Furnace pressure switches are part of the combustion controls of a modern gas furnace. We don’t want any products of combustion to enter the conditioned space. So an induced draft blower is used to ensure that the products of combustion flow out the furnace flue. The inducer also performs the function of pre-purging the heat exchanger. The furnace ignition control module must verify that the inducer is on and has established the proper draft before it signals the gas valve to supply gas to the burners. One of the devices commonly used to detect proper furnace inducer operation is the furnace pressure switch. The pressure switch provides an important safety function that helps to ensure proper furnace combustion. If the furnace ignition control module detects an unsafe condition associated with the pressure switch, the furnace is shut down.

Accurate Heating & Cooling is an established HVAC contractor serving the greater Omaha metro area. We have been in business since 1998 and we proudly perform furnace repairs and replacements. Call us today at 402-238-2425 for all your residential and light commercial HVAC needs. We also service geothermal units, mini split systems, heat pumps, and roof top units.

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What A Furnace Pressure Switch Looks Like

WARNING: Accessing components behind the furnace panels involve high voltage equipment that poses serious electrical shock risk. Follow all safe electrical work practices and ensure that only qualified, trained service technicians perform repair activities. Hazardous Voltage! Failure to disconnect power before servicing could result in death or serious injury. Disconnect all electric power, including remote disconnects, before servicing. Follow proper lockout/tagout procedures to ensure the power cannot be inadvertently energized.

The photo below shows an older pressure switch in an 80% AFUE furnace. The inducer is in the upper right hand corner of the photo. Near the bottom left hand corner of the photo is the pressure switch. It is a black plastic cylindrical object. A grey vacuum hose and two electrical leads are connected to the pressure switch.

Older Style Plastic Furnace Pressure Switch

 

A close up of part of the pressure switch is shown below.

Close Up of Older Style Plastic Furnace Pressure Switch

 

And another shot of the pressure switch is shown below. The connections for the vacuum hose and the two electrical leads are more visible.

Close Up of Older Style Plastic Furnace Pressure Switch showing vacuum hose connection

 

A metal pressure switch from an older Carrier furnace is shown below. The pressure switch is on the right and the furnace inducer is on the left.

Older Metal Carrier Furnace Pressure Switch

 

Older Metal Furnace Pressure Switches

The close up image below shows more of the pressure switch. Two electrical leads connect to the pressure switch.

Close Up of Older Metal Carrier Furnace Pressure Switch

 

Both electrical leads connect to the furnace ignition control module. In the case of this older Carrier furnace, the electrical leads run through this Molex plug to connect to the board.

Wiring Connections for Furnace Pressure Switch

 

Vinyl tubing connects the pressure switch with the heat exchanger as shown in the photo below. The clear vinyl tubing connects to the pressure switch. The black tubing in this photo is not associated with the pressure switch.

Vacuum Hose Connection for Furnace Pressure Switch

 

Principle of Operation

The furnace inducer pulls the products of combustion from the heat exchanger. Another name for the furnace inducer is the induced draft combustion blower. The inducer directs the products of combustion into the furnace flue. A negative pressure is created inside the furnace heat exchanger. The pressure switch contains a diaphragm that reacts to the negative pressure created by the inducer. Thus, when the inducer starts and establishes an adequate negative pressure in the heat exchanger, the pressure switch contacts make. An electrical signal is then sent to the furnace ignition control module to allow the remaining steps of the ignition sequence to resume.

 

Check out the video below which provides a good description how a furnace pressure switch works.

 

Newer High Efficiency Furnace Pressure Switches

Newer high efficiency furnaces have pressure switches like the one shown in the photo below. The inducer is the large object in the center of the photo. The pressure switch is the small cylindrical object near the lower right in the photo. There are two electrical connections to this pressure switch.

Newer style Vacuum Furnace Pressure Switch

 

And a close up of this newer style pressure switch is shown below.

 

Close Up of Newer style Vacuum Furnace Pressure Switch

 

Diagnosing Failures

One of the reasons for a furnace failure may be indicated as an LED flash code for a pressure switch issue. We often find that a pressure switch LED code indicates some other issue with the furnace. There may be obstructions in the combustion air pipes supplying the furnace. There may be a plugged condensate drain in the furnace. Or a weakening or failed inducer motor. And a plugged vacuum line may be the cause. Rarely is the cause of the issue a failed pressure switch.

Some of the older metal pressure switches can seize up. After being idle all summer long we sometimes see the older metal pressure switch not work the first time the furnace is turned on in the fall. In these cases, it can be an easy fix to get the pressure switch freed up so that it is working again. But usually, a pressure switch issue is caused by some other issue.

If your furnace will not operate and the ignition control module is flashing a pressure switch error code, give us a call. Our office phone number is 402-238-2425. We will come out and diagnose and repair the issue and get the heat back on. Also call us for all your light commercial HVAC needs.

Have Us Out For A Fall Furnace Clean & Check

We can identify when an inducer is beginning to weaken when we are doing a pre-season furnace clean & check. Many of our Omaha customers have us out in the fall to perform this important furnace service. This pre-season service helps to keep your furnace operating trouble free all season long. It gives us a chance to identify parts when they are weakening and get them replaced before they fail. For a list of HVAC maintenance items that homeowners can perform to help ensure their systems operate well, refer to this blog.

When It Is Time For Replacing HVAC Equipment

There comes a time when older HVAC equipment requires more repair work to keep it running and replacing the furnace or air conditioner becomes a priority. When this time comes, we can help you decide what replacement equipment is best for your particular application.

We proudly install Trane and Armstrong HVAC equipment.

For more information about the Trane equipment that we install, refer to our equipment page. It also may be handy to check out the Trane website for more information.

For more information about these 80% efficiency Armstrong furnaces, consult this Armstrong 80 Furnace Brochure 2019. Or the Armstrong website here also offers additional information on furnaces and other HVAC products. Information about Armstrong 90% and higher furnaces can be found in our blog as well.

Additional Information

Accurate Heating & Cooling has been in business for over 20 years. We are your residential furnace repair experts. Call us today at 402-238-2425 for all your HVAC heating needs. Also check out what our customers are saying about us on our customer testimonials page.

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Check our out Home Page here to learn more about us.

Information describing what the MERV rating on air filters means is available here.

For a discussion on what air filter would be best, refer to this blog.

For technical material on troubleshooting a low voltage transformer failure, refer to this blog.