Anyone can experience an interruption in heating during a cold winter, we rely on our furnaces to keep our homes warm, and when something goes wrong, it creates a host of problems. Sometimes the furnace may try to start, but it cannot run for much longer than a couple of seconds before everything, including the pilot light turns off. This is an intimidating situation; no-one wants to pay for a furnace and be without heating during an installation when it’s cold outside. But, there is good news; this could be a problem that’s simple to fix the possible cause could be a dirty flame sensor. When the flame sensor is cleaned or replaced, if it’s too dirty or damaged, your furnace will work again.

What is a Flame Sensor?

A flame sensor is a simple component in your furnace that has a very specific task. When you look at a flame sensor, it’s a thin metallic rod with a slight bend, and it’s situated in front of the pilot light in the furnace. When your thermostat informs the furnace that it’s time to turn on, the gas valve opens, and gas is forced into the combustion chamber where the ignitor or pilot light lights up the burners. The job of a flame sensor is to confirm that you have a flame present when that gas valve is opened.

If the furnace doesn’t have a flame sensor, the system will continue to fill the home with gas. Over time this would build up to dangerous levels, and it could result in gas poisoning or even an explosion. So, the flame sensor is an important safety measure; if there is no flame, it will automatically shut-off the entire furnace to ensure that everyone stays safe. The lastest furnace models go even further; if this shut-off sequence is repeated three times in a row, it will lockout for an hour. This is enough time to allow any gas in the home to dissipate safely.

Having a safety feature like a flame sensor is a great way to keep everyone in the home safe. But, if there is a flame and the flame sensor cannot detect it, this is a problem. In this case, the furnace is working fine, but the flame sensor can’t tell the difference, so the furnace is shut down, and you cannot get any heat. When the furnace turns off and on again repeatedly in this manner, it can create wear and tear on components that could be expensive to fix. The best way to approach this problem is to clean the flame sensor, and if that doesn’t work, it should be replaced.

What is a Thermocouple?

Many people use “thermocouple” and “flame sensor” as interchangeable terms, but they are quite different. Although they both carry out a similar function, they are found on different types of furnaces. If you have a furnace with an electronic controller, it will have a flame sensor. If you have a furnace that uses a standing pilot light that’s continuously lit, you will have a thermocouple. This component is located next to the pilot light, and it monitors how that component is performing. So, a thermocouple fulfills the same purpose as a safety feature, but it works in a different way.

What is a Dirty Flame Sensor?

The flame sensor can become dirty or corroded over time, and this reduces it’s sensing capabilities. A flame sensor spends a great deal of time inside a flame, and this means that it will become covered with carbon deposits and other contaminants. These materials are burned onto the surface of the flame sensor, and eventually, it cannot sense a flame leading to the aforementioned automatic shut-off sequence. Even small deposits of burned materials can cause this to happen, but this is a simple problem to fix.

What are the Signs of a Bad Flame Sensor?

A bad flame sensor is easy to detect, make sure your gas burners are lighting and going out in the manner discussed earlier. This is the most common cause, but you can confirm this by inspecting the flame sensor. If you can see soot or burned material on the surface of the flame sensor, it needs cleaning. If the tip of the sensor is completely black, you need to get your furnace serviced. If you can see any cracks in the porcelain flame sensor casing, you will need a replacement.

4 Steps to Clean a Flame Sensor

Cleaning your furnace flame sensor isn’t too difficult if your confident working with complex equipment. If you follow these four steps, you may be able to get your furnace running again. But, if you’re not sure about this process or you run into something that you can’t handle, it’s time to call your local HVAC specialist for expert help.

Step 1: Turn Off the Furnace

Working with gas appliances is inherently dangerous because of the risks of poisoning and an explosion. If your working on the furnace and it turns on unexpectedly, you could be burned, or a fire could start. The furnace can usually be turned off with a toggle switch, and if you don’t have one, you can turn the furnace off at your breaker box. If you have a furnace that lacks an electronic control system, you will need to turn off the gas valve that supplies your heating system.

Step 2: Remove the Flame Sensor

The flame sensor is easy to reach, and it’s usually secured with a ¼” hex head screw. Take out the screw, put it somewhere safe, and the flame sensor should slide out easily. Take care, there could be a wire on the back end, and you don’t want to break that connection.

Step 3: Clean the Flame Sensor

Use a light grain sandpaper, steel wool or a wire brush to rub at the metal surface of the flame sensor gently. Remove the build up of burned material and any light patches of corrosion on the surface. When the surface is clear, you can wipe the surface down with a paper towel or a soft cloth.

Step 4: Test the Flame Sensor

Place the flame sensor back into position, securing it with the screw, and turn the furnace or gas valve back on. Then test the furnace, and it should light as normal to provide heat to your home. If this process doesn’t work, you can repeat it to clean away any dirt that you may have missed. But if this doesn’t work, you will need to contact a local HVAC specialist and ask about a replacement flame sensor.