If your furnace seems to shut down on its own and no longer works as efficiently, chances are, your flame sensor needs to be cleaned.
Ideally, you want to call a professional HVAC cleaner to clean your flame sensor. The flame sensor is one of the most delicate parts of the furnace and has to be handled with care.
In most types of furnace setups, the flame sensor should be located near the burner assembly. It’s the thin, sometimes bent, metallic rod fitted in front of the flame stream of the furnace. The flame sensor works by monitoring if the gas valve is open and there’s fire. If a furnace kept emitting gas without combustion, the buildup of invisible gas could cause gas poisoning.
The slight buildup of carbon in the flame sensor could cause the furnace to shut down before turning to safety lockout. Carbon buildup is usually caused by dust — and since most furnace systems are installed in dusty areas of the house, the flame sensor should be cleaned regularly to prevent accidents as well as to prolong the life of your furnace system.
Not all furnace setups are installed the same way and if the instructions below do not match with your own setup, do not attempt to clean any parts of the furnace on your own.
How to Clean a Furnace Flame Sensor
First, you will need a 1/4 inch hex screwdriver or even a wrench. Get a small piece of sandpaper, emery cloth, or even steel wool, as well as several pieces of clean paper towels.
Step 1: Shut Off the Power
It’s important to cut the power before disassembling parts of the furnace. Turn the power straight from the breaker box. If your furnace is not controlled electrically, you will have to shut off the gas of the unit as well.
Step 2: Find the Flame Sensor
After making sure that the power to the furnace is shut down completely, look for the flame sensor. The flame sensor is usually attached at the rear of the furnace. It should be placed near the burner — attached with one or two sheet metal screws in the bracket.
The screws can be removed using your 1/4 inch hex head screwdriver. As the screws are removed, the sensor will glide out. Carefully remove the sensor. In some units, the flame sensor cannot be removed completely. You can still clean it while it’s semi-attached to the setup.
Step 3: Clean the Flame Sensor
With your sandpaper, lightly remove the carbon buildup from the flame sensor. Do not sand down the flame sensor! Then, with clean paper towel, remove any dust from the sanding.
Step 4: Assemble the Flame Sensor Back
After cleaning the flame sensor, remount it back to where it belongs and with your screwdriver, secure all screws tightly. If everything is where it should be, bring the power back to the furnace and check the results. If the system isn’t working immediately, don’t be alarmed for this is normal. Shutting down the power will reset the furnace system so it won’t start the usual way right away.