It happens that you hear that furnace starts, flames come on, works for a few seconds and all of a sudden: Click and silence. Then, in a minute or so this cycle repeats. Furnace Cycling On and Off. Then again in a loop. Furnace not starting at all or starting intermittently. You might wake up to a cold house or return from work and furnace is not doing anything to warm up your house.
In other words, the house temperature is lower than the temperature set. And the furnace is struggling to start or not doing anything at all. Pilot light ignites but the flame dies out right away.
Sometimes the furnace would finally start working and the flame stays on, heats up the apartment and after a period of time torturing sequence of lighting up repeats itself. The video below demonstrates the symptoms of furnace coming on and off repeatedly
If this seems to be similar to what you have, keep reading. The flames may stay on good several seconds longer, but it is very similar.
Short version is: Flame sensor needs cleaning
Long version is that 90% of the time the problem lays in a dirty flame sensor. The name of the device speaks for itself, but to describe the function briefly, this sensor is sitting in a direct way of flames when furnace is on. Its function is to feel the heat, so to speak.
Explanation of the problem
The normal sequence of furnace operation is as follows:
- Hot surface Ignitor or spark Ignitor comes on, ready to light the gas once it starts flowing
- Gas valve opens up and gas starts flowing into the chamber
- Gas ignites when coming in contact with hot surface ignitor or catching a spark
- Flames burn continuously
Imagine the ignitor is not working properly, disconnected, malfunctions. The control board send a signal/provides voltage for it to operate and it needs a means to make sure the gas is burning and not just flowing into the furnace without catching flame. The flame sensor is a bi-metal element that tells control board that there is flame in the chamber, in other words, it is hot.
The flame sensor is just simply a thin metal rod or a rod with 90 degree angle. With time the metal surface gets oxidized, collects impurities and dust, hence its sensitivity decreases drastically. Due to this it does not sense presence of flames correctly and the system shuts down as describe and shown in the video above.
Depending on make and model of the furnace the delay between gas starting flow into a chamber and when a control board needs a “flame on” confirmation differs, but generally it needs a few seconds tops. In the beginning, the symptoms might even differ, when a furnace lights up from the second or third try, but eventually the dirty flame sensor might prevent furnace from staying on altogether.
How to clean furnace Flame Sensor
Flame sensor is normally located on top or side of the chamber, close to the burners. As shown in the picture it can be recognized by the fact that only a single wire is connected to it. The sensor is normally connected by a single screw.
- Turn off the furnace
- Locate the flame sensor
- Unscrew it (don’t let the screw fall aside) and extract
- Grab a fine sand paper or a metal mesh and gently sand the rod surface. Don’t really go into it or you can easily damage it by applying too much force. Avoid touching the rod with bare hands and do not put any cleaning chemicals on it.
- Insert back in the hole and secure with a screw.
If you correctly diagnosed the problem and accurately followed the instructions, the furnace should start functioning correctly right away.
If this did not solve the problem, there might be something else, give us a call and we’ll be happy to save the day for you.
Please note that furnaces differ and diagnosing problems might be sometimes tough even for a professional, so these instructions here are provided for information purposes only. They worked when repairing this particular Ducane furnace. Doing it yourself does not entitle you to any sort of warranty or support. You can do it on your own risk or we can diagnose for your peace of mind.