Just like Shrek, I like to think about the heating sequence of operations as an onion—it has layers. And each layer builds upon the previous one. We have written previously in great detail about gas furnaces, from top to bottom. I want to focus today on why there is a sequence of operations and how I think about that process. 

The Sequence of Operations Keeps Us Safe

This is the main reason for the sequence of operations—it keeps us safe. When the heat turns on, we want to make sure we have a couple of things: a clear flue pipe, a working inducer motor, and a functional igniter and blower motor. We don’t want the heat to come on without these components working. 

Inducer Motor

If the inducer motor wasn’t working—or the flue pipe wasn’t clear—and the furnace fired up, what would happen? Well, the flames wouldn’t draw into the heat exchanger. Stuff would start to melt inside the burner cabinet. If the flue was blocked up, the combustion gases would begin to back up in the flue and probably end up in the house.

So, in a sequence of operations, we want to make sure that doesn’t happen. We need to clear that hurdle before we move on to the next layer. But the board doesn’t measure inducer motor operation directly. It uses a pressure switch to sense the air pressure differential to ensure the inducer motor is running and the flue is clear.

Igniter/Flame Sensor

The flame sensor proves that the igniter has ignited the gas. Imagine if we had a broken igniter and no flame sensor. Gas would just be dumped into the heat exchanger. That would not be safe. Again, direct measurement isn’t possible, so the flame sensor fulfills this role. 

Limit Switches

Now come the limit switches. If a limit switch is open when the thermostat calls for “W,” then the system will not enter the sequence of operations. It will usually run the blower motor and (depending on the brand) the inducer motor in an attempt to cool off the system. 

But let’s say the limit switch was closed on a call for heat. The switch should open if the system overheats. Imagine what would happen if it didn’t. What if the fan motor failed and the furnace had no way of knowing that there was no air flowing through the system? The system would get continually hotter until it caught on fire. 

Layers of Safety

Each layer of the furnace sequence of operations is about ensuring homeowner safety, and they all build upon each other. The sequence will not begin if a limit switch is open. Because why would you want it to? The igniter will not turn on if the pressure switch doesn’t close first. Lighting a fire with a clogged flue pipe won’t do you much good! The flame sensor must sense a flame—otherwise, it will cut off the sequence and begin again. Why would you want the blower motor to come on if the igniter wasn’t lit and burning gas?

It took me a while to realize this, but the people who designed these furnaces are smart and have a reason for putting things together the way they did. Understanding the logic behind the basic sequence helps me troubleshoot problems and understand the big picture. 

—Matt Bruner