Why Is My Furnace Pressure Switch Stuck Closed 

A furnace pressure switch stuck in a closed position isn’t as common as “stuck open” cases. However, it happens. You’ll need a few tests to verify that the unit is stuck. But, ultimately, it’s easier to fix than one stuck open. 

Below we discuss everything you need to know about “stuck closed” pressure switches and what you can do about the issue. We also touch on several other things you may want to know about furnace pressure switches, so you’re fully prepared in case of an emergency.  

What’s a Pressure Switch and What Does it Do?

Furnace Pressure Switch
Image: diy.repairclinic.com

The pressure switch is a relay type of circuit designed to protect your loved ones from a dangerous backdraft of combustion gases. 

The furnace produces lots of waste/combustion gases during the combustion process. These include carbon dioxide, carbon monoxide, and sulfur dioxide. All of these gases can cause poisoning and asphyxiation in large quantities.  

Furnaces depend on the draft inducer fan to vent the gases. The blowing fan directs the gases out of the furnace and outside the house. 

However, there has to be a way to tell that the inducer motor removes the exhaust gases, right? More specifically, there needs to be a way to check whether the said gases are moving outside the house and not back into the house (back-drafting). 

That’s where the pressure switch comes in handy. It essentially tells you that combustion gases are moving out of the furnace smoothly. As long as it’s closed, you can rest easy knowing that there’s no risk of poisoning from backdraft.  

However, if it refuses to close, i.e., stays open, you immediately know that there’s a problem with the combustion gas exhaust process, but you have to know where the pressure switch is located.

More importantly, when the pressure switch is open, it suspends furnace operation, preventing further production of exhaust gases. This way, it protects you and your family/guests from poisoning. 

How Do I Know if the Pressure Switch is Bad?

Unfortunately, the pressure switch can become defective and refuse to work. When this happens, the furnace may refuse to run, or if it does, it will usually run without producing heat.

As a result, the air coming out of the ducts will be at room temperature or slightly colder due to the breeze. 

The good news is that a bad pressure switch comes with several tell-tale signs that you can easily spot. These include;

No “Click” Sound at Ignition

The first sign that you have an issue with your pressure switch is a missing “click” sound when igniting the furnace. The clicking happens when the gas valve opens and is the ultimate sign that your pressure switch is OK. If it’s missing, your pressure switch is not opening. Maybe it’s stuck. 

Furnace Blows Air at Room Temperature

As we saw earlier, the furnace won’t run if you have a bad pressure switch. However, since the exhaust fan may be running already, you’ll still probably get air from your ducts. But it will be cold air – not warm. 

Physical/Visible Damage/Wear

The pressure switch doesn’t wear or get damaged too often since 90% of it is metal. However, the rubber parts tend to wear slowly with age, the nylon hose connecting the switch to the inducer housing being the biggest culprit. So, if you notice cuts or other damage on the hose, the switch might be compromised. 

Dirty Hose Port

The hose that connects to the pressure switch is a port specially designed to prevent air leakage. It ensures an airtight connection to prevent external interference in the pressure building inside the switch. Unfortunately, a dirty port might not be as airtight as required, which can compromise the switch’s reliability. 

What Happens if the Pressure Switch Goes Bad?

A lot can go wrong if the pressure switch goes bad. Indeed, you can almost certainly say bye to home heating on that day until the issue is fixed. So, what exactly happens;

  • The switch may refuse to open/close: The most common outcome when you have a bad pressure switch is that the switch may refuse to open or close, depending on the original position. For example, if you’re starting the furnace, meaning the switch is open, it may refuse to close. If it’s closed, it may refuse to open.
  • The furnace may refuse to heat: This is almost a guarantee if you’re trying to turn on the furnace. It will not produce heat. Why? Because when a malfunctioned pressure switch rarely generates enough pressure to close the switch necessary to complete the furnace circuitry. So, the exhaust fan may run. But the burner likely won’t.
  • The furnace may enter “lockout”: Lockout essentially means that the furnace is unresponsive to certain external controls. A “hard lockout” essentially puts the furnace in a state where it won’t even respond to electrical power input when you turn ON the unit at the switch.
  • It may get “stuck open/closed”: Finally, a bad pressure switch can also get stuck in the open or closed position. A “stuck closed” switch is closed even when the furnace is off, the opposite of what’s expected. In a system working, the pressure switch is usually open when the furnace is OFF and closed when the unit is ON. However, if the switch is faulty, sometimes you may find it ON when it should be OFF. Again, a “stuck open” switch is the opposite.

