How to Clean a Furnace Flame Sensor

Is your furnace going through the start up process and just as it finally fires up, shuts down just as quick? Does it do this a couple times and then shut down completely? If so, you're likely having the same problem that many homeowners do every year...a dirty flame sensor. HVAC is a complex industry but you don't have to have a career in it to make this repair.

Though we will be using a standard gas fired furnace for our example, boilers and other gas burning appliances often also use flame sensors. The following could be used for these types of appliances too.

Before getting into the simple process of cleaning your flame sensor, know that the key word here is simple. Most furnace related service work is not this simple and should be performed by an HVAC professional.

The flame sensor is a rather simple device located at the burner assembly. Not much more than a thin, usually bent, metallic rod that sits in front of the flame stream when the furnace is running. It's purpose is to confirm to the system that fire is present when the gas valve is open. This prevents the unit from continually emitting gas that has not been ignited and allowing a potentially dangerous build up. When your furnace begins to start up and the burners are ignited, the flame sensor has a very short window of time to detect the flame. If no flame is detected, it automatically shuts down the unit. Most units will allow this shut down 3 times before going into a "safety lockout" for a specified amount of time (approx. 1 hr.) before trying again. Now, not only are you without heat but this repetitive start process is hard on certain parts thus reducing their lifespan and the efficiency of the unit.

Not to say a flame sensor never goes bad however more often than not, it is just dirty from carbon build up. The flame sensor has a very low tolerance for variations in the reading it takes therefore the slightest amounts of carbon can cause it to misread and shut down. Since many units are located in basements, attics, and laundry areas, you can see how debris in the air could build up on the sensor and burn. Thus creating carbon or dirt build up.

* 1/4" Hex Driver or Wrench (This could vary based on your type of mounting screw)
* Small piece of light grit sandpaper, steel wool, or emery cloth
* Dry, Clean Paper Towel

Let's get started...

1) It is imperative that whenever maintaining your furnace (or other appliance), you shut off the power to the unit. Usually, there will be a toggle switch (light switch) mounted on or near the unit. If not, furnaces are to be installed with dedicated breakers, so locate your furnace breaker and shut it down from there.

NOTE: Turning off the thermostat does not shut down power to the unit. Also, there are many different types of furnaces and appliances. If your gas valve is not electrically controlled, you will likely need to shut off the gas to the unit as well. Should any of the information provided here not match up to what you see on your unit, STOP! Do not guess or assume anything when dealing with your heating system. Call an HVAC professional in your area to repair the unit.

2) Remove the Sensor
The sensor is rather easily accessible and typically mounted by one 1/4" hex head screw. Upon removing this screw, the sensor will slide out to where you can more easily clean it. Carefully remove the sensor. It may be necessary to detach the wire fixed to the end of it for even more room to work but not usually.

3) Clean the Sensor
Once you have removed the sensor, gently rub the metal rod (only) with a very light grit sandpaper. Remember, your not sanding down an autobody here, just ridding the sensor of any build up it may have. Then, use a clean paper towel to wipe clean any dust left behind by the sanding.

4) Replace the Sensor
Once you've cleaned the sensor, simply reconnect the wire if you've taken it off, remount the sensor to the burner assembly using the 1/4" screw, replace the door(s) on the unit, and turn the power back on.

5) Check Your Results
If the unit takes a few extra seconds to start up or the fan immediately kicks on and runs for a bit, this is normal. Shutting down the power to the unit can cause it to reset and run through a short series of checks before trying to fire again. Once this check is complete, the unit should again begin to operate by command of the thermostat. Make sure the unit refires and runs properly until the thermostat is satisfied at least once before being confident your problem is solved.

If cleaning your flame sensor did not work, it's possible that the sensor is broken and that the furnace is operating as it should. You may need to replace the sensor in which case you can follow these same instructions with the exception that no sanding is involved; just remove one sensor and replace it with an acceptable new sensor. Of course something else could be causing the problem and you should contact a service repair technician.

It's likely your local hardware store will not have this part available for you however, you likely have a contractor supply facility nearby that can help. There are also many web sites that offer flame sensor replacements. You will need the make and model of your furnace in order to find an acceptable match.