Thermostatic Point-Of-Use Mixing Valves; Gas Supply Systems; Gas Pressure Requirements - A.O. Smith 300 Series Instruction Manual

A high quality power supply filter/suppressor must be installed if the
above conditions exist. Call the technical support phone number
listed on the back cover of this manual for more information.
Note: Malfunctions caused by the power supply and the costs to
install power supply filters are not covered under the limited
warranty.

THERMOSTATIC POINT-OF-USE MIXING VALVES

Water heated to a temperature which will satisfy clothes washing,
dish washing, and other sanitizing needs can scald and cause
permanent injury upon contact. Short repeated heating cycles
caused by small hot water uses can cause temperatures at the
point of use to exceed the water heater's temperature setting by
up to 20°F (11°C).
Some people are more likely to be permanently injured by hot water
than others. These include the elderly, children, the infirm and the
physically/mentally disabled. Table 3 shows the approximate time-
to-burn relationship for normal adult skin. If anyone using hot water
provided by the water heater being installed fits into one of these
groups or if there is a local code or state law requiring a certain
water temperature at the point of use, then special precautions
must be taken.
In addition to using the lowest possible temperature setting that
satisfies the demand of the application a means, such as a mixing
valve, for example, can be used at the hot water taps used by these
people to reduce the water temperature.
Check State and/or local codes for mixing valve requirements and
installation practices.
Because of the increased risk from
scalding, if you set the water heater's
gas control knob higher than 120°F,
install thermostatic mixing valves at
each point of use. Due to the increased
risk of scalding, do not set the tempera-
ture of the thermostatic mixing valves
above 120°F.
Mixing valves are available at plumbing supply stores. Consult
a Qualified Installer or Service Agency. Follow mixing valve
manufacturer's instructions for installation of the valves.
Water temperature over 125°F (52°C)
can cause severe burns instantly
resulting in severe injury or death.
Children, the elderly and the
physically or mentally disabled are at
highest risk for scald injury.
Feel water before bathing or
showering.
Temperature limiting devices such as
mixing valves must be installed
when required by codes and to
ensure safe temperatures at fixtures.
Burn Hazard
In all cases, the following burn table must be used.
Table 3. Time to Burn at Different Temperatures
Water Temperature
°F (°C)
110 (43.3)
116 (46.7)
116 (46.7)
122 (50)
131 (55)
140 (60)
149 (65)
154 (67.8)
(U.S. Government Memorandum, C.P.S.C., Peter L. Armstrong,
Sept. 15, 1978)

GAS SUPPLY SYSTEMS

Low pressure building gas supply systems are defined as those
systems that cannot under any circumstances exceed 14" W.C.
(1/2 PSI Gauge). These systems do not require pressure regulation.
Measurements should be taken to insure that gas pressures are
stable and fall within the requirements stated on the water heater
rating plate. Readings should be taken with all gas burning equipment
off (static pressure) and with all gas burning equipment running at
maximum rate (dynamic pressure). The gas supply pressure must
be stable within 1.5" W.C. from static to dynamic pressure to provide
good performance. Pressure drops that exceed 1.5" W.C. may cause
rough starting, noisy combustion or nuisance outages. Increases or
spikes in static pressure during off cycles may cause failure to ignite
or in severe cases damage to water heater gas control valves. If
your low pressure system does NOT meet these requirements, the
installer is responsible for the corrections.
High pressure building supply systems use pressures that exceed
14" W.C. (1/2 PSI Gauge). These systems must use field supplied
regulators to lower the gas pressure to less than 14" W.C. (1/2 PSI
Gauge). Appliances require gas regulators that are properly sized
for the water heater input and deliver the rating plate specified
pressures. Gas supply systems where pressure exceeds 5 PSI often
require multiple regulators to achieve desired pressures. Systems in
excess of 5 PSI building pressure should be designed by gas delivery
professionals for best performance. Water heaters connected to gas
supply systems that exceed 14" W.C. (1/2 PSI Gauge) at any time
must be equipped with a gas supply regulator.

GAS PRESSURE REQUIREMENTS

All models require a minimum gas supply pressure of 3.5" W.C.(0.87
kPa) for natural gas and 8.0" W.C. (1.99 kPa) for propane. The
minimum supply pressure is measured while gas is not flowing
(static pressure) AND while gas is flowing (dynamic pressure). The
supply pressure (static and dynamic) should never fall below 3.5"
W.C.(0.87 kPa) for natural gas or 8.0" W.C. (1.99 kPa) for propane.
The supply pressure should be measured with all gas fired water
heaters connected to the common main firing at full capacity. If the
supply pressure drops more than 1.5" W.C. (0.37 kPa) as gas begins
to flow to the water heater then the supply gas system including the
gas line and/or the gas regulator may be restricted or undersized.
See Supply Gas Regulator (page 19) and Gas piping (page 35) of
this manual. The gas control valve on all models has a maximum gas
supply pressure limit of 14" W.C.(3.48 kPa) The maximum supply
pressure is measured while gas is not flowing (static pressure) AND
while gas is flowing (dynamic pressure).
18
Time for Permanent
Time for 1st Degree
Burn
2nd & 3rd Degree
(Less Severe Burns)
(Most Severe Burns
(normal shower temp.)
(pain threshold)
35 minutes
1 minute
5 seconds 25 seconds
2 seconds
1 second
instantaneous
Burns
45 minutes
5 minutes
5 seconds
2 seconds
1 second