What Does a Pressure Switch Do on a Furnace?

Furnaces have many components that work together to generate a heat source, but a pressure switch is one part of the system that a furnace operator might be most familiar with. Without a properly functioning pressure switch, a furnace can’t function at all.

A pressure switch is a safety feature that is designed to automatically shut the furnace off if it senses negative motor pressure. If the draft inducer introduces exhaust fumes back into the system, this can lead to backdrafting. Backdrafting in a furnace can cause fires or even an explosion. 

Learning to identify and troubleshoot the pressure switch is an important skill for furnace owners to master as a part of regular home maintenance. Read on to learn more about the pressure switch on a furnace and how it works. 

What Is A Pressure Switch Designed to Do? 

In any mechanical device where there is a build-up of pressure that could threaten the device, pressure switches are installed to help prevent the pressure from building too high. Using sensors, these switches can adjust the amount of pressure in the system and keep it on an even keel. 

In furnaces, a pressure switch works on a similar concept but prevents the backdrafting of air and exhaust fumes into the ignition system.

In operation, this means that the pressure switch will stall out the ignition system in the furnace and cut it off to keep it from running. (Source: Repair Clinic)

Pressure Switch Types

There are only two main types of pressure switches:

  1. Electronic
  2. Mechanical (electromechanical)
 Electronic Pressure SwitchesMechanical Pressure Switches
How do they work?Such switches have electronic pressure sensors. A lot of solid-state switches can not only open and close the contacts, but also transmit the value of the implied pressure.These switches measure pressure changes with the help of mechanical means (pistons, Bourbon tubes, diaphragms, etc.).
Main advantagesHighly vibration- and shock-resistant; long lifespan; flexibility (delay point, switch point, etc. can be adjusted); can work in low-voltage applications.High reliability; can operate without auxiliary power; in general, can handle higher voltages.

When it comes to furnaces, the pressure switches in different units are going to have their own peculiarities.

A single-stage conventional furnace has one switch with a hose that leads to the draft inducer fan.

A single-stage condensing unit has a switch with two hoses.

Finally, two-stage units have two pressure switches. 

What Is the Difference Between Pressure Switch and Pressure Transmitter?

These are the devices in the pressure sensing industry. They are extremely accurate, but you should know that each of them has its own function.

A pressure sensor is an all-encompassing term that includes devices that are able to measure pressure and convert it to an electrical signal. Pressure switches and transmitters, for example, are pressure sensors.

A pressure transmitter can ‘translate’ the pressure into a current and send the signal over a relatively long distance. Such a device is used for measuring and communication, but it can’t control or change the pressure.

A pressure switch is a device that gets triggered once a certain pressure is reached (it doesn’t translate the pressure values). The pressure that triggers the switch can be high or low and can either disengage or engage a circuit. 

What Happens When a Pressure Switch Goes Bad?

When you discover that your furnace has suddenly stopped working, a pressure switch is usually the first suspect to look at it if the furnace still has electrical power.

Due to its nature as an automatic safety cut-off, a pressure switch will not allow the furnace to start back up until the component’s malfunction is addressed. 

Several different causes that can contribute to a pressure switch on a furnace going bad. Here are some of the factors that may lead to a pressure switch failure: 

  • Age: Like any mechanical component, a pressure switch doesn’t last forever and will eventually wear out from being used under load. Age and the environmental exposure that goes with it can eventually cause the pressure switch to stop functioning.
  • Polluted pressure switch diaphragm: A pressure switch diaphragm can sometimes become coated or stiff. This in turn can cause the switch’s sensors not to function properly.
     
  • Physical damage: Cracks or other physical damage to the pressure switch diaphragm will prevent the pressure switch from working correctly. This kind of malfunction can usually be observed by inspecting the switch during the troubleshooting process.
  • Debris: In some cases, debris can get caught in the pressure switch and this can cause the pressure switch to become stuck open

To figure out which cause is contributing to the malfunction of a furnace pressure switch, owners need to access the furnace control panel and doing a troubleshoot on the system.

For furnace owners that aren’t comfortable doing this process themselves, there are furnace repair technicians that can perform this test as part of diagnosing a furnace malfunction. 

Testing the furnace pressure switch yourself is usually the first step in checking over a failed furnace.

If the pressure switch is bypassed and the furnace operates, then the switch itself has failed. But if bypassing the switch doesn’t cause the furnace to operate, the problem lies elsewhere in the system. 

How Do I Know If Pressure Switch Is Bad?

There are plenty of systems that can have a pressure switch. If you have a gas furnace, the chances are high that, at one point, you would have to deal with your furnace’s pressure switch.

If you have a bad pressure switch:

  • The furnace will not operate at all
  • The furnace will shut down
  • There will be a flashing light on the circuit board that serves as an indication of such an error code
  • You might have only room temperature air coming from the vents

How Do You Check or Test Pressure Switch on a Furnace?

