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On-Demand Water Heater Service Handbook for 240, 340 & 540 Condensing Models • 21
•
Multi-Link System:
Push the Alarm Reset button on
the multi-unit controller to clear
the code.
Code 941: Abnormal Exhaust
Temperature
(Indoor models only.) This
code indicates that the exhaust
temperature exceeded limits and that
the heater was not able to reduce
the exhaust temperature, forcing the
exhaust high limit switch to trip and
shutdown the burners.
1. Check for excessive vent length and
for any blockage in the intake air
and/or exhaust.
2. Check for dust or lint in the
burner and heat exchanger in
contaminated installation areas.
3. Check the incoming water
temperature. If it is above 140°F,
the exhaust temperature may be
too high. Reduce the supply water
temperature.
4. Check the resistance of the thermistor.
Refer to “Code 311, 321, 331 or 341:
5. Check the manifold pressure.
See “Checking and Adjusting the
Manifold Gas Pressure,” page 34.
Code 991: Abnormal
Combustion
This code indicates imperfect
combustion. This can be caused
by insufficient intake air or exhaust
obstructions.
See also “Error Code: 101.”
WARNING!
Familiarize yourself with the
components before attempting these
procedures. Use the Installation Manual
and reference the component schematic
on the last few pages of the manual.
Safety first! These procedures must be
performed by licensed, qualified service
professionals only. Failure to observe
this warning could lead to personal
injury or death.
A.O. Smith is not liable for the actions of
the technician if he/she fails to adhere to
any or all safety procedures, specifically in
regards to electrical and gas safety.
1. Check the rating plate to verify that
the gas type of the water heater
matches the gas type applied to
the unit. Next, make sure that
the gas-type DIP switch setting is
2. Verify that the venting meets the
requirements specified in the
installation manual (e.g., vent
diameter and length).
3. Verify the DIP switch
settings.
3.1 Are the DIP
switches set for
the correct vent
size and length?
Refer to Dip
p. 23 for the
correct settings.
Each 90-degree
elbow is equal to
5 feet of straight
pipe. Refer to
the installation
manual for vent
requirements.
3.2 Is the heater set
to the correct
elevation? The
default setting
for your water
heater is 2,000
ft. above sea level. A DIP
switch change is required for
installations that are higher
than 2,000 feet above sea
level. Refer to “Dip Switch
of its tables lists the correct
DIP switch settings for each
elevation range.
4. Check the following:
Is the gas supply pressure within
specified limits when the heater is
in standby and when it is running?
If the pressure difference between
standby and maximum firing is greater
than 1 inch W.C., the supply line,
gas meter, and/or regulator may be
undersized. NOTE: In Canada, do
not exceed the maximum allowable
pressure drop permitted by B149.1.
Verify that the gas lines are sized
240 (T-H3J) / 340 (T-H3S) model
MAX bu
tt
on
MIN bu
tt
on
Increase bu
tt
on
Decrease bu
tt
on
540 (T-H3) model
MIN bu
tt
on
MAX bu
tt
on
Increase bu
tt
on
Decrease bu
tt
on
Manifold port
Computer board
To manometer
Tube
Green LED
Green LED
240 (T-H3J) / 340 (T-H3S) model
MAX bu
tt
on
MIN bu
tt
on
Increase bu
tt
on
Decrease bu
tt
on
540 (T-H3) model
MIN bu
tt
on
MAX bu
tt
on
Increase bu
tt
on
Decrease bu
tt
on
Manifold port
Computer board
To manometer
Tube
Green LED
Green LED
Figure 7.
Figure 8.
