This part is out of stock right now, so it cannot be added to the cart.
This part proves there is a flame once the burner lights. If it does not detect a signal, the system shuts the gas off for safety.
Specifications
- Part number: 100109908
- Brand: A.O. Smith
- Part type: Flame sensor
- Height: 0.95 in / 2.41 cm
- Length: 9.15 in / 23.24 cm
- Width: 7.35 in / 18.67 cm
- Rod length: about the same as your old one
- Mount: same bracket and screw spacing
- Connector: same terminal style
From the maker's and parts sellers' listings for this number. Compare them with the label on your old part.
What the Flame Sensor Does
A flame sensor sits in the path of the flame. When the burner ignites, the sensor sends a signal to the control board to confirm the flame is lit.
If the sensor is dirty or faulty, the system will not receive this confirmation. Without a signal, the unit shuts the gas off to keep unburned fuel from collecting.
Dimensions and Physical Specs
When checking your old part against this number, compare the overall package and mounting dimensions. The listed measurements for this part are:
Make sure your mounting area and bracket have sufficient space for these dimensions.
- Height: 0.95 in / 2.41 cm
- Length: 9.15 in / 23.24 cm
- Width: 7.35 in / 18.67 cm
Confirming a Replacement Match
A replacement flame sensor must physically line up with the burner assembly to work as intended. A mismatched part may fail to sense the flame or fit into the mounting location.
Look at your old sensor closely and verify these points before installing a replacement:
- Rod length: about the same as your old one
- Mount: same bracket and screw spacing
- Connector: same terminal style
Signs of Failure and Replacement Steps
If your burners light and then go out after 3 to 10 seconds, a dirty flame sensor is a common cause. A light cleaning fixes many sensors. You should replace the part if the metal rod is bent or worn, or if the white porcelain is cracked.
Follow these steps to safely swap the sensor once you confirm it needs replacement:
- Turn off the electrical power at the breaker.
- Shut off the gas supply valve completely.
- Take a photo of the wire connected to the sensor so you know how it connects.
- Remove the mounting screw that holds the flange bracket in place.
- Carefully disconnect the wire terminal from the old sensor.
- Position the new sensor in place, line up the bracket, and secure it with the mounting screw.
- Reattach the wire to the terminal, restore the gas and electrical power, and observe a test cycle.
Read your old part
- Turn off power and remove the old sensor from the burner assembly.
- Wipe away any soot and locate the stamped part number on the mounting bracket or flange.
- Confirm the stamped number reads 100109908.
- Compare the bracket shape, rod length, and wire terminal to verify the physical build matches your old part.
When to call a pro
If you are not sure you can complete any step safely, stop work. Gas valve work and fuel line handling require special care. Hire a licensed HVAC technician to inspect your system and install the part.
Questions
What happens if a flame sensor fails?
The sensor proves there is a flame. If it cannot send a clear signal, the unit shuts the gas off for safety. A common sign is burners that light and then shut down after 3 to 10 seconds.
Can I clean my flame sensor instead of replacing it?
A light cleaning fixes many sensors. However, you must replace the sensor if the rod is bent or worn, or if the white porcelain insulator is cracked.
Can I bypass the flame sensor to keep my heat running?
No. Never bypass a flame sensor, limit switch, or any other safety device. The sensor prevents unburned gas from building up inside your unit.
What should I do if I smell gas while working on the unit?
If you smell gas, do not touch any electrical switch. Leave the house immediately and call your gas company from outside.
Related part numbers
- 100113151Water Heater 100113151 Flame Sensor Guide, A.O. SmithSame brand and type
- 100074673Flame Sensor, A.O. SmithSame brand and type
- 100110899Water Heater 100110899 Igniter, A.O. SmithSame brand
- 100108267Thermocouple, A.O. SmithSame brand
- 25957Flame Sensor Reference, NiecoSame part type
- 100112456Igniter, A.O. SmithSame brand
Browse: A.O. Smith part numbers
All part numbers · Part number replacements · Shop flame sensors · Repair guides
Brand names and part numbers identify the part and show fit. HVAC Fitter is not affiliated with, authorized by, or endorsed by any equipment maker. If you are not sure a repair is safe for you, call a licensed HVAC technician.