Hot Surface Igniter / Flame Sensor / Control Timing - A.O. Smith BTH 120 Series Service Handbook

HOT SURFACE IGNITER / FLAME SENSOR / CONTROL TIMING

HOT SURFACE IGNITER
This water heater is equipped with an electric hot surface igniter. The igniter material is silicone carbide and
should not be handled with bare hands because of possible damage to the igniter.
The normal ohm reading at 77 °F (25 °F) is listed between 40 and 70 ohms. The minimum igniter current
monitored by the CCB is 2.7 amps. Once this threshold is met a check mark will appear by the igniter icon
in the UIM (display).
FLAME SENSOR
This water heater is equipped with an electronic flame sensor. The flame sensor senses flame by passing
a small electric current through the burner flame. This type of flame sensing is also known as flame
rectification. The CCB is looking for a minimum current of .7 micro amperes for the water heater to operate.
Once this current is established the UIM will display a flame icon.
CONTROL TIMING
Pre-purge
Igniter Warm-up
Trial for ignition Gas
Control Open
Inter/Post Purge
40
25 seconds
17 seconds
5 seconds
30 seconds
Servicing should only be performed by a Qualified Service Technician