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A flame sensor is an important instrument for the safety of a furnace. Its job is to make sure that the gas is burning in the furnace instead of escaping and building up elsewhere. A flame sensor can detect a weak or non-existent flame and shuts down the furnace to avoid gas-related issues. A dirty or broken flame sensor can lead to gas build-up or an emergency service call for a furnace that won’t turn on. This can end up costing money, so regular testing and cleaning is the best idea to avoid unexpected costs or emergencies.
Modern gas appliances, especially gas furnaces, employ electronic igniters instead of standing pilots (which are common on older appliances), so they do not require thermocouples to keep the pilot lit. They still require a safety sensor to ensure that the gas ignites when the appliance is turned on, and this function is served by the flame sensor. Although designs vary somewhat, a typical flame sensor is a 2- to 4-inch metal rod with a ceramic base and attached by a wire to a gas valve. It is mounted on the burner housing in such a way that the metal rod is in contact with the flame when the gas is burning.
The job of the flame sensor is to send a small electric signal to the gas valve when the gas ignites to keep the valve open, and when the sensor malfunctions, the valve will close and the furnace will shut down. This prevents unburned gas from circulating through air ducts and igniting elsewhere in the home. You can usually identify a faulty flame sensor just by looking at it, but if that’s not conclusive, you can also test it with a multimeter.
What is flame sensor? What does a flame sensor do?
A flame sensor is an essential safety device used in your furnace. It is connected to the pilot light and ensures that a flame is always burning in the furnace. This is a really important feature as it ensures that your furnace is not emitting gas when the light is out. Without a pilot flame, the gas does not burn and can create a dangerous build-up of gases inside your home. The flame sensor uses an important process called flame rectification. To accomplish this process, the flame sensor unit sends electrical currents through a flame rod which then detects the shape of the pilot light. If the flame is too low or not there, it turns off the gas. If your flame sensor turns off the gas, it doesn’t always mean something is wrong with the furnace. Sometimes this can indicate a flame rod that needs cleaning. You need to do a test to determine if the unit is dirty or broken.
How to test flame sensor
To determine if the flame sensor is sending a signal to the gas valve, you must connect a multimeter than measure microamps (μA) in series between the sensor terminal and the wire leading to the gas valve. This test must be conducted with the power on as you need to take the readings when the furnace is started. Remove the lead from the flame sensor and connect it to one of the probes on the meter. Then, connect the other meter probe to the sensor terminal. Turn on the thermostat to call for heat and check the meter reading when the burner ignites. It should read between 5 and 10 µA. If it reads less than this, the sensor is defective and needs to be replaced.
How to clean furnace flame sensor
There are some steps for cleaning flame sensor-
Step 1: Power Down the Furnace
Turn off the furnace switch. If you can’t find the switch, turn off the circuit breaker in the main electrical panel that controls the furnace. In addition to preventing shaking, this will also prevent the furnace from cycling while you’re working.
Step 2: Locate the Flame Sensor
Remove the access panel. If you’re working on a furnace with top and bottom panels, remove the top ones. The flame sensor should be right in front of you. It consists of a white porcelain base that may be yellow or brown, and has a single wire attached to it.
Step 3: Remove the Flame Sensor
Disconnect the wire by pulling it from the terminal. The wire has spade connectors, and these sometimes get stuck, so you may have to pull hard. Loosen the screw holding the sensor to the wall of the burner housing using a flat-head screwdriver. Secure the screw in place, and then lift out the sensor.
Step 4: Clean the Probe
Rub the probe with emery cloth or fine steel wool to remove carbon deposits. The probe will probably be a pale black color before you start cleaning it, and when you’re done, it should be a shiny silver color. The probe is delicate, so don’t rub too hard. If you find it difficult to clean the probe, it may be time for a replacement.
Step 5: Replace the Flame Sensor
Insert the probe into the hole in the burner housing and screw in to secure it. Push the wire connector back onto the terminal.
Step 6: Test the Sensor
Turn on the furnace and turn the thermostat on its highest setting to demand heat. When the furnace starts up, make sure the burners are lit and then watch for a while to see if the burners are on. If the burners will not light, replace the flame sensor. If they won’t light at all, the problem is something other than the flame sensor. call for service.