If you've never opened up your furnace before, it might seem like a bit of a mystery. While modern furnaces contain numerous components and sensors, they're still relatively simple devices. Even if you don't plan on performing maintenance and repairs yourself, it's helpful to understand a bit about how they work so you can know why they sometimes fail.
One common component that can cause trouble for homeowners is the flame sensor. This device serves a critical safety function, but it can also leave you out in the cold.
What Is a Flame Sensor?
Several things happen in relatively short order when you turn up the setpoint on your thermostat. Your furnace relies on your home's thermostat (or thermostats, if you have multiple heating zones) to act as its eyes, ears, and brain. Instead of thinking for itself, your furnace waits patiently for the thermostat to request heat and then waits again for the instruction to shut off.
While the furnace doesn't need to decide when to heat or for how long, it doesn't need to monitor its own operation for potential issues. In particular, your furnace needs to avoid situations where it may pump excess gas into the combustion chamber. Allowing too much unburnt gas to flow into the chamber wastes fuel and can even result in a catastrophic failure or explosion.
The flame sensor is the first line of defense against these possibilities. This sensor detects the presence of a flame, shutting the furnace down and stopping the gas flow if it doesn't find one. The flame sensor fails "safe," which means that a dirty or faulty sensor typically won't allow any gas into the chamber, preventing your furnace from producing heat.
Why Do Flame Sensors Fail?
Flame sensors must endure high temperatures and close proximity to your furnace burners. While manufacturers design them to withstand these conditions, soot and dust can cause the sensor to become dirty. Dirty sensors often fail to read the correct temperature, preventing the furnace from turning on or causing it to shut down prematurely.
Old sensors can also fail outright, and no amount of cleaning will get them running again. Issues in the combustion chamber can also cause premature sensor failure or sensors that frequently become too dirty to operate. Since inefficient combustion produces more soot, a poorly running furnace may sometimes cause numerous flame sensor failures.
If your furnace doesn't ignite, you can try checking and cleaning the flame sensor yourself, but always be sure to cut power to the unit before opening it up. However, frequent sensor failures or a sensor that requires cleaning more than once per year may indicate a deeper problem. In these cases, it's best to bring in furnace repair contractors to address the situation.