Signs your Furnace Pressure Switch is “Stuck Closed”

A “stuck closed” pressure switch signs are almost similar to a “stuck open” one, except for the error code.

  • The furnace will not start: The furnace’s first thing when the thermostat requests heating is to check to see if the pressure switch is in the expected position, i.e., open. If so, the ignition process goes on smoothly. Otherwise, it will halt the ignition process.
  • A flashing error code: Most furnaces won’t even allow you to try again if it senses that the starting position of the pressure switch is “closed.” Instead, the unit will flash an error code warning you of a fatal problem.
  • Furnace lockout: The error code is typically accompanied by furnace “lockout,” meaning you must manually reset the appliance to get it back running.

What Causes the Pressure Switch to Get Stuck Closed?

“Stuck closed” pressure switches are very often the result of an internal malfunction. However, it can also be any of the following other reasons;

  • Water in the hose(s)

This only applies to condenser furnaces, which make up about 90% of all domestic furnaces. The water collecting in the condensate pan can back up into the hose(s) connecting the switch to the inducer box. It can cause false pressure when this happens, causing the fan to permanently stick in the “closed” position even when the furnace is off. 

  • Bad switch (short-circuiting)

Since the pressure switch is an electrical relay, short-circuiting is never inconceivable. Although rare, debris and other elements within the circuit area can cause short-circuiting. When the switch short-circuits, it loses its memory and may get “stuck closed” for good. 

  • A bad control board

This is even less likely but possible nonetheless. If the circuit board fails, perhaps due to a power surge, it may lose its capacity to determine the correct position of the circuit. Thus, it may not even tell when the switch is closed while the furnace is OFF. 

Here is how to know if your furnace control board is bad a nd what you can do to fix it.

How Do You Fix a Furnace Pressure Switch on a Furnace?

The best advice is to follow the error codes and check with the owner’s manual to find the appropriate remedy for each situation. 

That said, though, you need to replace the pressure switch 80% of the time if it gets “stuck closed” more than once over a short period. The moisture is enough to compromise the switch, even if it’s just water backed up into the hose. It can even cause short-circuiting. 

What Else Do You Need to Know?

How Long Should a Furnace Pressure Switch Last?

The furnace pressure switch is pretty reliable. It lasts at least five years and may get to seven with good maintenance. However, the longevity also depends on the type of furnace and whether it’s an OEM part. Universal switches rarely last two years. 

How Much Does it Cost to Replace a Pressure Switch on a Furnace?

The cost to replace a furnace pressure switch ranges from $50 to $350, depending on multiple factors such as the type of furnace, type of switch (low-pressure or high-pressure), and so forth.

It also depends on whether you opt for DIY or professional replacement. Professional replacement rarely costs below average service charge, i.e., $70. 

How Do You Bypass a Pressure Switch?

First off, yes, you can bypass the furnace pressure switch. It’s not recommended to do so. However, if you must do it, perhaps to confirm that the pressure switch is bad, then so be it.

More importantly, bypassing the pressure switch is extremely risky as you’re essentially starting the furnace without regard for exhaust gases. For the above reasons, you should never bypass it for long or consider it a permanent solution. 

With the above points in mind, you can now go ahead and bypass the switch. Again, it’s a straightforward process. Disconnect the two wires connecting to the furnace. Then touch the wires directly. That’s all. 

Hopefully, you’ve learned a few important lessons about pressure switches “stuck closed.” But, again, we must stress the need to call an HVAC professional whenever you get stuck. The rule of thumb is, if you don’t feel confident enough about it, don’t do it.