Closed Water Systems; Thermal Expansion; Temperature-Pressure Relief Valve - A.O. Smith 300 Series Instruction Manual

To reduce the risk of unusually hot
water reaching the fixtures in the
house, install thermostatic mixing
valves at each point of use.
Note: Solar water heating systems can often supply water with
temperatures exceeding 180°F (82.2°C) and may result in
water heater malfunction.

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 plumbing service agency to have a thermal expansion
tank installed.
CAUTION
Property Damage Hazard
● Avoid water heater damage.
● Install thermal expansion tank if necessary.
● Do not apply heat to cold water inlet.
● Contact qualified installer or service agency.
Note: To protect against untimely corrosion of hot and cold water
fittings, it is strongly recommended that di-electric unions or
Burn Hazard
20
couplings be installed on this water heater when connected
to copper pipe. See Figure 14 (page 20) also.
Figure 14. Water Pipe Connections
Figure 14 show the typical attachment of the water piping to the
water heater. The water heater is equipped with 3/4 inch NPT water
connections for 50 gallon (189.27 L) unit and 1 inch NPT water
connections for 75 gallon (284 L) unit.
Note: If using copper tubing, solder tubing to an adapter before
attaching the adapter to the water heater connections. Do not
solder the water lines directly to the water heater connections.
It will harm the dip tube and damage the tank, T & P Valve
and Pipe Insulation (if supplied).
Fit pipe insulation over the incoming cold water line and the hot
water line. Make sure that the insulation is against the top cover
of the heater. Fit T & P valve insulation over valve. Make sure that
the insulation does not interfere with the lever of the T & P valve.
Secure all insulation using tape.

TEMPERATURE-PRESSURE RELIEF VALVE

This water heater is provided with a properly rated/sized and certified
combination Temperature-Pressure Relief Valve (T&P relief valve) by
the manufacturer. The valve is certified by a nationally recognized
testing laboratory that maintains periodic inspection of production of
listed equipment of materials as meeting the requirements for Relief
Valves for Hot Water Supply Systems, ANSI Z21.22 • CSA 4.4, and the code
requirements of ASME.
If replaced, the new T&P valve must meet the requirements of local
codes, but not less than a combination Temperature-Pressure Relief
Valve rated/sized and certified as indicated in the above paragraph.
The new valve must be marked with a maximum set pressure not to
exceed the marked hydrostatic working pressure of the water heater
(150 psi = 1,035 kPa) and a discharge capacity not less than the
water heater Btu/hr or kW input rate as shown on the water heater's
model rating plate.
Note: In addition to the factory installed Temperature-Pressure
Relief Valve on the water heater, each remote storage tank
that may be installed and piped to a water heating appliance