High Limit Switch (E.c.o.); Dishwashing Machine Requirement; Circulating Pump; Installation Instructions - A.O. Smith BTH 120-250(A) Installation & Operating Instructions Manual

that the water temperature be set for the lowest temperature which satisfies
your hot water needs. This will also provide the most energy efficient
operation of the water heater and minimize scale formation.
SETTING THE WATER HEATER TEMPERATURE AT 120°F/49°C WILL
REDUCE THE RISK OF SCALDS. Some states require settings at
specific lower temperatures.
Figure 1 shows the approximate time-to-burn relationship for normal adult
skin. Short repeated heating cycles caused by small hot water uses can
cause temperatures at the point of use to exceed the thermostat setting by
up to 20°F/11°C. If you experience this type of use, you should consider
using lower temperature settings to reduce scald hazards.
Temperature
Setting
180°F / 82°C
170°F / 77°C
160°F / 71°C
150°F / 66°C
140°F / 60°C
130°F / 54°C
120°F / 49°C
Valves for reducing point-of-use temperature by mixing cold and hot water
are available (see Figure 2). Also available are inexpensive devices that
attach to faucets to limit hot water temperatures. Contact a licensed plumber
or the local plumbing authority.

HIGH LIMIT SWITCH (E.C.O.)

The top immersion well of the dual bulb controller also contains the high limit
(energy cutoff) sensor. The high limit switch interrupts the main burner
gas flow should the water temperature reach approximately 202°F/94°C.
Should the high limit switch activate, it must be manually reset by depressing
the Temperature Adjustment Button/Reset Button on the display board. The
water temperature must drop below 160°F/71°C before the controller can
be reset.
Time to Produce 2nd & 3rd
Degree Burns on Adult Skin
Nearly instantaneous
Nearly instantaneous
About 1/2 second
About 1-1/2 seconds
Less than 5 seconds
About 30 seconds
More than 5 minutes
FIGURE 1
FIGURE 2
Continued manual resetting of high limit control, preceded by higher than
usual water temperature is evidence of high limit switch operation. The
following are possible reasons for high limit switch operation.
• A malfunction in the thermostatic controls would allow the gas valve to
remain open causing water temperature to exceed the thermostat setting.
The water temperature would continue to rise until high limit switch
operation.
Contact your dealer or servicer if continued high limit switch operation
occurs.

DISHWASHING MACHINE REQUIREMENT

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 between these limits.
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 means of the
plug cock in the circulating line. (See installation diagrams.)

CIRCULATING PUMP

A circulating pump is used when a system requires a circulating loop or
there is a storage tank used in conjunction with the heater. Refer to the
piping diagrams at rear of manual for electrical hookup information and
install in accordance with the latest version of the National Electric Code
NFPA 70 or Canadian Code CSAC22.1.
All bronze circulators are recommended for used with commercial water
heaters.
Although circulators are oiled and operated by the manufacturer some
circulators must be oiled again before operated. Please refer to
manufacturer's instructions.

INSTALLATION INSTRUCTIONS

REQUIRED ABILITY

INSTALLATION OR SERVICE OF THIS WATER HEATER REQUIRES ABILITY
EQUIVALENT TO THAT OF A LICENSED TRADESMAN IN THE FIELD
INVOLVED. PLUMBING, AIR SUPPLY, VENTING, GAS SUPPLY AND
ELECTRICAL WORK ARE REQUIRED.

INSULATION BLANKETS

Insulation blankets available to the general public for external use on gas
water heaters are not approved for use on your A.O. Smith water heater.
The purpose of an insulation blanket is to reduce the standby heat loss
encountered with storage tank water heaters. Your A.O. Smith water
heater meets or exceeds the ASHRAE/IES 90.1 1999 standards with
respect to insulation and standby loss requirements, making an insulation
blanket unnecessary.
Should you choose to apply an insulation blanket to this heater,
you should follow these instructions. Failure to follow these
instructions can result in fire, asphyxiation, serious personal
injury or death.
• Do not apply insulation to the top, or the upper 15" (38 cm) of
the water heater, as this will interfere with safe operation.
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WARNING