A.O. Smith 400A - Manual (Page 38)

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NOTE: Always turn off and disconnect main supply wiring before
servicing the unit.
4. Check connecting wire to the flame sensor. Verify that the flame
sensor has not been damaged in any way.
5. Verify that the air supply is adequate. The air inlet screen or blower
wheel may be restricted. Also, check the installation for proper
ventilation. See AIR REQUIREMENTS.
6. Check the venting system for the correct sizing, excess or
insufficient. See VENTING.
7. Check for a maximum inlet pressure of 11" Natural Gas
(2.74 kPa) W.C. and the manifold pressure as listed in Table 4:
that is, for Natural Gas, 4.0" (0.996 kPa) W.C. Please note that
the manifold pressure listed is the maximum value for the manifold
pressure. Do not set the manifold pressure higher than the value
shown for your heater in Table 4. Overfiring the heater will result
in rough ignition and/or noisy operation.
8. Confirm that the air inlet screen (see Figures 7 & 8) is free of
obstructions.
CAUTION
Do not reach into the burner housing or combustion chamber if the
heater is still hot. Allow the heater to cool and always use gloves as
the combustion chamber and the burner sleeve and housing can
become very hot after operation. Overfiring is a dangerous condition
that must be corrected immediately.
NOISY BURNER OPERATION
The exhaust of this unit produces some noise during ignition period.
When combustion is too lean or too rich, start up noise will get worse.
Check for flue gas CO reading, adjust gas valve manifold pressure
if required to bring the flue gas CO to 9–9.5% level for natural gas,
10.75–11.25% for LP. This noise may be objectionable if the exhaust
terminal is located too close to the opening of an occupied space.
The noise may also be objectionable if the exhaust is located in the
confined space between two buildings such as an alley or walkway.
Since everyone's perception and tolerance for background noise
varies, there is no standard recommendation. The best option
when the noise is a concern is to fine-tune the air/gas ratio of the
combustion system or locate the exhaust as far away from any
occupied spaces as practical.
GAS FAILS TO SHUT OFF
Check for defective gas valve or thermostat. If operation is
incorrect, replace.
REPLACEMENT PARTS
Replacement parts may be ordered through A.O. Smith dealers,
authorized servicers or distributors. Refer to the Yellow Pages for
where to call or contact the A.O. Smith Water Products Company,
125 Southeast Parkway, Franklin, TN 37068, 1-800-433-2545.
When ordering parts be sure to state the quantity, part number
and description of the items including the complete model and
serial number as it appears on the product. Refer to the parts list
for more information.