Everything You Need to Know About a Furnace Pressure Switch

Updated on April 30, 2018
Cre8tor profile image

Dan has been in the HVAC industry for 22 years with experience ranging from installation and service to sales and distribution.

The pressure switch in a furnace detects proper ventilation of the unit.
The pressure switch in a furnace detects proper ventilation of the unit.

Why Won't My Furnace Light?

I get this question a lot from readers and while there are various possibilities, they usually believe it's the igniter or the gas valve that is failing. Though these are valid suspicions of the failure cause, there is one that is often overlooked because it's not understood: the pressure switch. This

Let's take a closer look at this part: what it does, how it works, and what the purpose of it is so that if your furnace ever fails to light you might be able to figure out if the pressure switch is your problem and what you can do to fix it. Or perhaps understand why that blinking red light says "the pressure switch is open" and yet might not be the problem.

What does a pressure switch do?

A pressure switch is a pretty simple component. Like all switches it makes a connection so that power can pass through it. Think of all switches like a drawbridge but instead of cars it has electricity waiting to pass over it. When the bridge is open nothing goes across but once closed, traffic can pass.

In the case of a light switch, no lights will come on until you flip the switch by hand and allow power to the light but your pressure switch is activated by air pressure or in this case suction. The suction your furnace's pressure switch needs to work is created by the inducer fan or draft inducer. When that fan comes on it will pull a vacuum inside the pressure switch through that rubber tube you see connecting the two which then pulls the switch in an closes (connects) the circuit to allow power to pass to the next component which is the igniter.

The Pressure Switch Connects to the Inducer Fan Housing

Image of a pressure switch connected to the inducer fan housing on a furnace.
Image of a pressure switch connected to the inducer fan housing on a furnace.

The Purpose of Your Furnace's Pressure Switch

The pressure switch in your furnace is used as a safety mechanism. It is there to confirm the unit is drafting properly. If the unit is not drafting properly then it does not operate as efficiently and creates a safety risk by not removing unwanted gases that can make us sick or cause small explosions inside the heat exchanger. This is why if the pressure switch isn't satisfied with the right amount of suction it will not allow the unit to ignite.

What is a high pressure switch?

Different furnaces may have different types of pressure switches in them. Based on the size of the unit (meaning heat output capability, not the physical dimensions) the amount of suction the pressure switch requires to work will vary. But when talking about high and low pressure switches it typically has to do with the number of stages a unit can operate in.

While many of us still have a single stage furnace that runs at full capacity every time it turns on, there are two stage units that will run at a lower heat output and then when needed, kick into a 2nd stage for more heat. These types of furnaces require a dual pressure switch so it can monitor the level of suction in low fire and then another in high fire since they require different amounts of ventilation to function properly.

Some two stage furnaces will use what is commonly referred to as a pancake switch since the two pressure switches are "pancaked" together while others will have two separate pressure switches. Either way, one of these switches is called the low pressure switch and the other a high pressure switch.

Click thumbnail to view full-size
This is what a "pancake" type pressure switch looks like and if you need to replace it you just replace them both since that is likely how you will have to buy it.This is an example of separated high and low pressure switches which can be bought in singles.
This is what a "pancake" type pressure switch looks like and if you need to replace it you just replace them both since that is likely how you will have to buy it.
This is what a "pancake" type pressure switch looks like and if you need to replace it you just replace them both since that is likely how you will have to buy it.
This is an example of separated high and low pressure switches which can be bought in singles.
This is an example of separated high and low pressure switches which can be bought in singles.

Causes of Pressure Switch Failures

Bad/Shorted Switch
Inducer Fan Running Too Slow
Inducer Fan Motor Is Bad
Blockage In Inducer Fan
Blockage In Flue/Chimney
Crack/Hole In Suction Tube
Bad/Loose Wire Connection
Water Built Up In The Inducer Fan Housing (High Efficiency Furnaces)
Loose Suction Tube
 
 
 

A Pressure Switch Can Be Tricky...

This part is finicky so it can fail and then work again making it hard to pinpoint. A downdraft of air can cause temporary failure. A "tired" switch may work today and not tonight, then again tomorrow. Intermittent failures are harder to diagnose so watching and listening the unit run, or try to run, may be helpful in pinpointing your problem.

What To Do When Your Pressure Switch Fails

The table above gives you quite a few causes of pressure switch failure that are easily confirmed or eliminated as the problem either audibly or visually. You should start with checking those things since they are the easiest, don't require special tools, and cost nothing.

