Lennox Furnace Primary and Secondary Limit Switch Open: Causes, Diagnosis, and Solutions

When a Lennox furnace experiences a primary or secondary limit switch open condition, it can lead to heating interruptions, safety concerns, and system inefficiency. Limit switches in furnaces are vital safety devices that prevent overheating by shutting off the burner if temperatures exceed safe limits. Understanding why the primary or secondary limit switch opens and how to diagnose and resolve these issues is essential for maintaining optimal furnace performance and safety.

Component	Function	Common Issues	Typical Solutions
Primary Limit Switch	Shuts off burner if heat exchanger gets too hot	Switch opens due to overheating or electrical faults	Clean or replace switch; check airflow and ductwork
Secondary Limit Switch	Acts as backup; guards against excessive heat in furnace	Switch opens during high temperature spikes	Inspect for blockages, test switch, and replace if faulty

What Are Primary and Secondary Limit Switches in Lennox Furnaces?

The primary and secondary limit switches are critical safety devices in Lennox furnaces designed to monitor internal temperatures within the heat exchanger and furnace cabinet. The primary limit switch detects high temperatures as soon as they exceed predefined thresholds to turn off the burner immediately. The secondary limit switch acts as a backup safety measure, protecting the furnace in case the primary switch fails or additional overheating occurs.

Both switches are usually mounted on or near the heat exchanger or furnace plenum and are connected to the control board. They close under normal conditions, completing the burner control circuit, and open when temperatures rise beyond safe limits, stopping the burner and preventing potential damage or hazards.

Common Causes of Limit Switch Open Conditions in Lennox Furnaces

The limit switch opening generally signals a safety shutdown due to unsafe operating conditions. Common causes include:

• Blocked or dirty air filters: Reduced airflow causes heat exchanger overheating, triggering the switch.
• Restricted or closed vents and registers: Limit airflow through the furnace leading to excessive heat buildup.
• Faulty blower motor or fan: Failure or malfunction reduces air circulation through the heat exchanger.
• Dirty heat exchanger: Dirt or debris increases heat retention, causing higher temperatures.
• Improper thermostat settings or wiring faults: Can cause the furnace to run longer than necessary, elevating temperatures.
• Malfunctioning limit switch: A failing or aging switch may open prematurely even under normal conditions.
• Closed or blocked return air ductwork: Restricts airflow and raises internal temperatures.
• Overloaded furnace or undersized system: When the furnace is too small for the home, it may overexert, causing overheating.

How to Diagnose an Open Primary or Secondary Limit Switch on a Lennox Furnace

Diagnosing an open limit switch requires methodical inspection and testing:

Visual Inspection

Start by inspecting the furnace area:

• Check air filters: Replace if dirty or clogged.
• Inspect vents and registers: Ensure they are open and not obstructed.
• Examine heat exchanger: Look for corrosion, cracks, or excessive dirt buildup.
• Assess blower motor: Listen for unusual noises or check if it runs continuously.

Electrical Testing

Use a multimeter or continuity tester to examine the limit switches:

• Power down the furnace and disconnect from power source.
• Locate the primary and secondary limit switches.
• Test continuity of each switch when cool; both should be closed (continuity present).
• If a switch shows open circuit when cool, it is likely faulty and needs replacement.

Temperature Testing

Monitoring furnace temperature with a thermometer or thermal camera can determine if overheating is the root cause:

• Measure temperature near heat exchanger while furnace operates.
• Compare readings against manufacturer’s safe operating temperatures.
• High temperatures confirm overheating triggering limit switch open condition.

Steps to Fix Lennox Furnace Primary or Secondary Limit Switch Open Issues

Effective fixes depend on addressing the root causes behind the limit switch opening:

Improve Airflow

• Replace or clean air filters regularly.
• Open all supply vents and registers to allow airflow.
• Clear obstructions from return air ducts and registers.

Ensure Blower Motor Functionality

• Inspect blower motor for mechanical defects or electrical issues.
• Lubricate motor bearings if applicable.
• Replace blower motor if it fails to operate properly.

Clean or Repair Heat Exchanger

• Remove debris and dust buildup on heat exchanger surfaces.
• Replace heat exchanger if cracked or severely corroded.

Test and Replace Limit Switches

• Replace any malfunctioning limit switch as per Lennox specifications.
• Use OEM parts to ensure compatibility and performance.

Check Thermostat and Control Wiring

• Verify thermostat wiring matches Lennox furnace wiring diagrams.
• Ensure thermostat settings are correct and not causing extended run cycles.

Maintenance Tips for Preventing Limit Switch Problems in Lennox Furnaces

Routine maintenance significantly reduces the chance of limit switch issues. Key measures include:

• Change air filters every 1-3 months.
• Schedule annual professional furnace inspections and cleanings.
• Keep vents and registers unobstructed year-round.
• Ensure proper furnace sizing when installing or replacing units.
• Inspect and replace aging limit switches proactively to avoid sudden shutdowns.

When to Call a Professional HVAC Technician

If troubleshooting above does not identify or resolve the cause of the limit switch open status, or if there are signs of significant heat exchanger damage, professional assistance is recommended. A certified Lennox technician has the tools and expertise to:

• Safely test and replace limit switches and control board components.
• Perform detailed combustion and airflow analysis to optimize performance.
• Identify hidden ductwork or blower motor issues.
• Ensure furnace safety compliance and reliable operation.

Hiring a licensed technician also protects the homeowner’s warranty and adds peace of mind that the furnace is safe to operate.

Understanding Lennox Furnace Error Codes Related to Limit Switches

Lennox furnaces may display diagnostic codes or LED flash patterns when limit switches open. Common codes indicate:

• Error code 12 or 18: Often signify limit switch open condition.
• Flashing LED lights: Specific flash sequences correspond to primary or secondary limit switch faults.

Refer to the Lennox furnace user manual or technical guide for system-specific error meanings and troubleshooting steps.