Agency (page 3)
to have a thermal expansion tank installed.

GAS PRESSURE REQUIREMENTS

Table 1 shows supply and manifold gas pressure requirements for BTR 500/A 120 - 121
model water heaters. Supply gas pressure is the pressure of fuel gas being supplied to the
water heater. Manifold gas pressure is the pressure of the gas being supplied to the water
heater's burners.
Supply gas pressure should be measured twice. One measurement while the water
heater is not firing (static) and again while the water heater is firing (dynamic). If the
supply gas pressure drops more than 1.5" W.C. between the static and dynamic pressure
measurements, this may indicate the gas line and/or gas regulator to the water heater is
undersized. See the gas line requirements in the Instruction Manual that came with the
water heater.
INSTALLATION CONSIDERATIONS
Servicing should only be performed by a Qualifi ed Service Agent
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