  • First, as the unit begins to try and run (meaning after the inducer has ran for 10-30 seconds) do you hear the gas valve "click" open? The gas valve will not open if the pressure switch doesn't work so the "click" sound of the gas valve opening tells us that the pressure switch is fine and the igniter is likely our problem since we should also notice it didn't try and light just prior to the "click" we heard. If we don't hear the click...
  • Listen to see if you can hear anything rattling around in the inducer fan. If so, see if you can remove it. (Be sure to turn the power off first) If you hear the motor whining, grinding, or running seemingly slow then you may need to replace it and the switch is likely doing what it's supposed to do.
  • Look to see if the switch hose is tight and in tact. If it's loose try to tighten it up a bit and if it's cracked or has a hole, see if you can seal it or cut away the cracked material and reconnect. Be careful as the inducer housing may be hot if the unit has ran recently. Again, this would show the switch doing it's job and it should work now that the hose is fixed.
  • Look for any blockage in the flue you can see. If one is found, remove it and your problem should be solved.

If you've taken a few minutes to rule these things out and still have a problem then we can confirm the switch is bad by testing the circuit with a voltmeter and maybe get the unit working temporarily for a little heat at least.

  • If you have a voltmeter, shut off the power to the unit. Remove the wires from the pressure switch and test for voltage across the 2 terminals. If it measures "0" then the switch is closed when it shouldn't be (shorted or stuck). If it measures "infinite" (O.L.) the switch is not shorted and could just be sticking open. Either way we'll need to replace the switch but perhaps we can still get it to work temporarily and get the heat back on for a bit. To do this...
  • Remove the hose from the inducer housing (again being careful since the inducer housing may be hot if the unit has ran recently) and gently suck and blow on the end of it. You should be able to hear the switch opening and closing. Do this a couple of times and then reconnect the tube to the inducer housing and see if it will work. Sometimes the switch will get stuck and just needs a little help loosening up. Even if this works, replace the switch soon. It is quite likely the switch will stick again and it's always better to fix it on your terms than the typically inconvenient timing it will choose to breakdown again.

Click thumbnail to view full-size
Voltmeter showing resistance meaning our pressure switch is closed and since there is no vacuum, it shouldn't be and therefore has shorted.Voltmeter showing "infinite" (O.L) meaning there is not connection so there is "infinite" resistance because the switch is open and not shorted.
Voltmeter showing resistance meaning our pressure switch is closed and since there is no vacuum, it shouldn't be and therefore has shorted.
Voltmeter showing resistance meaning our pressure switch is closed and since there is no vacuum, it shouldn't be and therefore has shorted.
Voltmeter showing "infinite" (O.L) meaning there is not connection so there is "infinite" resistance because the switch is open and not shorted.
Voltmeter showing "infinite" (O.L) meaning there is not connection so there is "infinite" resistance because the switch is open and not shorted.

Replacing A Pressure Switch

The actual work involved in replacing a pressure switch is quite simple and doesn't require any specialty tools that you don't likely have laying around the house.

  • Turn off the power to the unit
  • Disconnect the wires from the switch
  • Disconnect the hose from the switch
  • Remove the switch from the bracket (or bracket too if your new one has it mounted already)
  • Fasten the new switch back in place
  • Reconnect the hose to the switch
  • Reconnect the wires to the switch

See how easy that is?

How Much Does a Pressure Switch Cost?

A single stage pressure switch whether from the factory or universal can cost anywhere from $20-$50 while pressure switches for a 2 stage unit can run in the $30-$75 range.

Finding and Buying A Replacement Pressure Switch

Like many furnace components, the pressure switch is not an item typically found in the "big box" stores like Lowe's and Home Depot. To find a replacement you will likely have to do one of two things: contact the contractor supply house that sells your brand of furnace or obtain a universal replacement from a contractor supply house or online.

Furnaces are much like cars and have parts that are specific to their brand. That is why if you want a direct factory replacement you have to find the supplier that carries your brand. It's a "don't call a Chevy dealer for Ford parts" sort of thing. Once you've found the supply house that carries your brand then be prepared to tell them what part you are looking for and have your unit's model and serial number handy so they can help you. That said, many of these suppliers are "contractor only" and will not sell to the general public so the more prepared you are, the more likely you are to get them to sell to you or at least give you a part number so you can look online for it.

A universal pressure switch however may be easier to find and purchase at any supply house or online. I keep an NS2 universal pressure switch handy so that if my or a family member's unit breaks down at an inopportune time, I can make the repair without going hunting for one. These are nice because they can handle single stage and dual stage units as well as be set to various pressures. They also contain brackets that make mounting possible in more than one unit. The instructions walk you through the process of setting up the switch to suit your unit's needs.

Click thumbnail to view full-size
NS2 Universal Pressure SwitchWhat is included in an NS2 universal pressure switch.
NS2 Universal Pressure Switch
NS2 Universal Pressure Switch
What is included in an NS2 universal pressure switch.
What is included in an NS2 universal pressure switch.

