What Is a Furnace Pressure Switch, and Why Does It Keep Tripping?
A furnace pressure switch is a safety switch that confirms the draft inducer motor is pulling combustion gases out through the vent before the burners are allowed to light. If the switch does not close, the furnace will not fire. Carrier's troubleshooting guidance points to causes such as a restricted vent, a defective inducer motor, and disconnected or obstructed pressure tubing.
Winter brings its own cause in Somerset County: snow. Trane's installer guide for its concentric vent kit, written for a range of its gas furnaces, requires the vent and intake terminations to sit at least 12 inches above the highest anticipated snow level. NOAA's 1991 to 2020 normals show Somerset averaging 96.2 inches of snow a year, with 5 inches or more on the ground on about 35 days. When drifts reach a low sidewall vent, the pressure switch is the part that stops the furnace. This guide explains how the switch works, what trips it, the codes it can trigger, and what you can safely check yourself.
What Does a Furnace Pressure Switch Do?
Induced-draft gas furnaces, such as the Carrier two-stage furnace covered in its troubleshooting guide, use a small fan called a draft inducer to pull combustion gases through the furnace and out the vent. The pressure switch connects to that system through a small tube. When the inducer creates enough draft, the switch closes and tells the control board it is safe to continue.
Carrier's troubleshooting guide for one of its two-stage 80% furnaces lays out the order:
The thermostat calls for heat.
The control board checks that the pressure switch contacts are open.
The inducer motor starts.
The pressure switch closes once there is enough draft.
The control runs a 15-second prepurge.
Ignition begins.
If the switch fails to stay closed, the inducer keeps running and the furnace never reaches ignition. That pause is the safety feature at work: the burners will not light until the switch proves the vent is drawing.
Why Does a Furnace Pressure Switch Trip?
Carrier's guide lists these checks when a pressure switch does not close or reopens during a cycle:
A vent that is undersized or restricted
A defective inducer motor
Low voltage to the inducer motor
A defective pressure switch
Excessive wind at the vent outlet
Not enough combustion air
Pressure tubing that is disconnected or obstructed
The reverse problem has its own code. When a switch is stuck closed or its tubing is blocked, the control sees a closed switch before the inducer even starts and refuses to continue.
Notice how many of these causes are outside the switch itself. A technician checks the draft pressure before replacing parts, and Carrier's guide includes a pressure check diagram for that purpose. A new switch on a blocked vent will trip just like the old one. Trane's guide to furnace hard lockouts also names blocked vent pipes or flues among the common causes of a furnace that shuts itself down.
Can Snow and Ice Block a Furnace Vent?
Yes. On high-efficiency furnaces, the vent pipes deserve the closest look. Trane notes that a condensing furnace uses PVC intake and exhaust pipes. Those pipes can exit through a sidewall, where drifting snow can reach them.
Trane's vent kit instructions spell out the protections an installer should build in:
Terminations at least 12 inches above the highest anticipated snow level, or above grade, whichever is greater
Ribs removed from the rain cap on vertical vent kits, because ice or snow can form on the cap and leave the furnace inoperable
Vent kits never installed one above the other, to prevent condensate freeze-up
The U.S. Consumer Product Safety Commission adds a homeowner step in its December 2025 winter safety release: clear snow away from the outside vents of fuel-burning appliances such as furnaces so carbon monoxide does not build up in the house. The carbon monoxide and furnace safety guide explains why that matters and where alarms belong.
Carrier's guide shows why a snow-blocked vent can leave you cold for a while even after you clear it. On the furnace it covers, if the low-heat pressure switch stays open for longer than five minutes, the inducer shuts off for 15 minutes before the furnace tries again.
Which Error Codes Point to a Pressure Switch Problem?
Codes vary by brand and model, so always read the chart on the inside of your furnace door. As an example, Carrier's guide for its two-stage 80% furnace uses these:
Code | Meaning in that Carrier guide |
23 | Pressure switch did not open |
31 | High-heat pressure switch or relay did not close or reopened |
32 | Low-heat pressure switch did not close or reopened |
43 | Low-heat pressure switch open while high-heat switch is closed |
Write down the number of flashes and the furnace model before you call. It gives the technician a head start, and the list of furnace problems that need immediate professional help explains which symptoms should not wait.
What Can You Safely Check Yourself?
A few checks are safe for any homeowner:
Look at the vent pipes outside. Clear snow, ice or leaves from around the intake and exhaust openings without forcing anything into the pipes.
Change the air filter. Trane says that is the only repair step a homeowner should try when a furnace locks out.
Note the error code. If your furnace has a viewing window or exterior status light, write down the flash pattern. Trane advises against removing the front panel while the furnace is running.
Leave the rest to a technician. The pressure tubing, switch and inducer motor sit inside the burner compartment, close to gas and electrical parts. Never jumper or bypass a pressure switch. Carrier's guidance for other furnace safety parts, including the flame sensor and limit switch, is never to bypass or defeat them, and the same caution applies here.
When Should You Call a Technician?
Call for service if the furnace keeps cycling the inducer without lighting, if the code returns after you clear the vents, or if the inducer motor runs but the burners never light. Also call if water is collecting in the vent pipes or near the furnace.
If a carbon monoxide alarm sounds at any point, get everyone outside first and call 911, as CPSC advises. If you smell gas, leave the house and contact your gas utility from outside.
For condensing furnaces, the guide to whether high-efficiency furnaces are worth the cost explains how their venting differs from older models. If the furnace will not start at all, the guide to a furnace that won't turn on covers the simpler checks first.
FAQ
Can I bypass a furnace pressure switch to get heat?
No. The pressure switch keeps the burners from lighting without a working draft. It is a safety control, and Carrier's guidance for furnace safety parts is never to bypass them.
Does wind make a pressure switch trip?
It can. Carrier's troubleshooting guide lists excessive wind among the causes of a pressure switch that will not close or reopens.
What is the inducer motor?
It is the small fan that pulls combustion gases through the furnace and out the vent before the burners light. The pressure switch watches the draft it creates.
Why does my furnace trip the pressure switch only in winter?
Snow and ice around sidewall vents can cause it. Trane's vent instructions also warn about ice forming on rain caps and condensate freezing when vent kits are stacked.
How long will my furnace wait after a pressure switch fault?
It depends on the model. On the Carrier furnace covered in its two-stage guide, the inducer shuts off for 15 minutes before retrying if the switch stays open for more than five minutes.
Is a pressure switch fault dangerous?
The fault itself means the safety system is working. The cause behind it, such as a blocked vent, needs attention quickly.
Van Bremen's Heat & Air Repairs Furnace Venting and Draft Problems
Van Bremen's Heat & Air services residential and commercial furnaces from 214 Stoystown Rd in Somerset, covering Somerset, Fayette, Cambria and Westmoreland Counties and Morgantown, WV. A technician can test the draft, inspect the vent run and terminations, and replace a failed switch or inducer when that is the true cause. See the heating services page and maintenance services page to learn more.
Visit Van Bremen's Heat & Air or call (814) 483-1735 today. Contact us for more information.
Comments