So the furnace sequence of operation is the order of how the furnace starts up. So you have power coming from your breaker panel to your electrical junction box. The power goes down, into the control board. You always have power just sitting on your control board, and this control board sends power to your thermostat. When there's a call for heating, for example, from my thermostat, it closes a switch. It sends power back down to the control board, saying that there's a call for heat, and then the control board starts the process of heating the house.
So the way it works is the power goes to the thermostat from the main control board. Then back down from the thermostat, turning on your inducer motor; your inducer motor turns on and starts sucking air through the burners and then out your exhaust pipe. The inducer motor turns on these pressure switches hooked up by a hose in the back of the inducer motor housing. The purpose is to prove that there is indeed a draft.
These pressure switches will not close because normally there open when they're off, but when the inducer motor turns on those pressure switches, they close, and they let the circuit through. Once the Control Board senses that the pressure switch is completed, it sends a signal to the igniter to glow.
Once it starts to glow, the control board sends power to the gas valve opening it up to let gas come through and lights all the burners.
On the other side, we have the flame sensor, and that's just there to prove that there is a flame. Once the control board realizes that there is a flame, it leaves the burners on. If the flame sensor is not sensing any flame, then the burners will shut off after about three seconds. If the burners turn on and about twenty or thirty seconds later, depending on what kind of furnace you have, your blower motor will come on.
Now let's make sure the power is turned on the furnace and increase the thermostat's heat to see the furnace in action. So the thermostats calling for heat, the first thing that should energize is the motor, the motor turns on. You can't physically see it, but the pressure switch will close. The igniter is starting to glow bright yellow or bright orange. Then you're going to hear the gas valve click, the burners then light, and now the furnace is going to be preheating for a while.
Once it warms up the heat exchanger enough, the blower motor will come on and start distributing the air. The reason for that is if the furnace was cold and the blower fan came on right away, you're going to feel cold air coming out of the vents instead of warm air, so it does a little preheat before it turns on the blower motor.
Once the thermostat is satisfied, and what I mean by that is, let's say you set it to 74, and it finally gets to 74, everything goes backward from there. Hence, the gas valve gets de-energized, and the burners turn off. Simultaneously, the blower motor stays on for like 30 seconds to cool off the furnace. Everything shuts off, and the furnace goes into standby mode.
Well, that's the furnace sequence of operation. Now I want to simulate a few problems to show you why is it important to know the operation sequence depending on where the sequence is interrupted? That's where you should be looking. For example, if your igniter is glowing, but the gas is not going through, you know that the problem revolves around the gas valve. Either it's not getting power, it's not getting gas or the gas valve is bad, or if your inducer motor is running, but your igniter does not start to glow. That means one of two things, either your igniter is bad, or for some reason, your pressure switch is not closing.
I hope this information is useful, helping you understand your heating system. Understanding the series of events makes it simple to locate the problem if your furnace is not working and gives you direction towards diagnosing the issue.