Homeowner's Guide to the Parts of a Furnace

Posted by James Clark on Mar 13th 2020

With 50% of homes in the United States relying on a furnace to warm their homes, we believe it's important to know as you can. Repairing an appliance can be expensive and time consuming, so we’ve decided to share with you the Parts of an 80% Furnace. Understanding how the parts of a furnace and how they work together can help you troubleshoot your issue.

Source: U.S. Energy Information Administration,2015 Residential Energy Consumption Survey

Parts of a Gas Furnace and Order of Operations

Control Board -> Draft inducer Motor -> Pressure Switch

The part of the furnace that begins the whole process is the control board or ignition module, think of this as the brains of the operations. The control board sends the electrical currents to the different parts of the furnace in order to function properly. The first part of the furnace that is triggered by the control board is the draft inducer motor, to ensure that any of the combustion gas will be eliminated from the furnace and out of the house through the chimney. The change in pressure causes the pressure switch or switches to close which sets up for the next series of actions.

Ignition Sequence -> Gas Valve Opens -> Ignitor Switch -> Flame Sensor

The next section in the anatomy of a furnace is what controls the ignition sequence. Once the control panel senses that the pressure sensor has closed it signals the ignitor to turn on and prepare for the gas to be lit. While the ignitor begins to heat the gas valve opens and allows gas to flow through the gas orifices and through the burners lighting the ignitor. The flame sensor will ensure that there are flames coming from the burners. If no flames are present the furnace the flame sensor will send a message to the control board that the furnace must be shut off.

Heat Exchanger Preheats -> Exhaust Vents from Home

After the burners have been ignited and there are flames present the air within the furnace will begin to preheat and go through the heat exchanger. This part of the furnace separates the heated air that will be forced through the ductwork of the house from the combusted air that will be forced out of the house through the chimney. Moments after the bruners preheat the heat exchanger the next sequence can begin.

Heat Exchanger -> Run Capacitor -> Blower Motor -> Pushes Air through Ductwork of the Home

Once the furnace has enough heat within the heat exchanger the run capacitor will receive command from the control panel to set off the operations of the remaining parts of the furnace. Once the run capacitor gets the signal it will send an electrical current to the blower motor to begin its job. The motor wheel that lives within the motor will begin to spin which pushes air through the heat exchanger which then goes through the ductwork to heat your home.

Fail Safe and Safety Furnace Parts

There are several parts of the furnace that are designed as fail safe measures or safety measures to keep you and your family safe. The heat inducer motor ensures that any dangerous gases are pushed out of your house through the chimney.

The pressure sensor will stall any further operations of the furnace unless it senses a pressure change because of the draft caused by gases leaving through the exhaust stack. Without of the pressure sensor engaging the ignitor will not begin to heat or spark.

There are two parts of a furnace that work with the flame from the burner to ensure safety. The first part is the flame sensor, this is a small piece of metal that can sense whether or not there is a flame present in the furnace, if there is no flame detected the furnace will shut off ensuring that no natural gas will leak into your home. The flame rollout switch will ensure that the flames from the burner are only pushed back into the heat exchanger.

The main safety feature is the limit switch, this measures the temperature of the interior of the furnace. This part of the furnace will automatically shut the entire furnace off if the limit switch is triggered because the internal temperature is too hot.

Parts of a Gas Furnace

Draft Inducer Motor

The draft inducer, also referred to as a combustion blower, sucks combustion gas through the heat exchanger to begin heating air to be distributed through the house. Pushes combusted gas out through the chimney.

Heat Exchanger

The heat exchanger separates heated air that will go through your house from combusted gas that will leave through the chimney. Heats air so that the blower fan can push heated air through your supply duct to the rest of the house.

Pressure Switch

The pressure switch proves combustion gasses are being eliminated to make sure that there is enough air flow for your furnace to begin the ignition process. If there isn’t adequate air flow the furnace will not ignite. Stops combustion air from entering your house due to inadequate flue draft.

High Limit Switch

makes sure the furnace will not overheat inside.

Flame Rollout Switch

The flame rollout switch makes sure that flames are only being sucked into the heat exchanger. If these are tripped it usually means that there is an issue with combustion gases being able to leave the furnace and can be indicative of a bad heat exchanger or flue problems.

Gas Valve

The gas valve opens and closes to allow flow of natural gas or propane in order for the furnace to ignite.


Gas is pushed through the orifice to be ignited so it can burn in the burner. The burner is shaped so that the flame can be formed into a cone.

Gas Orifices

Gas orifices are the small holes that gas is pushed through to be ignite. These are included on gas conversion kits when switching from propane to natural or vice-versa.


The ignitor lights gas to create flame. They come in two styles, spark electrode and hot surface ignition.

Flame sensor

The flame sensor is a critical safety measure to ensure furnace will only run if ignitor was switched and there are flames present. Without it, unlit gases can enter the home.

Control board

The control board is the brains of operation sends power to the the draft inducer, blower motor and monitors safeties and the sequence of operation.

Run Capacitor

Induces the phase shift to start the blower motor and helps blower motor run. When they fail, they can cause the blower motor to overheat and prematurely fail.

Blower Motor

The blower motor pushes air past the heat exchanger and through the air supply ducts. There are many types of blower motors, including PSC, ECM, Inverter (DC) and 3-Phase.

Blower Wheel

The blower is driven by the blower motor and pushes air past the heat exchanger and through the air supply ducts.

If you have any further questions on the repair process, or need help selecting the parts you need for this repair,contact us!

About the Author

James Clark

James Clark is the HVAC Controls Specialist and Ecommerce Manager for Technical Hot & Cold. He has been with the company since 2014 and enjoys helping homeowners save money by providing help articles that walk them through various DIY HVAC repairs. In his spare time, he's playing music with his children and spending too much time working on his lawn.


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