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ULTRA HIGH EFFICIENCY POWER VENT/POWER DIRECT VENT - SERVICE MANUAL
Technical Literature Department 24 of 52 Ashland City, TN © 2008
Servicing should only be performed by a Qualified Service Agent
FLAME SENSING TEST
To prove burner flame during ignition the control system monitors flame sensing current
through the flame sensor. It is a DC micro amp (µA) current that flows through the flame
sensor. The control system must sense a minimum level of current to “prove” flame. The
minimum flame sensing current is approximately 1.0 µA. If flame sensing current remains
lower than 1.0 µA the control system would lock out after 3 failed trials for ignition and
display “Ignition Failure.” Flame sensing current typically runs much higher than minimum
when the flame sensor is clean; between 8.0 µA to 12.0 µA.
The most common cause of “Ignition Failure” lock out is a contaminated or corroded flame
sensor. Rust/corrosion will accumulate on the flame sensor over time. Cleaning the flame
sensor is a common maintenance procedure that should be performed anytime the water
heater is being serviced. Clean the flame sensor with ultra fine steel wool or an ultra fine
Scotch-Brite pad. DO NOT use a coarse abrasive material such as sand paper.
The burner not being grounded can also cause ignition failure - see page 8.
Measuring Flame Sensing Current
Measuring flame sensing current requires a multi meter with DC micro amp functionality.
See tool requirements on page 3. The meter’s selector is set to DC micro amps. The flame
sensor wire is disconnected and one of the meter probes is inserted in the wire connector.
The other meter probe touches the flame sensor terminal. There must be constant contact at
both points to measure current. Start the water heater - when ignition occurs the meter will
read flame sensing current in real time. See the image below.
If flame sensing current is lower than 8.0 µA, remove the flame sensor and clean it with ultra
fine steel wool. Perform a close visual inspection of the flame sensor. If the sensor appears
to be damaged or shows signs of excessive wear, replace the flame sensor. If the ceramic
insulator on the flame sensor has any visible cracks, replace the flame sensor. When
finished inspecting and cleaning the flame sensor, reinstall it and measure flame sensing
current again.
Flame Sensor