If you want to test the functionality of your furnace pressure switch at home without having to call a technician, the process is pretty simple.

The first step is to shut off your furnace power via the disconnect switch or—if the furnace doesn’t have one—the breaker box circuit that controls the furnace power source.

Once the furnace is shut down, you can check the pressure switch using the following steps: 

  • Check the pressure switch hose. If there are any holes or leaks in the pressure switch hose, this can prevent the pressure switch from functioning. If the pressure switch hose is damaged, it should be replaced.
  • Check the pressure switch hose port. If the hose port is cracked, this can cause a pressure leak that prevents the pressure switch from working properly. If this is the problem with the pressures witch, the port must be replaced or the switch itself.
  • Check the furnace cover. Ensure that any ventilation openings are not obstructed by dust or other debris, as this can affect the operation of the pressure switch. (Source: Spruce)
  • Check the ventilation pipe. Any obstructions in the pipe opening can cause a backup of exhaust fumes in the motor that will deactivate the furnace pressure switch. 
  • Use a multimeter to check the switch resistance. Toggling a multimeter to measure Olms and hooking it up to the furnace pressure switch will allow you to get a resistance reading. Anything above a reading of 0 indicates that the electrical components of the switch have failed and it needs to be replaced. 

Once you determine that the pressure switch on the furnace is the source of the furnace malfunction, you can make a plan to move forward.

Some furnace pressure switches can be easily replaced by hand with just a few simple tools and a little electrical know-how. Others are more complicated and would be better installed by an HVAC technician

How Much Does Pressure Switch Replacement Cost?

The cost of replacing a pressure switch varies wildly depending on which model of furnace you have and whether or not you can diagnose and replace the pressure switch on your own without a technician.

For most models, the price of replacing a pressure switch on a furnace falls between $40 and $350. (Source: Whipple Plumbing)

How Do I Find a Replacement Pressure Switch?

The easiest way to find a replacement pressure switch is to contact the manufacturer of your furnace and order a new part. There is also the option with some models to check online at contractor supply outlets for 

Will A Furnace Technician Put In a Pressure Switch I Bought? 

It depends on the technician. In many cases, a furnace technician will not use parts supplied by the owner for a few reasons: 

  • Furnace owners often purchase the wrong part, model, or variant.
  • The pressure switch might not be the problem. A furnace repair technician will want to diagnose the furnace rather than just replace parts.
  • Using an owner’s own parts opens the furnace technician up to liability if the furnace fails after installation.
  • Furnace technicians make a profit on selling parts at markup cost. 

If you want a furnace technician that is willing to put in a pressure switch that you bought, you may be able to find one if you look long enough.

This is a way that you can save money on the pressure switch replacement. But in many cases, it can be easier to let the furnace technician handle sourcing the parts themselves.  

Does A Pressure Switch Keep a Furnace From Exploding? 

With a furnace, many people worry about the possibility of an explosion. With gas-heated furnaces, there is the chance of a fire or explosion.

However, the likelihood of this happening is very low. The pressure switch is designed to cut furnace action off well before any malfunctions in the ignition system would lead to an explosion. 

The larger threat with a furnace comes from carbon monoxide leaks due to age or defect. This is one of many reasons why a carbon monoxide detector is important to keep in your furnace-heated home. 

As for a furnace exploding due to the pressure switch, the switch ensures that the furnace would shut itself down long before it ever became a serious risk.  

How to Keep a Pressure Switch Working on a Furnace

The major factor that contributes to the continued operation of a furnace in an HVAC setup is regular maintenance.

Fluctuating temperatures put a significant amount of strain on both electrical and mechanical components, as does exposure to humidity and condensation. 

How Often Should a Furnace Be Maintained?

A gas furnace should be serviced at least once a year, but preferably twice—once in the autumn, and once in the spring. (Source: Bell Bros HVAC) During furnace service, a technician inspects the entire furnace for any parts that may need repair or replacement. They also take the time to clean any components where debris might interfere with the operation of the mechanisms inside. 

How Long Does a Pressure Switch Last?

It is challenging to measure the life of a pressure switch in years. However, you can expect the component to last for around 50.000 cycles.

If you are not constantly turning the unit on and off, the pressure switch is going to last you for over 10 years.

How Do You Bypass a Pressure Switch?

Bear in mind that bypassing a pressure switch is not a solution to the problem. It is just a way of checking whether or not there is an issue with the actual switch. 

Be extremely careful as the furnace has to be on when you perform the test.

  • The pressure switch has two wires. Disconnect one of them.
  • Connect this wire to the end of your jumper wire (a regular wire might work as well).
  • Turn the furnace on. Proceed only once you hear the draft inducer motor start.
  • Connect the other end of your jumper wire to the switch’s second wire.
  • Let the system run for a couple of minutes. If everything seems to be ok, then it looks like your pressure switch would have to be replaced