What Happens When a Furnace Pressure Switch Goes Bad?

If you’re like most people, you don’t think much about what happens inside your furnace when you turn up the thermostat. What seems like a simple process is quite complex, with a finely-tuned series of parts and processes working together to produce safe and reliable heat. 

One of those critical parts is the furnace pressure switch. Here, Complete Comfort Control, the most trusted heating repair experts in the Hamptons, NY, explains what happens when a furnace pressure switch goes bad and why it’s critical to call a repair professional for help immediately. 

What Is a Furnace Pressure Switch, and What Does It Do?

As the name implies, a furnace pressure switch is the part inside a gas furnace that controls air pressure. It’s a safety feature that prevents harmful carbon monoxide and other harmful gases from entering your home and reduces the risk that the unit will catch on fire or explode.

The switch detects air pressure when the furnace is running. If it senses negative pressure, it automatically stops combustion, which (depending on your specific furnace) either turns the unit off entirely or stops it from producing warm air. Negative pressure creates a backdraft, meaning the furnace sucks contaminated air back into your home instead of safely directing it outside. 

How Pressure Switches Work 

To understand what happens when a furnace pressure switch goes bad, it’s helpful to have a basic understanding of how the unit works. 

Inducer Motor Startup 

When the thermostat tells the furnace to start, the control board signals the inducer motor to kick on. This motor creates negative pressure for up to 60 seconds to clear the heat exchanger of harmful gases and ensure safety and efficiency. If everything works correctly, the pressure switch will detect the negative pressure created by the draft inducer motor and engage, allowing the furnace to continue the startup process. 

Heat Production Starts 

When the inducer motor startup process completes, the ignitor activates, and the gas valve releases gas. The resulting flame ignites all the burners, creating the warm air that the blowers push through the ductors, warming your home. 

What Happens When a Furnace Pressure Switch Goes Bad?

Typically, once the inducer motor fires up and gets running, it takes less than 10 seconds to create the backdraft that engages the pressure switch. If the switch doesn’t close, the furnace will automatically shut down and try again in a few minutes. However, if the switch still doesn’t close after three to five tries, the control board will stop sending power to the inducer, creating a safety lockout. 

When this happens, you’ll notice that the furnace isn’t running and that there’s either cool air or no air from the vents. You need an HVAC technician to investigate and determine the source of the problem. Attempting to run the furnace if you suspect a faulty switch is dangerous, as it can create excessive amounts of carbon monoxide. 

What Causes Switch Failure 

Like any mechanical component of a furnace, the pressure switch can wear out over time. The average lifespan is about 10 years, meaning most homeowners must deal with this issue at some point. The part’s role as a critical safety feature means that, unlike some other components, a failed switch means the furnace won’t work until a technician replaces it.

Beyond normal wear and tear, other reasons that the switch can fail include:

  • A ruptured diaphragm (the part inside the switch that controls pressure)
  • Damage to the pressure switch hose
  • Water in the hose 
  • An obstruction in the flue 
  • A clogged switch pressure chamber port 

Fixing Pressure Switch Problems 

Replacing a faulty switch is not a DIY project. Furnaces have different switch types and configurations; a single-stage furnace, for example, has one switch, while two-stage furnaces have multiple. 

Sometimes, the issue isn’t with the switch at all, but rather, negative motor pressure occurs because your home is too airtight. You need a professional HVAC expert to investigate and find the source of the problem to ensure the correct solution and a safe home. 

Call Complete Comfort Control for Help if Your Furnace Stops Working 

The short answer to the question, “What happens when a furnace pressure switch goes bad?” is that you won’t have any heat. This is the last thing you want on a cold winter day, so if you suspect trouble, dial (631) 910-7649 to schedule a visit from one of Complete Comfort Controls’ Hamptons-based specialists. We can solve the trickiest problems, from broken switches to determining what causes a furnace circuit board to burn, and we’re here when you need us most. 

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