Mixing Valves; Dishwashing Machines; Closed Water Systems; Thermal Expansion - A.O. Smith BTH-120 Instruction Manual

mixing 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.
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 6 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 mixing Valve should be
installed at the water heater (see Figure 8) or at the hot water
taps to further reduce system water temperature.
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.
taBle 6
Water Temperature Time for 1st Degree Burn
°F (°C) (Less Severe Burns)
110 (43) (normal shower temp.)
116 (47) (pain threshold)
116 (47) 35 minutes
122 (50)
131 (55) 5 seconds
140 (60) 2 seconds
149 (65) 1 second
154 (68) instantaneous
(U.S. Government memorandum, C.P.S.C., Peter L. Armstrong, Sept. 15, 1978)

dishwashing machines

All dishwashing machines meeting the National Sanitation
Foundation requirements are designed to operate with water
flow pressures between 15 and 25 pounds per square inch (103
kPa and 173 kPa). Flow pressures above 25 pounds per square
inch (173 kPa), or below 15 pounds per square inch (103 kPa),
will result in improperly sanitized dishes. Where pressures are
high, a water pressure reducing or flow regulating control valve
should be used in the 180°F (82°C) line to the dishwashing
machine and should be adjusted to deliver water pressure
between these limits.
Time for Permanent Burns
2nd & 3rd Degree
(most Severe Burns)
45 minutes
1 minute 5 minutes
25 seconds
5 seconds
2 seconds
1 second
TEMPERED WATER
OUTLET
COLD
WATER
INLET
The National Sanitation Foundation also recommends circulation
of 180°F (82°C) water. Where this is done, the circulation should
be very gentle so that it does not cause any unnecessary
turbulence inside the water heater. The circulation should be just
enough to provide 180°F (82°C) water at the point of take-off to
the dishwashing machine.
Adjust flow by throttling a full port ball valve installed in the
circulating line on the outlet side of the pump. Never throttle flow
on the suction side of a pump. See the Water Piping Diagrams
beginning on page 71.
note: To comply with NSF Standard 5 installation requirements
the bottom of the water heater must be sealed to the floor with a
silicone based sealant or elevated 6 inches above the floor.

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 must 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.
See Water Line Connections on page 42 and the Water Piping
Diagrams beginning on page 71.
15
HOT WATER
OUTLET
12" TO 15"
(30-38 cm)
CHECK
VALVE
MIXING
CHECK
VALVE
VALVE
TO TANK
INLET
figure 8