Ignition Failure - A.O. Smith BTH 120 Service Handbook

4. IGNITION FAILURE

Ignition Failure
Fault occurred 2 mins ago
Flame not detected. Clean flame rod.
Check gas supply.
Call a service professional:
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CHANGE
CHECK/ REPAIR
• Determine whether or not the burner is igniting by looking through sight glass. Because the control system
will shut off the Gas Control Valve if 2-3 seconds if ignition is not detected, you must look in the sight glass
at the exact moment the Gas Control Valve icon appears on the display screen.
TESTING FOR IGNITION (FLAME):
• Set the Set Point Temperature high enough so there is a call for heat.
• Recycle the unit off and back on by turning the Enable/Disable switch to Disable for one second and then
back to Enable.
• The unit will begin an ignition cycle.
• When the Gas Control Valve icon appears on display screen, immediately look through the sight glass for
a brief blue flame (if you are getting an Ignition Failure error, the flame, if the burner ignites, will only be
present for 2-3 seconds).
BURNER IGNITES (BRIEFLY)
• If the burner ignites briefly then shuts off with an "Ignition Failure" error message, follow these steps:
• Remove the connector from flame sense rod [picture of connector showing location on unit]. Inspect
the connector for corrosion. Replace if corroded or damaged. Remove Flame Sensor and clean metal
end with steel wool.
• Check insulator for soot, cracks or other damage and replace if necessary.
CONTROL SYSTEM
APPLIES AC CURRENT
TO FLAME SENSOR
FLAME SENSOR
CERAMIC INSULATOR
METAL CONDUCTOR
AC CURRENT FLOWS FROM THE FLAME
SENSOR TO THE BURNER. THE BURNER
MUST BE GROUNDED FOR CURRENT TO
FLOW. DURING THIS PROCESS THE AC
CURRENT IS "RECTIFIED" AND BECOMES
A DC CURRENT. DC MICRO AMP CURRENT
THEN FLOWS BETWEEN THE FLAME SENSOR
AND THE BURNER.
BACK ADVANCED
Servicing should only be performed by a Qualified Service Technician
CAUSES:
• On an "Ignition Failure" error, either the burner is
not igniting or the burner is igniting, but the Flame
Sensor is not detecting ignition (flame).
SPARK
IGNITER
BURNER
FLAME
BURNER
(MUST BE GROUNDED)
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