Blower Runs Nonstop? What That Means for Limit Switches and Overheating Risks

Discover why your furnace blower runs nonstop and how limit switches, airflow, and overheating issues could be the cause. Learn what to do next.

A furnace blower that never shuts off can be frustrating and even a little worrying. Many homeowners think it’s just the fan running to keep air moving, but in many cases, a nonstop blower means the system is trying to protect itself from overheating or a sensor is misreading the situation. Either way, it’s not something to ignore.

The blower motor is designed to cycle on and off as the system heats or cools the home. When it runs nonstop, it wastes energy, adds unnecessary wear to the motor, and can be a warning sign that something deeper is wrong with the furnace’s safety system. One of the key parts involved in this issue is the limit switch.

Let’s break down what causes a blower to run constantly, how the limit switch is connected to this problem, and what homeowners can do to prevent potential damage or overheating risks.

Understanding What the Blower Does

The blower is the part of your furnace that moves air through the ductwork. It doesn’t create heat itself but pushes warm air throughout the house once the burners or heating elements have warmed the air in the heat exchanger.

Under normal operation, the blower:

• Turns on a short time after the furnace starts heating.

• Runs continuously until the thermostat temperature is reached.

• Shuts off a few minutes after the burners stop.

  
  

If it never shuts off, that sequence is interrupted. Something is telling the blower to keep running, even though the furnace has stopped producing heat. The most common culprit is the limit switch or a miscommunication between it and the control board.

The Role of the Limit Switch in Furnace Safety

The limit switch acts as the furnace’s built-in temperature guardian. It monitors the air temperature inside the furnace and ensures the system doesn’t overheat.

Here’s how it works:

• When the furnace starts heating, the limit switch senses the rising temperature.

• Once the air inside the plenum reaches a safe and warm temperature, the switch signals the blower to turn on.

• As the furnace cools after the heating cycle, the limit switch shuts the blower off when the temperature drops to a safe level.

  
  

The limit switch also serves as a high-temperature safety shutoff. If the furnace’s internal temperature climbs too high, it cuts power to the burners to prevent overheating. This safety function keeps the heat exchanger from cracking and stops potential fire hazards.

If the limit switch malfunctions, gets stuck, or senses abnormal temperatures, it can cause the blower to run continuously as a protective measure.

Why the Blower Keeps Running

Several issues can cause a blower to keep running after the furnace stops heating. Some are simple fixes, while others point to deeper problems within the system.

1. The Limit Switch Is Stuck in the “Closed” Position

A limit switch has electrical contacts that open and close based on temperature. Over time, these contacts can stick or fail. When the switch gets stuck in the closed position, it constantly signals the blower to stay on, even when the furnace doesn’t need to move air.

This problem usually develops slowly, with the blower taking longer to shut off after heating cycles until it eventually runs continuously.

2. Overheating Furnace

If the furnace overheats, the limit switch triggers the blower to run non-stop to help cool the system. Overheating can be caused by:

• Dirty air filters that restrict airflow.

• Closed or blocked vents.

• Dust buildup on the blower motor or coils.

• A failing motor that struggles to move air efficiently.

  
  

In this case, the blower is doing its job to protect the furnace, but the root cause needs to be fixed quickly. Repeated overheating can damage the heat exchanger, which is costly to replace and can cause dangerous carbon monoxide leaks.

3. Thermostat Fan Setting

Sometimes, the issue is simple. The thermostat may be set to “Fan On” instead of “Auto.” In this setting, the blower runs continuously, independent of heating or cooling cycles. Switching it back to “Auto” returns the blower to normal operation.

It’s a small detail that can easily go unnoticed, but can save hours of unnecessary troubleshooting.

4. Faulty Control Board

Modern furnaces rely on electronic control boards to manage signals between the thermostat, blower motor, and safety switches. A malfunction in the control board can cause it to misread the signals coming from the limit switch or thermostat, leading to continuous blower operation.

