How a Furnace Works: 13 Things You Need to Know

Have you ever wondered how a furnace works? Well, it’s really cool . . . well, hot. Unless it’s not working, then that’s not cool.

Even though furnaces and air conditioners can be a little intimidating. The more you understand about how a furnace works, the better-prepared you’ll be when it comes to making repair or replacement decisions.

Now, for ease of explanation, we’ve broken your furnace down into two main parts:

The furnace where the air gets heated.
The living space where the warm air is distributed throughout the house.

Even though there’s more to how a furnace works than the basics we’re going to go over today, this should give you enough information to answer the question, how does my furnace work?

CIRCUIT BOARD
INDUCER MOTOR
IGNITOR
GAS VALVE
FLAME SENSOR
HEAT EXCHANGER
EXHAUST VENT
COMBUSTION AIR
BLOWER FAN
FILTER

Now, this is a high-efficiency unit, you can tell because it’s got PVC piping for the combustion and the exhaust. If this was not a high-efficiency furnace, you would notice a metal vent going out of the top and you might notice louvered doors because that’s how it brings the combustion air in.

Either way, if yours is high-efficiency or not, the information we will tell you about how a furnace works will apply to either one.

In the living space, you’ll find:

Thermostat
Supply Vents
Return Air Vents

There are a lot of things that have to happen in a very specific order in order for your furnace to work. I’m going to try and strike a balance between not enough information and too much information. Hopefully, it’s just right.

Now, back to how a furnace works! It all starts when the temperature in a room drops below the thermostat setting. The thermostat sends a signal to the circuit board calling for heat. The inducer motor turns on and draws combustion air into the furnace. This helps to make sure the gas is burning efficiently.

Next, an ignitor will either glow or spark to ignite the gas that starts flowing. This gas ignites into flames while the hot air and obnoxious gases are drawn into the heat exchanger. To make sure that the gas ignites, there’s a flame sensor that’s placed right into the path where the flame should be. If a flame is detected, the heating process continues while all of the hot air is being drawn into the heat exchanger. The heat exchanger gets very hot and as the air blows across it, it heats up and is blown out through the house.

Before we get to that, we need to talk about where the fumes, gases, and carbon monoxide goes. The heat exchanger tubes have openings on either end. The flames superheat the air that enters into one end, then all of the dangerous gases exit out the other end which is connected to an exhaust which vents outside of the house.

Back to the air that’s blowing across the heat exchanger. This is the real secret of how a furnace really works. With help from the blower fan, the cool air in the home is drawn through the return vents, through the filter, and across the heat exchanger. Then, the newly warmed air is blown through the ducting and out the supply vents all-around your home. The air cools off and is drawn back through the return vents and the process repeats itself until the room is warm enough and the thermostat sends a signal to stop. And that’s how a furnace works!

These 13 things are designed to work together and if even one of them stops working, you can experience a variety of problems, most of which result in your furnace blowing cold air (which is not what you want in the middle of winter)

Oct 30th 2019

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