In Lennox furnaces, the pressure switch works with the inducer to confirm safe venting and airflow before ignition. When the inducer runs but the pressure switch reads open, the furnace will not light and may enter a fault state. This article explains what an open pressure switch with the inducer on means, common causes, and practical steps to diagnose and resolve the issue. It covers safety considerations, diagnostic procedures, and guidance on replacement options for common Lennox models used in American homes.
Understanding How The Lennox Furnace Pressure Switch And Inducer Work
In a Lennox furnace, the inducer motor starts to purge combustion gases and establish a draft. The pressure switch monitors this draft by sensing negative pressure through a small hose connected to the inducer housing. A closed, proper vacuum signals safe conditions and allows ignition. If the switch does not sense adequate draft, it opens the circuit, preventing gas valve operation. An open switch while the inducer is running indicates a venting or sensor problem that must be addressed before the furnace can operate.
Symptoms Of An Open Pressure Switch When Inducer Is On
The following signs commonly accompany an open pressure switch with the inducer active:
- Furnace fails to ignite and stops after the inducer runs for a short period
- Vent-related fault codes or blinking LED indicators on the furnace control board
- Repeated lockouts or intermittent ignition attempts
- Audible inducer running without flame or heat
- Unusual odors from the furnace or venting system
Common Causes Of An Open Pressure Switch When Inducer Is On
- Blocked or dirty venting—debris, bird nests, or snow can obstruct exhaust or intake paths
- Cracked, disconnected, or softened vacuum hose from the inducer to the pressure switch
- Dirty or restricted air filter causing reduced inducer flow and improper pressure
- Faulty, misadjusted, or aging pressure switch diaphragm or switch itself
- Inducer motor issues such as worn bearings or electrical faults
- Condensate trap or discharge line blockage in high-efficiency systems
- Improper vent sizing or venting configuration after renovations or additions
Diagnostics And Troubleshooting Steps
- Ensure safety first. Turn off the furnace at the breaker, lockout/tagout if available, and never bypass the pressure switch to force operation.
- Inspect the air filter and replace if dirty to restore proper airflow through the inducer.
- Carefully examine the venting system for blockages, cracks, or disconnections. Look at both the combustion vent and intake paths for high-efficiency systems.
- Check the condensate trap and discharge line for blockages or standing water that could affect pressure readings.
- Inspect the vacuum hose from the inducer to the pressure switch for cracks, kinks, or disconnections. Replace any damaged hoses.
- Verify the pressure switch wiring connections are secure and free of corrosion. A loose connection can mimic an open switch condition.
- For models with a reset pressure switch, press the reset button if present, after confirming no blockages or hose issues.
- Test the pressure switch with proper tools. A technician may use a manometer or a pressure switch tester to confirm the diaphragm seals and vacuum ratings align with Lennox specifications.
- Attempt a controlled ignition sequence after addressing obvious blockages and hose issues. If the furnace still reports an open switch, replacement of the switch may be necessary.
- Document any error codes shown on the control board and refer to the Lennox service manual for model-specific diagnostic steps.
When To Call A Professional
If basic checks do not resolve the issue, or if the technician identifies a faulty pressure switch, cracked hoses, or venting problems, a licensed HVAC professional should handle repair or replacement. Incorrect venting or bypassing safety controls can create dangerous carbon monoxide risks or fuel leaks. Seek professional help if there is any hesitation about working with 24-volt controls, burners, or gas lines.
Replacement Considerations For Lennox Furnaces
Replacing a pressure switch is a common repair for Lennox furnaces when the diaphragm fails or the switch no longer closes under proper vacuum. Use OEM Lennox replacement parts or equivalents recommended by the model’s service manual to maintain warranty coverage and compatibility. Labor costs vary by region and model, typically ranging from a few hundred dollars including parts in the United States. After replacement, a professional should verify proper venting, switch operation, and safe ignition with a full system diagnostic.