The Lennox furnace pressure switch plays a critical role in ensuring safe and efficient operation of the heating system. When the inducer motor is running but the pressure switch remains open, the furnace will fail to ignite, causing heating interruptions. This issue can be frustrating for homeowners and HVAC technicians alike. Understanding why the pressure switch stays open despite inducer operation is essential for troubleshooting and repair.
| Component | Function | Common Issues |
|---|---|---|
| Lennox Furnace Inducer Motor | Creates draft to vent combustion gases | Motor runs but inadequate draft, preventing switch closure |
| Pressure Switch | Detects proper draft pressure before ignition | Fails to close if pressure is insufficient or sensing tube clogged |
| Pressure Tubing | Connects inducer to pressure switch for sensing | Leaks or blockages can prevent switch activation |
| Vent System | Channels combustion gases outside | Blockages or damage reduce airflow, causing switch to stay open |
How the Lennox Furnace Pressure Switch Works
The pressure switch is a safety device that verifies the inducer fan generates sufficient negative pressure in the vent system. When the furnace thermostat calls for heat, the inducer motor starts to establish proper airflow. The pressure switch monitors this airflow via a small vacuum line connected to the inducer housing or vent pipe.
If the vacuum pressure meets the manufacturer’s specifications, the pressure switch closes its electrical contacts, signaling the control board that it is safe to ignite the burners. If pressure is weak or absent, the switch stays open, preventing ignition and protecting against unsafe combustion conditions.
Common Causes of Pressure Switch Open Fault Despite Inducer Running
1. Blocked or Obstructed Vent Pipe
A primary cause for the pressure switch staying open is a venting problem. Leaves, bird nests, snow, or debris can block the vent pipe, limiting airflow. This blockage means the inducer motor cannot create the required draft, so the pressure switch never senses enough vacuum to close.
2. Cracked or Damaged Pressure Hose
The small flexible tubing connecting the inducer housing to the pressure switch is vulnerable to cracking or disconnection. A leak in this hose causes loss of vacuum pressure detection, leaving the switch open even if the inducer motor runs normally.
3. Faulty Pressure Switch
Although less common, the pressure switch itself may be defective. Internal contacts can wear out or get stuck in the open position, causing false readings. Testing the switch’s continuity with a multimeter will assist in its diagnosis.
4. Weak or Malfunctioning Inducer Motor
The inducer motor might run but fail to generate adequate airflow due to mechanical wear, damaged fan blades, or electrical issues. Insufficient draft means the pressure switch will not sense pressure and remains open.
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5. Improper Furnace Installation or Vent Design
Errors during installation such as wrong vent pipe size, excessive length, or poor routing can reduce inducer effectiveness. This leads to insufficient pressure formation, preventing the pressure switch from closing.
Step-By-Step Troubleshooting for Pressure Switch Open Issue
- Turn off power and gas supply to the furnace before beginning diagnostics to ensure safety.
- Inspect the vent pipe outlet for obstructions like debris, nests, or ice. Remove anything blocking airflow.
- Check the pressure hose for cracks, disconnections, or blockages. Replace if damaged or obstructed.
- Test the pressure switch using a multimeter for continuity when the inducer is running. The switch should close under proper vacuum.
- Verify inducer motor performance. Listen for unusual noises and check that the fan blades spin freely without obstruction.
- Examine vent pipe installation for compliance with Lennox specifications, ensuring proper size, slope, and length.
- Consult the furnace’s error codes or control board diagnostics to gain additional insights.
Tips For Preventing Future Pressure Switch Failures
- Schedule regular furnace maintenance to keep vent pipes and pressure lines clear and intact.
- Inspect and clean vents seasonally to prevent debris accumulation and blockages.
- Replace old pressure tubing proactively, as it degrades over time from heat and moisture.
- Confirm inducer motor functionality annually to avoid airflow issues.
- Follow manufacturer specifications strictly when installing or servicing venting components.
When to Call a Professional HVAC Technician
If basic troubleshooting does not resolve the pressure switch open condition, professional service is recommended. A qualified technician has the tools to perform detailed diagnostics such as manometer pressure tests, internal component inspection, and accurate multimeter evaluations. Attempting advanced repairs without expertise can risk further damage or unsafe furnace conditions.
Hiring a certified Lennox specialist ensures the furnace is safely restored to optimal operation, preventing heating disruptions and protecting home safety.