Hand with multimeter testing furnace pressure switch

How to Check a Pressure Switch on a Furnace: Complete Guide

The pressure switch, responsive to the negative pressure created by the draft within a single-stage conventional furnace, is a modest but key element of your furnace system. Acting as a safety component, the pressure switch monitors and regulates the pressure levels in the flue of a single-stage condensing furnace, ensuring your furnace operates safely by preventing potential hazards.

But how can we tell if a normally open switch is doing its job right, or if it needs some attention? Our ‘How to Check a Pressure Switch on a Furnace’ guide will unravel the mystery behind testing and maintaining a furnace’s pressure switch and the role switch terminals play in this process.

Key Takeaways

  • In a single-stage condensing furnace, the pressure switch plays a significant role in observing and managing pressure levels, responding to a heat call to ensure safe operations.
  • Regularly checking and maintaining the pressure switch can help prevent furnace breakdowns and increase energy efficiency.
  • Common problems with the pressure switch include short-cycling, failure to start, and false negatives caused by environmental factors or a dirty/clogged furnace.
  • While checking a pressure switch with a volt meter, use a multimeter to test for continuity and resistance, and if any part of this process is uncertain, seek professional help or prioritize safety.

Understanding the Role of a Pressure Switch in Your Furnace Systems

How to Check a Pressure Switch on a Furnace: Complete Guide featuring a person with multimeter testing furnace pressure switch

Often, the pressure switch, a safety part in a furnace system that manages the flow of gas to the system’s burner, is overlooked despite its significant role. It works as a safety mechanism, continuously monitoring the pressure conditions within the furnace. If the pressure levels aren’t optimal, the gas furnace pressure switch triggers a shutdown, preventing potential hazards.

There are two types of pressure switches—open and closed. An open switch will close if the pressure is at the correct level, kickstarting the furnace’s operation. In contrast, a closed switch, a safety measure linked to the flue pipe, will open if the pressure in the pipe is too high or too low, stopping the combustion operation.

Understanding the role of a furnace pressure switch, whether in an open or closed state, is essential for keeping your furnace in top-notch shape. A faulty switch could lead to serious complications, from inefficient heating to potential gas leaks. Hence, understanding how a furnace startup works and how to test a furnace pressure switch, which can condense the gas back into a liquid, is fundamental.

Recognizing Common Furnace Pressure Switch Problems

How to Check a Pressure Switch on a Furnace featuring a Close-up of multimeter use on furnace pressure switch

A few common problems can arise with the pressure switch in the furnace’s venting system. One leading sign of a faulty switch or diaphragm is ‘short-cycling’, where the furnace starts but quickly shuts off due to the exhaust fumes, indicating that the switch is failing to stay closed. This could indicate the pressure switch isn’t detecting the right level of venting pressure at the condensate collector.

Another common problem is when the furnace doesn’t start at all. This could mean that the pressure switch is stuck in the open position, which would prevent the furnace from igniting when pressure at the draft builds up.

Environmental factors like excessive wind or a blocked exhaust vent can create a false negative on the switch, causing it to shut off the furnace. This situation is often created by the draft inducer. In a situation where a furnace malfunctions due to being dirty or clogged, it can lead to the switch sensing incorrect pressure levels.

Tools and Preparations for Checking a Furnace Pressure Switch

Tools arrayed next to furnace with detached pressure switch

Before checking a furnace pressure switch and the connected diaphragm, gather the right tools, including a cracked hose, and understand the necessary safety precautions to deal with negative pressure. For a single-stage conventional furnace, you’ll need a multimeter to check the continuity and resistance of the pressure switch, a screwdriver to remove the furnace panels, and potentially a torch to find the switch within the furnace.

Before commencing any tests, make sure the furnace is powered off to prevent accidental shocks or burns – this includes the pressure switch that could be stuck open, which is a basic safety feature. Locate the pressure switch, which is typically round, about a couple of inches in diameter, and located near the draft inducer motor and the hose leading to the blower.

Step-by-Step Guide to Testing Your Furnace’s Pressure Switch

Hands using multimeter on furnace pressure switch

With our tools ready and safety measures in place, let’s walk through the process of testing your furnace’s pressure switch and understanding how it responds to the negative pressure created.