I hope the information I've given you regarding your furnace's pressure switch has helped you deduce whether or not the pressure switch is the problem you're having or if another part was a cause. If your still not sure what's going on then I recommend you contact a technician. We don't want to make matters worse or get in over our head.

~ We're all in this together ~


Feedback Is Important to Me

Did this article help you with your questions or issue involving a pressure switch?

See results

Questions & Answers

    © 2018 Dan Robbins

    Comments

      0 of 8192 characters used
      Post Comment

      No comments yet.

      working

      This website uses cookies

      As a user in the EEA, your approval is needed on a few things. To provide a better website experience, dengarden.com uses cookies (and other similar technologies) and may collect, process, and share personal data. Please choose which areas of our service you consent to our doing so.

      For more information on managing or withdrawing consents and how we handle data, visit our Privacy Policy at: https://dengarden.com/privacy-policy#gdpr

      Show Details
      Necessary
      HubPages Device IDThis is used to identify particular browsers or devices when the access the service, and is used for security reasons.
      LoginThis is necessary to sign in to the HubPages Service.
      Google RecaptchaThis is used to prevent bots and spam. (Privacy Policy)
      AkismetThis is used to detect comment spam. (Privacy Policy)
      HubPages Google AnalyticsThis is used to provide data on traffic to our website, all personally identifyable data is anonymized. (Privacy Policy)
      HubPages Traffic PixelThis is used to collect data on traffic to articles and other pages on our site. Unless you are signed in to a HubPages account, all personally identifiable information is anonymized.
      Amazon Web ServicesThis is a cloud services platform that we used to host our service. (Privacy Policy)
      CloudflareThis is a cloud CDN service that we use to efficiently deliver files required for our service to operate such as javascript, cascading style sheets, images, and videos. (Privacy Policy)
      Google Hosted LibrariesJavascript software libraries such as jQuery are loaded at endpoints on the googleapis.com or gstatic.com domains, for performance and efficiency reasons. (Privacy Policy)
      Features
      Google Custom SearchThis is feature allows you to search the site. (Privacy Policy)
      Google MapsSome articles have Google Maps embedded in them. (Privacy Policy)
      Google ChartsThis is used to display charts and graphs on articles and the author center. (Privacy Policy)
      Google AdSense Host APIThis service allows you to sign up for or associate a Google AdSense account with HubPages, so that you can earn money from ads on your articles. No data is shared unless you engage with this feature. (Privacy Policy)
      Google YouTubeSome articles have YouTube videos embedded in them. (Privacy Policy)
      VimeoSome articles have Vimeo videos embedded in them. (Privacy Policy)
      PaypalThis is used for a registered author who enrolls in the HubPages Earnings program and requests to be paid via PayPal. No data is shared with Paypal unless you engage with this feature. (Privacy Policy)
      Facebook LoginYou can use this to streamline signing up for, or signing in to your Hubpages account. No data is shared with Facebook unless you engage with this feature. (Privacy Policy)
      MavenThis supports the Maven widget and search functionality. (Privacy Policy)
      Marketing
      Google AdSenseThis is an ad network. (Privacy Policy)
      Google DoubleClickGoogle provides ad serving technology and runs an ad network. (Privacy Policy)
      Index ExchangeThis is an ad network. (Privacy Policy)
      SovrnThis is an ad network. (Privacy Policy)
      Facebook AdsThis is an ad network. (Privacy Policy)
      Amazon Unified Ad MarketplaceThis is an ad network. (Privacy Policy)
      AppNexusThis is an ad network. (Privacy Policy)
      OpenxThis is an ad network. (Privacy Policy)
      Rubicon ProjectThis is an ad network. (Privacy Policy)
      TripleLiftThis is an ad network. (Privacy Policy)
      Say MediaWe partner with Say Media to deliver ad campaigns on our sites. (Privacy Policy)
      Remarketing PixelsWe may use remarketing pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to advertise the HubPages Service to people that have visited our sites.
      Conversion Tracking PixelsWe may use conversion tracking pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to identify when an advertisement has successfully resulted in the desired action, such as signing up for the HubPages Service or publishing an article on the HubPages Service.
      Statistics
      Author Google AnalyticsThis is used to provide traffic data and reports to the authors of articles on the HubPages Service. (Privacy Policy)
      ComscoreComScore is a media measurement and analytics company providing marketing data and analytics to enterprises, media and advertising agencies, and publishers. Non-consent will result in ComScore only processing obfuscated personal data. (Privacy Policy)
      Amazon Tracking PixelSome articles display amazon products as part of the Amazon Affiliate program, this pixel provides traffic statistics for those products (Privacy Policy)