A Short Guide to a Furnace Pressure Switch

Image of a man working on a furnace with a screwdriver
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Furnaces are complex devices. That’s why understanding the functionality of certain important features is critical to maintaining a comfortable home and a healthy furnace. Here’s a quick overview of your furnace pressure switch.

What is a Furnace Pressure Switch?

Your pressure switch is a built-in feature designed to monitor the negative pressure created by the inducer motor. When you change your thermostat, you send a signal to your control board. Your control board, in turn, checks the centrifugal switch to ensure it’s open and non-energized. Your pressure switch then runs a diagnosis of your draft motor and begins moving heat throughout your home. In the event of incorrect pressure levels in your blower motor, the switch will shut down the furnace ignition.

The Importance of a Pressure Switch

Your pressure switch is designed to guard against dangerous gas leaks and prevent furnace cycling in the event of a mechanical failure. This improves the safety of your home and prevents ongoing furnace damage.

Types of Pressure Switches

The design of your pressure switch will vary according to the type of furnace you have in your home. Conventional, single-stage furnaces will have one hose leading to the body of the draft inducer furnace. Condensing furnaces have two hoses leading to the pressure switch. One hose is for sensing pressure at the draft inducer, and the other serves as a venting-pressure sensor. Two- and three-stage furnaces can have to up three hoses.

Problems with Your Pressure Switch

Pressure switches can malfunction due to age, which can prevent accurate readings and result in faulty furnace cycling. The diaphragm in your pressure switch is a common location for problems. Over time, your diaphragm can become coated or stiff, which reduces its effectiveness. Holes, tears and other damage to the diaphragm can also cause pressure switch malfunctions. A buildup of dust and dirt is another common source of pressure switch problems. Small bits of contaminants can jam the switch and cause a shutdown. Identifying the origin of your pressure switch problem can be difficult. It’s best to replace a damaged unit.

Replacing Your Pressure Switch

Furnace issues can sometimes stem from a faulty pressure switch. Here’s a quick guide to identifying and swapping out your pressure switch.

  1. Power down: Turn off the power to your furnace. Most furnaces have a power switch on the outside of the unit. If you can’t find a switch, flip the breaker that supplies power to your furnace.  
  2. Open the combustion chamber: Remove the cover to your furnace’s combustion chamber. This is usually on the top half of the unit. Unscrew or loosen the panel and set it aside.
  3. Disconnect the wires: Pressure switches have a set of wires connected to the front of the unit. Simply pull the wires toward your body to disconnect them.
  4. Detach the vacuum tube: The vacuum tube is located on the side of the unit. Some models have several tubes. Be sure to disconnect them all before continuing.
  5. Remove the mounting screws: Your switch should have several screws on the front of the unit. Loosen these screws and set them aside. The pressure switch is now free of the draft inducer motor.
  6. Install the new switch: Slide the new switch onto the motor and replace the screws.
  7. Reconnect the switch: Reconnect the wires and vacuum tube to the pressure switch.
  1. Turn on the power: Replace the combustion chamber panel and flip the power switch or breaker back on.

When to Call a Pro

If you’re still experiencing problems after replacing your pressure switch, the issue is likely elsewhere in your furnace. Call a professional to examine your unit. Troubleshooting other parts of your furnace will require working with gas and electrical systems, which can be dangerous.