  1. Access the Pressure Switch: Find the pressure switch and disconnect it from your furnace control board.
  2. Set up the Multimeter: Set it to test for resistance (ohms). If the switch has failed to discern if it’s open or closed, there should be little to no resistance. This would mean you need a new pressure switch.
  3. Test the Pressure Switch: Attach the multimeter probes to the open or closed terminals of the switch in your single-stage conventional furnace. If the pressure switch is a safety device and is in good condition, the voltage reading should be close to zero.
  4. Interpret the Results: If your reading is significantly more than zero, this indicates that the pressure switch is open when it should be closed, suggesting something is wrong.

Our guide offers an innovative, hands-on approach to maintaining your furnace’s pressure switch and blower system, ensuring that the switch closes at the right time, but always prioritize safety when doing any testing.

Fixing Common Pressure Switch Issues and General Furnace Maintenance

Acquiring knowledge on how to check, clean, reinstall and repair the hose port of the pressure switch is essential for ensuring the furnace’s efficiency. Here’s a comprehensive table showing the steps to address general pressure switch issues, including a clogged hose port.

1. InspectCheck the pressure switch for any visible damage.
2. CleanUse a soft brush to gently clean the pressure switch.
3. TestUse a multimeter to test if the switch is functioning properly.
4. ReplaceIf the switch is defective, replace it with a new one.
5. Test AgainCheck the new pressure switch to ensure it’s working.

Periodically inspect and clean the condensate collector box and the clogged hose port in your furnace. This includes cleaning or replacing the filter, lubricating moving parts, checking the thermostat, and the hose connected to the pressure switch work properly in the furnace system. However, if you’re uncomfortable performing these tasks, such as troubleshooting the hose that is connected to the pressure switch or dealing with the suction causing negative pressure, it’s always best to seek professional HVAC help.

Do LED lights affect the pressure switch on a furnace?

Yes, LED lights indeed can affect the pressure switch on a furnace if not properly managed. It is important to understand the specific led lighting ballast requirements to ensure that the LED lights do not interfere with the functioning of the pressure switch on the furnace.


We’ve covered the basics of understanding the role of pressure switch in your furnace, which includes responding to a call for heat, recognizing common problems, preparing tools, and the procedure of testing and resolving issues with the furnace pressure switch, such as when a furnace hasn’t turned on, is best left to a qualified technician.

With this guide, we hope you’re now equipped to handle any pressure switch or diaphragm problems with confidence. Remember, regular maintenance is key to prevent these issues.

If you’re ever in doubt, for instance, if something is wrong and your furnace still isn’t working right, don’t hesitate to call a professional.

Stay warm and safe.

Frequently Asked Questions

What is a pressure switch on a gas furnace?

A pressure switch on a gas furnace is a safety device that ensures the proper operation of the furnace’s venting system. It detects whether the draft inducer fan is running and whether the pressure in the venting system is within the correct range.

How does a pressure switch work on a gas furnace?

The pressure switch on a gas furnace is designed to open when it senses negative pressure in the venting system. When the draft inducer fan starts, it creates negative pressure, which closes the pressure switch and allows the furnace to ignite and operate.

What are the signs of a faulty pressure switch on a furnace?

Common signs of a faulty pressure switch on a furnace include the furnace not starting, the furnace’s igniter not glowing, or the furnace’s main gas valve not opening. These issues can be indicative of a malfunctioning pressure switch.

How can I test a furnace pressure switch?

To test a furnace pressure switch, you can use a multimeter to check for continuity or voltage across the switch terminals. You can also use a small screwdriver to manually actuate the switch and observe the readings on a multimeter.

Can a pressure switch on a furnace be bypassed?

Bypassing a pressure switch on a furnace is not recommended as it can compromise the safety of the system. The pressure switch is essential for ensuring proper venting and preventing the furnace from operating if there are issues with the venting system.

Lucy Dearing

Greetings! I'm Lucy Dearing, passionately immersed in the world of home improvement. Together with my husband, Danny, we strive to create spaces that are both delightful and practical. We believe in offering accurate and transparent advice, engaging with our readers on a journey to bring their dream homes to life. Trust us to guide you every step of the way.

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