Furnace Pressure Switch: Why Your Furnace Won't Light and What Causes It

The thermostat calls for heat. You hear the small fan inside the furnace spin up, it runs for a while, and then nothing. No click of the igniter, no burners. Eventually the fan stops, the furnace tries again, and the house keeps getting colder.

That pattern usually points to the pressure switch. It's a small safety device, but it stops a lot of furnaces on cold Minnesota mornings. Here's what it does, why it trips, what you can safely check, and why it should never be bypassed.

What a pressure switch does

Before a gas furnace lights, a small motor called the inducer fan starts and pulls air through the burners and pushes exhaust out the vent pipe. The pressure switch is connected to that system by a thin rubber hose. When the inducer creates enough draft, the switch closes and tells the control board it's safe to light the burners.

If the switch doesn't close, the board won't open the gas valve. It's a check that the exhaust has a clear path out of the house before any fire is lit. That's exactly the job you want it doing, because exhaust that can't get out can end up inside.

Signs the pressure switch is stopping your furnace

• The inducer fan runs, but the igniter never glows and the burners never light.
• The inducer runs for a minute or so, shuts off, and the furnace tries again.
• The furnace eventually locks out and the status light on the control board blinks a code. On many furnaces, that code specifically points to the pressure switch.
• The problem shows up after a snowstorm, during high wind, or in a deep cold snap.
• You hear gurgling from the furnace or see water around its base.

What trips a pressure switch in a Minnesota winter

In most cases, the switch itself is fine. It's doing its job because something is restricting the draft. These are the causes we find most often, roughly in order.

• Blocked intake or exhaust pipes. The white plastic pipes on the side of the house can be buried by drifting snow, iced over by frost buildup on very cold days, or blocked by leaves or a nest. This is the most common cause after a storm.
• A blocked condensate trap or drain. High-efficiency furnaces make water. If the trap clogs or the drain freezes, water backs up into the inducer housing or pressure hose and the switch can't read the draft.
• A cracked, kinked or water-filled pressure hose. The small rubber tube between the inducer and the switch can split, sag or collect water over time.
• A weak inducer motor. If the inducer's bearings are worn or the wheel is dirty, it may not pull enough draft to close the switch, especially on a cold start.
• A failed pressure switch. Switches do wear out, but it's less common than the causes above. A technician confirms it with a meter before replacing it.
• Venting that was undersized or too long. If the problem has happened since the furnace was installed, the vent design itself may be marginal. Our sheet metal and venting work can correct that.

What you can safely check

• Go outside and look at both white pipes. Gently clear snow, ice and debris by hand. Don't chip at the pipe. Our guide to furnace vent pipes, snow and ice shows what normal looks like.
• Check the condensate drain hose and pump. Make sure the hose isn't kinked and the pump isn't full or silent. Our post on a furnace leaking water explains the drain system.
• Replace a dirty filter. It isn't usually the direct cause, but it rules out a separate problem.
• Look through the small window on the furnace's lower door and count the flashes on the status light before you open anything. On many furnaces, removing the door cuts power and clears the code.
• Reset the furnace once by turning its power off for a minute and back on. If it locks out again, stop and call.

Never bypass a pressure switch

You'll find advice online about jumping the two wires on a pressure switch to get the furnace running. Don't do it, and don't let anyone else do it either. A jumped pressure switch lets the furnace light even when the exhaust has nowhere to go. That's how carbon monoxide ends up in a house.

If a technician needs to test the switch, it's done with a meter and a manometer, and the switch goes right back into service. If you ever find a pressure switch jumped or disconnected, turn the furnace off and call for service. Make sure you have working carbon monoxide alarms. Our post on carbon monoxide and Minnesota's CO alarm law covers where they belong.

What a repair involves

On a service call, the technician reads the code, checks the vent and intake terminals, clears and tests the condensate trap and drain, inspects the pressure hose, and measures the draft the inducer actually produces. That reading tells them whether the problem is a restriction, a weak inducer or a failed switch.

Clearing a trap or replacing a hose is a small repair. A new pressure switch is also a modest part. An inducer motor costs more, and on an older furnace it's worth weighing against the furnace's age. You'll get the price before any work starts. If repairs keep adding up, our post on whether to repair or replace an aging furnace can help.

Preventing pressure switch trips

A yearly furnace tune-up cleans the condensate trap, checks the pressure hose and inducer, and confirms the vent terminals are clear and correctly installed. After every big snow, take thirty seconds to check the pipes outside. And if your vent pipes sit low on a side of the house where snow drifts, ask about extending or relocating the terminals.

Inducer running but no heat?

If you've cleared the pipes and checked the drain and the furnace still won't light, schedule a furnace repair or call Heating & Cooling Two at 763-428-3677. Office hours are Monday through Friday, 7:30 a.m. to 4:30 p.m., and after-hours emergency service is available. Related reading: the furnace flame sensor, furnace blowing cold air and what to check when your furnace stops working.