Furnace Repair Concord, CA

Understanding how your furnace/heating system works to help with troubleshooting, diagnosing, and repair in Concord, CA.

Furnace error code indicator

Jim Jones, 11-22-2020,  Stars 5/5

I want to go over a furnace/heating system and the sequence of operations. Understanding the primary function of the individual components will make it easy to troubleshoot simple problems. Before we take the doors off and go over the inner workings, if your furnace isn't working, the first thing you want to do is look into your sight glass, which will be on the furnace's bottom door. Most of the newer furnaces will have them and if that's blinking some error code, remember what that code is before you take the doors off. I'm going to go ahead and take my doors off and show you why I just said that.

furnace/heater power switch

So here's my furnace power switch, and it's off right now. Usually, on most furnaces, another switch called the door switch is on the bottom door. Once you take that door off, the switch automatically pops open. It interrupts the power and, if you had some error code blinking on your control board, for example, I'd turn my power back on so I can see my lights blinking. You could see that through the sight glass. Usually, if you had some error code and flipped your furnace power switch off or took the bottom door off, that error code will clear out. If it's an intermittent problem, it might be a little hard to track down, but you know where to start if you have a code.

furnace/heater door cutoff switch

I use a piece of tape to keep the door switch on, so the furnace will be running while you have the doors off; otherwise, this switch, when it's popped open, will interrupt the power and, everything will be blank.

furnace/heating schematic on back of the door

On most heating systems, the explanation of those error codes will be on the back door; on mine, all I have is a schematic; there's no explanation of the error codes. It's frustrating as you can see nothing on the back of the door.

standard error codes for furnace diagnosing

Different heating systems will have other error codes and various diagnostic charts. Still, the error code names will be pretty similar from the furnace to furnace for the most part.
Here is an example out of my tech manual for standard error codes.

main control board with error codes listed

Sometimes the error codes will be located on the control board itself.
Alright, enough with the sidetracking back to the main topic, the furnace sequence of operation. Before we dive into that, I want to explain the components.

Furnace Components

electronic furnace/heater gas valve

The Gas Valve

So here is the gas valve; this particular one is an electronic gas valve, so when the power is applied, it opens up and lets the gas through.

2-stage pressure switch on furnace

Pressure Switches


These two are pressure switches. I have a two-stage furnace; therefore, I have two pressure switches, one for the low stage and one for the high stage.

furnace/heating venting motor

Venting Motor

I have an inducer motor or a venting motor, and it's a two-stage motor. If it's one stage, it'll have two wires coming out of it; usually, a black and a white; the motor I have has a black, white, and red wire that generally indicates a two-stage motor.

furnace/heating system burners

The Burners and Igniter

Here we have the burners that are situated diagonally. Now I'm going to take the cover panel off so we can see the igniter. Most of the newer furnaces will have this style of burner.

pulling a burner out of HVAC system

Popping a Burner Out

The individual burners pop right out.

clogged up furnace burner

Dirty Furnace Burner

The burner in the photo is a little dirty and requires regular maintenance.

cleaning the burner vent holes on a furnace

Burner Maintenance

If you want to clean your burner up real quick, all you have to do is get a brass brush to clean them up. If you're going to be diligent, you can also blow the burners off with some compressed air. OK, so let's, put the burners back in.

image of a cleaned furnace burner

Burner After Brushing

Clean burners will have a considerable impact on the performance of your furnace. If your burners look excessively dirty, it might be a good idea to have an AC Repair Team technician come out and go over your entire system.

flame sensor inside heating system

The Flame Sensor

The flame sensor is on the other side; it looks like a little steel rod.

main furnace electronic control board

Main Control Board

Here's a photo of a typical main control board or a circuit board. It will always have power going to it.

blower housing inside heating system

Blower Housing & Motor

Here we have the blower housing, and inside that is the blower motor.

Sequence of Operation

electronic thermostat

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.