What's a flame sensor?

The flame sensor on your system plays an important part in your furnace or boiler operation. When the system fires off and lights the burners, the flame sensor is there to verify that the system has indeed fired off and it's ok for the control board to continue energizing the gas valve and deliver fuel to the manifold/burner assembly. The flame sensor itself typically consists of little else than a steel rod with a spade terminal on the end, encased in porcelain insulation. The way these little devices work is through flame rectification. Within the flame there are free moving ions, and the ions in the flame allow current to pass through the flame just like current passes through a wire. The ignition control sends a voltage to the flame sensor, but because the burners themselves have much more grounded area than the flame sensor itself, the voltage passes to ground via the burners. During this process there is a small pulsating offset current in DC, and the flame sensor is actually able to "sense" (conduct) this tiny amount of current back to the control board. It's actually way more complicated than my mediocre attempt at describing this phenomenon, but for simplicity's sake, let's stick with that description and avoid more technical terms like postive ions, potential, and microamps.

So, now that you're thoroughly confused, I'll keep the rest fairly simple. Flame sensors sit in the flame itself. As they sit there in the flame, carbon deposits adhere to the surface of the steel rod, insulating it from the current it's supposed to "sense" (conduct) back to the control board. If the rod gets too dirty, the system will ignite the burners, run for about 2 seconds and then shut down the burners. The control board thinks that the flames haven't ignited, and will shut down the gas valve to prevent gas dumping into the manifold/burner assembly and creating a potentially explosive scenario. Removing the flame sensor is usually pretty easy (20 year old Lennox and certain Carrier units aside). Once the steel rod is cleaned with a bit of green scrubber pad or 000 steel wool (NOT SANDPAPER) and reinstalled, the system is power cycled (power supply off/on) and should then restart after a short while and attempt an ignition trial. If all goes well, burners fire off and stay on, blower motor comes alive after a short delay and starts moving air across the warm heat exchanger, delivering heated air to your house or office. Success! 

As such, there are procedures that professional technicians follow to diagnose this issue, and electrical tests performed to verify it has been successfully remedied. I highly suggest having your furnace or heating system checked out every year, not only to ensure reliability but also to verify safety and performance of the system. Flame sensors are always cleaned yearly with every Triple Point Maintenance, and are only replaced if they are damaged in some way, such as cracked porcelain insulation for example. 

So there you go, flame sensors in a couple paragraphs. Give us a call if you have concerns about your system, we are always happy to help! 

-August