What Does a Furnace Pressure Switch Do? Longmont Guide

Longmont winters can stay below zero for days, and that kind of cold puts real stress on your furnace’s venting system.

If you’re asking what a pressure switch does on a furnace, you’ve likely just seen your heat shut off during a cold snap. Whether you need immediate heating services to get your home warm again or just want a straight answer on what went wrong, you are in the right place.

A furnace pressure switch is a safety device. It confirms that air is moving safely through your furnace’s vent pipe before allowing the burners to light.

When it senses a problem, it shuts the furnace down rather than risk pushing exhaust gases into your home.

What a Furnace Pressure Switch Actually Does

To really understand what a pressure switch does on a furnace, it helps to look at the sequence a furnace follows before it ever produces heat.

Every modern furnace uses an inducer motor to pull combustion gases out through the vent pipe before fuel is allowed to ignite.

The furnace pressure switch measures the vacuum, or negative pressure, created by that motor. Here’s the basic sequence it controls:

• The thermostat calls for heat, and the inducer motor starts spinning.
• The motor pulls air through the vent pipe, creating negative pressure.
• The pressure switch monitors that vacuum through a small rubber tube.
• Once vacuum reaches the required level, the switch closes and signals the control board.
• Only then does the board allow the burners to ignite safely.

Most residential switches close only when the vacuum reaches between -0.5 and -1.5 inches of water column, depending on the furnace model.

If that pressure target isn’t reached within a few seconds, the switch stays open, and the control board locks out ignition.

Why Longmont Cold Snaps Trip the Switch

Longmont’s elevation near 4,979 feet and its history of extreme cold (the city has recorded lows near -38°F) create specific venting problems that warmer climates rarely see.

This is usually when the heat cuts out overnight and homeowners first start wondering how a furnace pressure switch actually works.

Many homes built in the 1990s and 2000s use 90%-plus efficiency furnaces with low, side-wall PVC vent terminations, which creates a few predictable weak points:

• Moisture inside the exhaust can condense and freeze directly at the vent cap.
• Wind-driven snow along the Front Range piles against low vent terminations within hours.
• Ice damming near the foundation can partially bury the vent opening overnight.
• Restricted airflow prevents the inducer motor from creating enough vacuum.

Once airflow is restricted, the furnace pressure switch stays open, meaning it won’t allow ignition. If your furnace keeps cycling off during cold weather, don’t wait for a full breakdown.

McCormick Heating and Cooling offers prompt furnace repair service across Longmont to get your heat working safely again before the next cold front moves in.

Common Furnace Pressure Switch Problems in Longmont Homes

Not every pressure switch issue is weather-related. Several furnace pressure switch problems show up repeatedly in Front Range homes, especially in furnaces over eight years old.

• Frozen or blocked vent terminations from snow, frost, or nesting insects
• Disconnected or cracked rubber tubing running from the switch to the vent pipe
• A failing inducer motor that spins but can’t build enough vacuum
• Condensate blockage inside the vent line, often linked to drainage issues
• Corroded or loose electrical connections at the switch itself

If your furnace has also shown signs of water pooling near the unit, it’s worth reading our guide on Furnace Leaking Water? 5 Condensate Causes, since condensate drainage problems and pressure switch faults often happen together in high-efficiency furnaces.

How Technicians Approach Furnace Troubleshooting Pressure Switch Issues

Properly troubleshooting a furnace pressure switch starts outside the house, not inside the furnace cabinet. A technician typically works through these steps in order:

• Inspect the vent termination for ice, debris, or snow buildup.
• Test the switch’s rubber tubing for cracks or disconnection.
• Use a manometer to measure actual vacuum pressure during an ignition attempt.
• Check the inducer motor’s amperage draw if pressure readings are low.
• Test the switch’s electrical contacts directly once airflow is confirmed.

This step-by-step approach matters because clearing snow from a vent cap fixes some furnaces instantly.

Before calling for service, grab a shovel, walk outside, and make sure the white PVC exhaust pipes near your foundation are completely free of snowdrifts and ice. If that doesn’t work, it’s time to look at parts replacement.

Furnaces that repeatedly lock out even after the vent is clear often point to a different root cause, similar to patterns we cover in Furnace Problems in Greeley Winters: Symptoms, Causes and When to Call.

