You might not think your furnace has much in common with a rocket getting ready for liftoff, but there's plenty you can learn from the detailed ignition sequences needed to blast off into space. Mission control follows a detailed checklist before launch to ensure that everything is in working order and ready to perform. These checklists also make it easier to discover problems when they do exist.
Although it's certainly not rocket science, your furnace's control board also follows a checklist before igniting its burners. Understanding how this countdown sequence works can help you diagnose issues with a furnace suffering from a failure to launch.
Just like a rocket prepping for launch, your furnace needs to go through several steps before it can ignite its burners. These steps help to prepare the burners for ignition while also ensuring that your furnace can operate safely. Before doing anything else, the furnace's control board will ensure that all safety switches are in their correct positions.
Assuming it doesn't detect any problems with your safety switches, the furnace can then begin to prepare to ignite the burners. First, it will engage the draft inducer motor to clear any lingering gases from the combustion chamber. All furnaces with draft inducers also include a safety switch to confirm the inducer blower is operating correctly and creating sufficient negative pressure in the flue.
Once this process completes, the furnace will engage its igniter and open the gas valves. At this point, the flame sensor will confirm that the burners are on and generating sufficient heat. If the flame sensor detects a problem, it will shut your furnace down to avoid leaking gas or allowing incomplete combustion. The burner will then continue to run until the thermostat no longer requests heat.
Houston, We Have a Problem
If your burner never ignites, you can use the countdown steps to help figure out the underlying cause. For example, if your burner ignites for a short while, you know that your furnace completed its initial safety check and the inducer motor is working correctly. In any other case, the control board would not attempt to light the burner at all.
By working your way through the ignition sequence from beginning to end, you can rule out many potential causes for an ignition failure, saving you time and money on your repair. Your furnace may also have error codes that it can display on its control board. Referencing these error codes with your user manual will provide additional clues to determine where in the ignition sequence a failure is occurring.
Of course, some furnace problems can be more challenging to solve than others. If you're unsure where to begin, it's best to call a professional for support. A trained furnace repair technician can figure out the exact cause of your problem so you can get your furnace back on the launchpad and ready for ignition.