Furnace Limit Switch

A furnace limit switch is an important safety component in any heating system. By monitoring the temperature inside your furnace, it prevents the furnace from overheating, and maintains safe operation and optimal performance. 

What is a Furnace Limit Switch?

A furnace limit switch, also known as the “limit switch on furnace,” is a device located inside the furnace that monitors the internal temperature. If the temperature exceeds a safe level, the limit switch signals the system to shut off the burner, protecting the furnace from overheating and potential damage. This switch not only plays a safety role but also helps control the operation of the blower fan, this allows for efficient air circulation in your home.

The furnace limit switch acts as a thermostat for the internal furnace components, acting as a safeguard and an efficiency controller.

 

What Does a Limit Switch Do on a Furnace?

The furnace limit switch is a dual-purpose device with both safety and efficiency functions:

1. Overheat Protection: When the furnace's internal temperature rises above a designated limit, the switch shuts off the burners, preventing overheating and potential damage to the system.
2. Fan Control: The limit switch also controls the blower fan. It activates the blower to circulate warm air once the furnace reaches a specific temperature, and turns it off when the furnace cools down.

Where is the Limit Switch on a Furnace?

The limit switch is typically located inside the furnace panel near the heat exchanger. It’s often positioned in a spot where it can accurately monitor the internal temperature. For those who may need to inspect or replace it, the limit switch usually appears as a small rectangular or circular device with a series of wires connected to it. However, it’s advisable to consult a professional for any repairs or inspections due to the potential hazards involved in working with furnace components.

Types of Limit Switches

Limit switches come in two main types, each with a specific role in furnace operation:

1. High Limit Switch: This switch prevents overheating by shutting off the burners when the furnace temperature exceeds a preset limit. The high limit switch is particularly important in avoiding damage to the furnace or, in extreme cases, preventing a potential fire hazard.
2. Fan Limit Switch: This switch regulates the operation of the blower fan. When the furnace reaches a certain temperature, the fan limit switch activates the blower to distribute warm air through the ductwork. It also turns off the blower once the furnace has cooled to prevent unnecessary energy consumption.

How Furnace Limit Switches Work?

A furnace limit switch functions through a simple yet essential process:

1. Temperature Monitoring: As the furnace operates, the limit switch continuously monitors the temperature. It’s preset to recognize specific temperature thresholds that are safe for the system.
2. Blower Activation: When the internal temperature reaches the limit set for the blower, the fan limit switch activates the blower fan to circulate warm air through the home.
3. Overheat Shutdown: If the furnace exceeds the high-temperature limit, the high limit switch automatically shuts off the burners. This stops additional heat generation and allows the furnace to cool down to a safe level.
4. Cycle Completion: Once the furnace cools to a safe temperature, the limit switch resets itself, allowing the furnace to continue its heating cycle if necessary.

This cycle repeats every time the furnace runs, maintaining a safe and controlled environment.

Importance of the Limit Switch

The limit switch is one of the most important components of a furnace for several reasons:

• Prevents Overheating: Without the limit switch, the furnace could easily overheat, damaging the system or posing a fire hazard.
• Ensures Safe Operation: The switch provides an essential safety mechanism, giving homeowners peace of mind.
• Improves Efficiency: By controlling the blower fan, the limit switch helps the furnace operate efficiently, avoiding wasted energy and extending the system’s lifespan.
• Reduces Repair Costs: A functioning limit switch prevents excessive wear and tear, reducing the need for costly repairs and replacements.

Symptoms of a Bad Limit Switch on Furnace

A faulty limit switch can lead to a range of furnace problems, reducing safety and potentially causing long-term damage. Here are common signs that indicate your limit switch may need attention:

1. Frequent On/Off Cycling: If the furnace frequently turns on and off (a process known as “short cycling”),it may indicate a faulty limit switch. This could lead to inefficient heating and increase wear on the furnace.
2. Blower Fan Runs Continuously: A bad limit switch may fail to shut off the blower fan, causing it to run constantly. This not only leads to higher energy bills but also can cause unnecessary wear on the blower motor.
3. No Heat or Blower Activation: If the furnace limit switch is damaged, it may prevent the furnace from producing heat or may block the blower fan from turning on, leaving your home without adequate warmth.
4. Error Codes on Digital Furnace Display: Many modern furnaces display error codes that can indicate a limit switch problem. If your furnace is showing an error, refer to the manual or consult a technician to diagnose the issue.

If you notice any of these symptoms, it’s best to contact a qualified HVAC technician to inspect your furnace and repair or replace the limit switch if needed. For peace of mind and to help protect your heating system from unexpected repair costs, consider a home warranty plan. 

Frequently Asked Questions (FAQs)

1. How can I test if my furnace limit switch is working properly?

Testing a furnace limit switch typically involves measuring its electrical continuity using a multimeter. It’s best to have a professional HVAC technician handle this, as working with furnace components can be hazardous if you’re not experienced.

2. What causes a furnace limit switch to go bad?

A limit switch can fail due to overheating, poor airflow, or age. Dust buildup in the furnace, a clogged air filter, or obstructions in the ductwork can lead to higher temperatures, which may damage the switch over time.

3. Can I replace a furnace limit switch myself?

While it’s possible to replace a furnace limit switch, it’s generally recommended to leave this task to a professional. Furnaces have high voltage components, and improper handling could lead to injury or further system damage.

4. How much does it cost to replace a furnace limit switch?

The cost to replace a furnace limit switch varies, but typically ranges from $100 to $300, depending on your furnace model and the labor costs in your area.