When a Reset Stops Working: Furnace Pressure Switch Stuck Open

Here’s where many Longmont homeowners get stuck. Resetting the furnace or clearing the vent works the first few times, but eventually the furnace won’t restart at all.

This usually signals a furnace pressure switch stuck-open failure, meaning the internal contacts no longer close even when correct vacuum pressure is present.

Common signs that point to a stuck switch rather than a venting problem include:

• The furnace locks out even with a completely clear vent pipe.
• You hear the inducer motor running normally with no ignition.
• The control board displays a repeated pressure switch error code.
• Resetting the breaker only buys a few minutes before shutdown returns.

A stuck-open switch is a mechanical or electrical failure, not a venting issue. Diaphragms inside the switch can wear out, corrode, or crack after years of pressure cycling, especially with repeated freeze-thaw stress typical of Longmont winters.

A furnace that won’t stay running through winter isn’t something to troubleshoot blindly. If your system refuses to start up at all, you may also find helpful troubleshooting advice in our guide: Furnace Not Turning On in Fort Collins? 3 Fast Fixes.

McCormick Heating and Cooling’s technicians carry replacement pressure switches on service calls throughout Longmont, so most repairs are completed in a single visit. 

Serving Longmont Homeowners With Honest Furnace Repairs

McCormick Heating and Cooling has worked on furnaces across Northern Colorado’s harsh winter climate for years, and pressure switch failures are among the most common cold-weather calls we receive in Longmont.

You can see verified reviews and service details on our Google Business Profile.

When it comes to replacing a pressure switch, the exact repair process depends on a few factors:

• Whether your system uses a standard single-stage switch or requires OEM-specific parts for your furnace brand

• Additional steps like vent clearing or inducer motor repairs are also needed.

A full diagnostic visit uncovers whether the real problem is venting, the inducer motor, or the switch itself.

Key Takeaways

• A furnace pressure switch confirms safe vent airflow before allowing ignition, protecting your home from exhaust buildup.
• Longmont’s cold snaps and low side-wall venting make frozen or blocked vent terminations a leading cause of switch trips.
• Repeated lockouts even after clearing the vent often mean the switch itself has failed and is stuck open.
• Professional diagnosis checks venting, vacuum pressure, and motor performance before replacing any part.

Protecting Your Furnace Through Longmont’s Next Cold Snap

Now that you know what a pressure switch does on a furnace, you can tell the difference between a quick vent fix and a part that needs replacing.

Longmont’s winters will keep testing these switches every season, especially in homes with low vent terminations exposed to wind and snow.

Catching the difference between a temporary vent blockage and a failing switch early can save you from a cold house and an emergency repair bill.

If your furnace keeps shutting down this winter, McCormick Heating and Cooling is ready to diagnose and repair the problem fast.

Contact McCormick Heating and Cooling team today to get your Longmont home’s heating system back on track before the next freeze hits.

FAQs About Furnace Pressure Switches in Longmont, CO

What does a pressure switch do on a furnace?

A furnace pressure switch ensures the inducer motor creates sufficient vacuum to vent exhaust safely before burner ignition. As a safety lockout, it prevents firing during restricted airflow, ensuring dangerous combustion gases cannot accumulate inside the furnace or your home.

What causes a furnace pressure switch to trip?

A furnace pressure switch trips when inducer motors lack vacuum. Causes include blocked pipes, loose wires, disconnected tubing, failing motors, condensate clogs, and frozen terminations. In Longmont, winter storms frequently block low vents with snow or ice, triggering repeated trips.

How do I know if my furnace pressure switch is bad?

A bad furnace pressure switch causes repeated lockouts even after clearing vents and resetting. Common signs include clicking without ignition, control board error codes, and running briefly before shutting off. A technician can confirm failure using a direct continuity test.

Can snow or ice on the vent pipe trip the pressure switch?

In cold climates like Longmont, snow or ice buildup on vent pipes frequently causes pressure switch trips. Ice at the vent cap or snow drifting against low sidewall terminations restricts airflow, stopping the inducer motor from achieving required vacuum levels.

Can a furnace run without a pressure switch?

No, a furnace should never run without a functioning pressure switch, preventing ignition when proper venting fails. Bypassing it eliminates critical safeguards against indoor exhaust gas buildup.

Because this can lead to severe hazards like carbon monoxide exposure, furnaces missing or bypassing this part require immediate inspection by a licensed technician.