What Does a Furnace Pressure Switch Do?

You may never have heard of a furnace pressure switch, but it plays a critical role in preventing carbon monoxide produced by your heating, ventilation and air conditioning (HVAC) system from building up in your home.

“Furnaces have all sorts of safety devices on them,” says Joshua Smith of Berkshire Heating and Air Conditioning. “One of those is a pressure switch.”

I asked Smith and Alexander Siv from Amherst Plumbing and Heating to explain what a pressure switch is, how it keeps you safe and what to expect if it goes bad.

About the Experts

• Alexander Siv owns Amherst Heating and Plumbing in Amherst, Massachusetts. He has a master plumber’s license in Massachusetts and more than 10 years of plumbing experience.
• Joshua Smith is operations manager at Berkshire Heating and Air Conditioning in West Springfield, Massachusetts. He has more than 20 years of HVAC experience and holds a Massachusetts oil burner and refrigeration license.

What Is a Furnace Pressure Switch?

It’s a safety mechanism found inside a gas-powered forced air furnace. It ensures the furnace exhaust system is drafting properly before allowing the burner to ignite.

“Everything happens in a sequence and everything has a safety behind it,” Smith says, “The pressure switch is the first safety mechanism in the system.”

If the system can’t vent carbon monoxide properly, the pressure switch shuts down the furnace and prevents it from igniting.

Pressure switches are typically located near the inducer motor. They’re round, with an attached tube that leads into the motor. “You might be able to see it if you remove the front panel, but they can look different across furnace brands and models,” Siv says.

What Does a Furnace Pressure Switch Do?

“Pressure switches prevent carbon monoxide poisoning due to a faulty furnace venting system,” Siv says.

Gas-powered, forced air furnaces combust natural gas or propane to create warm air that heats your home. Carbon monoxide is a by-product of this combustion process.

Because carbon monoxide is poisonous and potentially fatal, it cannot be allowed to accumulate inside homes. An inducer fan in the furnace removes these exhaust gases to the outside through a flue pipe.

To carry carbon monoxide outside, the flue pipe needs a certain level of air pressure, and the inducer fan needs to function properly. If there’s an issue with either, the pressure switch halts operation. “If there’s a blockage or leak in the flue pipe, the pressure switch senses that and shuts the furnace down for safety,” Smith says.

If the fan isn’t functioning, that also triggers the pressure switch. “It shuts it down before combustion even begins,” Siv says.

Can You Adjust a Furnace Pressure Switch?

No. Pressure switches detect the furnace’s designed pressure and can’t be adjusted. “They’re sent from the factory pre-programmed with the furnace’s pressure setting,” Siv says.

Signs of a Bad Furnace Pressure Switch

The most common sign of a faulty pressure switch is no heat. Most homeowners will notice this right away. There will still be airflow, but the furnace will only circulate unheated air.

This happens when the pressure switch fails and stays stuck in the open position, which prevents the igniter from lighting and heating air. If this happens, a bad pressure switch is a possible cause.

If your furnace short cycles, meaning it turns on and quickly off repeatedly, this is also a possible sign of a failing pressure switch.

Rarely, an activated carbon monoxide detector could be due to a faulty pressure switch.

If the switch fails and stays stuck in the closed position, the furnace will operate normally, even if there’s a problem with the exhaust system. Carbon monoxide could then back up into the home and trigger the detectors. If this happens to you, evacuate your home immediately and call your local fire department or gas company.

How To Test a Furnace Pressure Switch

HVAC professionals test a pressure switch several ways.

First, they check the furnace draft pressure with a manometer or vacuum pump. This tells them if it’s a problem with the switch itself or something else, like a failing inducer motor or blocked vent pipe. Electrical components of the pressure switch are also checked with a volt meter for proper voltage.

Both our HVAC pros say testing the pressure switch is not a DIY task.

How Long Do Furnace Pressure Switches Last?

They’re intended to last the life of the furnace, or 10 to 20 years. But there are always exceptions. “Like anything else, [pressure switches] can go bad,” Smith says.

A failing pressure switch could be caught early during a yearly maintenance visit from an HVAC pro. “We’re checking your volts and amperage, and checking the motor and switches to find those components that are getting weak to fix or replace them before you have to put in a no heat call,” says Smith.

What Happens When a Furnace Pressure Switch Fails?

“Everything stops, and you have no heat,” Smith says, “The furnace might try a few times, but the pressure switch keeps cutting it off.”

In that case, Siv adds it’s best to call an HVAC pro to troubleshoot the issue. “It could be the pressure switch, but the tech would have to do specific tests to determine that’s the cause,” Siv adds.

Can You Fix a Furnace Pressure Switch?

No. “It’s a matter of replacing the part. They can’t be repaired,” Smith says. Sometimes the pressure switch tubing could be cracked or disconnected, which could be a simpler fix.

How Much Does It Cost To Replace a Furnace Pressure Switch?

Expect to pay between $250 to $350. Siv says it’s generally a quick fix. “Parts are $30 to $50 and require less than an hour of tech time on site,” he says.

Do You Need To Call a Pro To Replace a Furnace Pressure Switch?

Absolutely. “You’ll need specialized tools and know-how to diagnose that it is a pressure switch that has failed,” Siv says, “This is one for a pro.”