Lennox Furnace Limit Switch

The limit switch in your Lennox furnace is a critical safety device. It protects your system from overheating and potentially causing a fire. Understanding how it works, how to troubleshoot it, and when to call a professional can save you time, money, and potentially your home. This guide will walk you through everything you need to know about Lennox furnace limit switches.

What is a Lennox Furnace Limit Switch and What Does It Do?

Your Lennox furnace uses a heat exchanger to warm the air that circulates through your home. The limit switch monitors the temperature of this heat exchanger. If the temperature exceeds a pre-set limit, the switch trips, shutting down the burners. This prevents the furnace from overheating and potentially damaging components or causing a fire. Think of it as a circuit breaker, but for heat.

Lennox furnaces typically have one or more limit switches, depending on the model and design. Some may be primary limit switches, while others are secondary or auxiliary limits, providing additional layers of protection.

Types of Limit Switches Found in Lennox Furnaces

While the function remains the same, limit switches come in different forms:

• Automatic Reset Limit Switch: This type automatically resets itself once the temperature drops below the safe threshold. If this type is repeatedly tripping, it indicates an underlying problem that needs addressing.
• Manual Reset Limit Switch: This switch requires manual intervention to reset it. You'll typically find a small button on the switch that needs to be pressed after the furnace cools down. A tripped manual reset limit indicates a more severe overheating issue that needs immediate investigation.
• Rollout Switch: A specific type of limit switch designed to detect flame rollout, where flames escape from the burner area. These are usually located near the burners and are often manual reset.

Common Causes of a Tripped Limit Switch in a Lennox Furnace

A tripped limit switch is a symptom, not the problem. Here are the most common reasons why your Lennox furnace limit switch might be tripping:

• Dirty Air Filter: This is the most frequent culprit. A clogged air filter restricts airflow, causing the furnace to work harder and overheat.
• Blocked Vents or Registers: Closed or obstructed vents prevent proper air circulation, leading to increased temperatures within the furnace.
• Faulty Blower Motor: A failing blower motor can't move enough air across the heat exchanger, resulting in overheating.
• Dirty Blower Wheel: A dirty blower wheel reduces airflow efficiency.
• Ductwork Issues: Leaks, collapses, or undersized ductwork can restrict airflow.
• Flame Sensor Problems: A malfunctioning flame sensor can cause the furnace to run inefficiently, leading to overheating.
• Overfiring: If the furnace is receiving too much fuel, it can overheat the heat exchanger. This usually requires professional adjustment.
• Faulty Limit Switch: While less common, the limit switch itself can fail, tripping even when the furnace is not overheating.

Troubleshooting a Lennox Furnace Limit Switch: A Step-by-Step Guide

Safety First! Before attempting any troubleshooting, turn off the power to your furnace at the breaker box. Also, turn off the gas supply to the furnace.

Tools You'll Need:

• Screwdrivers (Phillips and Flathead)
• Multimeter
• Vacuum cleaner or brush
• Work gloves
• Safety glasses

1. Check the Air Filter: This is the easiest and most common fix. Remove the air filter and inspect it. If it's dirty, replace it with a new one of the correct size and type.
2. Inspect Vents and Registers: Make sure all vents and registers are open and unobstructed by furniture or other objects.
3. Locate the Limit Switch: Limit switches are typically located near the blower motor or on the heat exchanger housing. Refer to your furnace's wiring diagram for the exact location.
4. Visually Inspect the Limit Switch: Look for any signs of damage, such as burning, melting, or loose wires.
5. Test the Limit Switch with a Multimeter:
   1. Set your multimeter to the continuity setting (usually indicated by a beep symbol).
   2. Disconnect the wires from the limit switch.
   3. Place one probe of the multimeter on each terminal of the switch.
   4. A closed switch (normal operating condition) will show continuity (the multimeter will beep). An open switch (tripped condition) will not show continuity.
   If the switch shows open when the furnace is cool, it may be faulty. Note that some limit switches are normally open and close upon reaching a certain temperature. Consult the furnace's documentation.
6. Reset the Limit Switch (If Applicable): If you have a manual reset limit switch, press the reset button. Then, restore power to the furnace and monitor its operation. If the switch trips again quickly, there's still an underlying issue.
7. Inspect the Blower Motor and Wheel:
   1. Turn off the power to the furnace.
   2. Access the blower motor and wheel.
   3. Visually inspect the blower motor for any signs of damage or wear.
   4. Clean the blower wheel with a vacuum cleaner and brush to remove any accumulated dust and debris.

When to Call a Professional HVAC Technician

While some limit switch issues can be resolved with simple DIY troubleshooting, it's crucial to know when to call a professional. Never attempt repairs you are not comfortable with. Improper repairs can be dangerous and could damage your furnace.

Call a qualified HVAC technician if:

• The limit switch continues to trip after you've cleaned the filter and checked the vents.
• You suspect a problem with the blower motor or ductwork.
• You are not comfortable working with electrical components or gas lines.
• You suspect the furnace is overfiring.
• You are unsure how to properly test the limit switch with a multimeter.
• You find any signs of burning or damage to the furnace components.

Lennox Furnace Limit Switch Replacement Cost

The cost to replace a Lennox furnace limit switch can vary depending on several factors, including:

• The type of limit switch: Some switches are more expensive than others.
• The furnace model: Some furnace models may require more specialized parts.
• The labor rate of the HVAC technician: Labor rates vary by location and company.

Generally, you can expect to pay between $150 and $400 to replace a limit switch in your Lennox furnace. This includes the cost of the switch and the technician's labor. It's always best to get multiple quotes from different HVAC companies before proceeding with the repair.

Important Considerations:

• Don't bypass the limit switch: Bypassing the limit switch is extremely dangerous and can lead to serious consequences, including fire and carbon monoxide poisoning.
• Use OEM parts: When replacing a limit switch, always use a genuine Lennox replacement part or an approved equivalent. This ensures proper fit and function.
• Regular Maintenance: Preventative maintenance, such as annual furnace inspections and cleaning, can help prevent limit switch issues and extend the life of your furnace.

By understanding the role of the limit switch in your Lennox furnace and following these troubleshooting steps, you can potentially save yourself time and money. However, always prioritize safety and call a professional when needed.