What is a Furnace Draft Inducer Motor and Pressure Switch?

By | June 7, 2022

What is a Furnace Draft Inducer Motor and Pressure Switch

  • A Furnace Draft Inducer Motor Vents Toxic Gases Outside.
  • A Furnace Pressure Switch Confirms the Draft Inducer Motor is Working.

A draft inducer motor and pressure switch work together to make sure toxic gasses are vented from a furnace.
What is a Furnace Draft Inducer Motor and Pressure Switch 2
When a furnace turns on, one of the first components to start is the inducer motor.

The inducer motor is a small fan that pulls air from the combustion chamber and vents it out of a home.

Since the flames in a furnace are toxic carbon monoxide, they must be vented outside.

When the inducer motor is running, it creates a small vacuum inside the furnace combustion chamber, which makes sure all toxic gases are vented.

For safety, a pressure switch is used to tell if the inducer motor has turned on. It does this by sensing the vacuum that the inducer motor creates and closes a switch.
Furnace Pressure Switch
If the inducer motor does not turn on, then the pressure switch stays open and keeps the gas off.

When an inducer motor goes bad, it will either stop spinning or get out of alignment and make noise when they spin.

The motor and the fan blade are balanced and aligned at the factory, so if you have to buy an inducer, you will likely have to buy the whole assembly.

To test the pressure switch a multimeter needs to be used and measure the continuity of the switch as it opens and closes.

This is done by taking the two wires off the pressure switch and touching them with the multimeter leads. The multimeter should be set to read continuity.

When the furnace is turned on, the draft inducer motor will begin to blow the air out the vent and create a vacuum. The vacuum should be measured by the pressure switch, which will close the switch. A multimeter will see if there is any change and if the switch closes or not.


Steps to Test the Furnace Pressure Switch

  1. The two wires on the pressure switch are disconnected.
  2. A multi meter is set to read continuity and the leads placed on the switch.
  3. The furnace is then turned on.
  4. If the switch is working, it will close and show on the meter.
  5. If it is not working, then it will not close, and the meter will not change.

If the pressure switch test bad keep in mind that the diaphragm needs to see a vacuum or it will not work.

Often when this happens something is clogging the line to the switch or stopping air from the inducer motor from being vented.

Category: Furnace repair

About Aaron H. Benetti

Aaron H. Benetti has worked in the HVAC field since 1991. He began his career as an HVAC installer and later began doing troubleshooting and repairs. Around 2010, he began to write books on HVAC topics, mainly tutorials and how-to information. Currently, Aaron writes many articles and posts for websites as well as books and videos on HVAC topics.

3 thoughts on “What is a Furnace Draft Inducer Motor and Pressure Switch?

  1. Mike Schawartskof

    My furnace is giving me 2 blinks as an error message, its Armstrong unit, the error in the manual means, Pressure switch closed while inducer not running, the problem that the inducer is running, that what is causing the pressure switch to close, but the control board is not receiving a signal that the inducer is running. Any ideas what is wrong here?

  2. Chrissy

    Recent had issues with my central air and called a repairman. He states unit has to be replaced and when he came to install it he then states that the heating portion was also damaged and needed to be replaced. He ordered just a part for it. Think it was damaged coils if I remember correctly Now he installed it and is saying that the pressure switch is bad. Does this sound right and is this something he should have known when he ordered the other parts.

  3. 55 Chevy Dave

    Yea, pressure switch should have been first thing checked along with inducer motor. Did you check your coils for pin holes? If it had any sign of a hole then inducer motor may not of created a vacuum to start furnace. – get it?
    Then the P/S wound not of activated but still be good. comprehendie partner.


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