Know Your Way Around Construction Equipment

How to Clean Your Gas Furnace Flame Sensor

Regular gas furnace maintenance is important to keep it running reliably and safely for years to come. Most people recognize the importance of changing air filters, for example, but you may not be aware that there is another regular maintenance task that should be performed. The flame sensor on your gas furnace is critical to its proper, safe operation, and a dirty sensor can cause the furnace to abort after startup. Below is more information on this small but important component and how you can clean it:

Gas furnace flame sensors—their purpose and how they work

Gas furnaces cycle on and off automatically based upon the thermostat setting, and despite minor differences between various models, furnaces follow a consistent pattern when starting up.

First, when the temperature inside the home drops beneath the thermostat's set point, the thermostat sends a "call" to the furnace control board for heat. The furnace then triggers the igniter, which begins to heat or spark, depending on the specific type used. Once the ignition conditions are met, the gas valve opens and permits gas to flow into the burners. There, the burners light and the system blower begins to push heated air through the ducts.

In the early stages, immediately after ignition, the flame sensor begins to check for the presence of flame proceeding from the burners. If the sensor fails to detect flame, it will shut off the gas valve and may restart the ignition sequence, depending on the furnace model. Eventually, repeated attempts at ignition will "lock" the furnace for a period of time as a safety measure.

Unfortunately, a dirty or charred flame sensor will often fail to detect the presence of flames and cause the unit to shut down on a false premise. This can prevent you from using your furnace until the flame sensor problem is addressed. Preventative cleaning can keep this potential problem from flaring up at an inopportune moment.

How to clean your gas furnace flame sensor

1. Switch off electric power to the furnace. Gas furnaces are wired into the house's electrical distribution system and can be shut down using a nearby electrical switch. Be sure to turn the switch to the "off" position, as you don't want to risk accidental startup of the unit while working. If you can't locate a switch, then identify the circuit breaker that controls your home's furnace and turn it off.

2. Remove the access cover. Once you have disconnected the power to the furnace, the next step is to access the inside of the furnace. Locate the screws that hold the cover in place and remove them. Gently pull the cover away from the furnace to prevent pinching or cutting wires. Set the panel aside to keep it out of the way and be careful not to damage the thin sheet metal.

3. Locate the flame sensor. After you have access to the interior of the furnace, locate the flame sensor; this consists of a thin rod set into a porcelain base which is attached by a single wire to the furnace control board. The sensor is located at the burner assembly and is often seen next to the igniter, which contains a pair of wires.

4. Remove the flame sensor. When you find the flame sensor, unscrew the screw or bolt holding it in place inside the burner. Keep track of the screw or bolt and pull the flame sensor away from the furnace interior. If necessary, unplug the sensor from the board to give yourself room to work.

5. Clean the flame sensor. After removing the sensor, be sure to treat it carefully to prevent accidentally breaking the porcelain. In a well-lit area, firmly grasp the base of the sensor near the porcelain and wrap a small piece of #0000 steel wool around the sensor. Next, vigorously rub the steel wool up and down the shaft of the sensor itself. Don't use harsh abrasives, such as sandpaper, files, or other potentially damaging items. Chips and gouges can cause the sensor to malfunction, which may require you purchase a new one. Once the flame sensor has been cleaned with the steel wool, is not caked with soot or other byproducts of ignition, and feels smooth to the touch, you may reinstall it.

6. Reinstall the flame sensor. The now-clean flame sensor is simple to replace. First, lead the sensor wire back to the sensor and re-attach it to the terminal on the sensor. Next, push the flame sensor back into position in the furnace and insert the screw that holds it in place. Lastly, replace the cover and power the unit back on to test its functioning.

If you do not feel comfortable with this process or if your furnace still doesn't work after you clean the flame sensor, look for furnace contractors in your area for help.