Electrical issues, moisture, or power surges can damage the board, and once it malfunctions, it may keep the blower running to prevent a perceived overheating situation.

5. Short Circuit or Wiring Issue

Loose or damaged wires in the furnace cabinet can create false signals that keep the blower running. A short in the wiring connected to the limit switch or blower relay can mimic the signal that tells the blower to stay on.

How Overheating Affects Your Furnace

Overheating is one of the most common and dangerous outcomes when the blower runs continuously. The system works harder than intended, the components stay hot for too long, and metal parts begin to expand and contract excessively.

That kind of stress can:

• Crack the heat exchanger, allowing gases like carbon monoxide to escape.

• Cause electrical parts to burn or fail prematurely.

• Dry out insulation on wires, increasing fire risk.

• Create inconsistent heating and air distribution.

  
  

The limit switch tries to protect against overheating, but if it’s dirty or faulty, it may react too late or too often, which shortens its lifespan and the furnace’s reliability.

How to Diagnose the Issue Safely

A furnace blower that won’t stop running can have multiple causes, so it’s important to approach the problem methodically and safely.

1. Check the Thermostat Settings

   Make sure the fan is set to “Auto.” If it’s on “On,” switch it back and wait a few minutes to see if the blower shuts off.

2. Inspect the Air Filter

   A dirty air filter restricts airflow, causing overheating and limit switch activation. Replacing or cleaning the filter is one of the easiest and most effective ways to solve airflow issues.

3. Observe the Furnace Operation

   Watch the furnace as it runs. Does it heat up and shut off quickly? Are there long blower cycles with no heat? These patterns help narrow down whether the issue is airflow, sensor-related, or electrical.

4. Listen for Unusual Sounds

   Grinding, rattling, or high-pitched whirring can point to motor or bearing problems that affect blower operation.

5. Avoid Resetting the Furnace Repeatedly

   Some people try resetting the furnace multiple times to stop the blower. This can make things worse if the system is overheating or has an electrical fault. It’s better to identify and address the actual cause.

   
   

If the blower keeps running after these checks, it’s best to have a professional inspect the limit switch, control board, and wiring.

How Technicians Fix a Constantly Running Blower

A technician starts by testing the thermostat and checking whether it’s sending a continuous fan signal. Then they move to the furnace to measure temperature readings and test the limit switch for continuity.

If the limit switch is stuck closed, they may clean or replace it. Dust and soot buildup can affect how accurately it senses temperature. If the readings are off but the switch itself is clean, the issue might be a wiring problem or a faulty control board.

In cases of overheating, the technician inspects filters, ducts, and blower components for obstructions or signs of poor airflow. The repair often involves cleaning components, lubricating moving parts, or recalibrating the temperature sensors to prevent the limit switch from tripping unnecessarily.

Preventing Blower and Limit Switch Problems

Prevention is always better than repair when it comes to furnace issues. Simple habits can protect your system from overheating and extend its lifespan.

• Change filters regularly. Dirty filters are the leading cause of airflow restriction and limit switch activation.

• Keep vents open and unblocked. Closing too many vents traps heat and pressure in the system.

• Schedule seasonal maintenance. Annual inspections help detect weak motors, faulty switches, and buildup before they become serious problems.

• Check ducts for leaks. Leaky ducts force the blower to work harder to maintain airflow.

• Keep the area around the furnace clean. Dust, lint, and debris can settle inside the cabinet and affect temperature sensors.

  
  

Routine care ensures that the limit switch only activates when it truly needs to, not because of preventable issues.

Why Ignoring a Constantly Running Blower Is Risky

A nonstop blower might not seem urgent since it still moves air, but ignoring it can lead to bigger problems. It raises your energy bills, wears down the motor, and increases the chance of heat-related damage. More importantly, if the cause is overheating, it poses real safety risks.

A properly functioning furnace cycles predictably and maintains steady indoor comfort. If yours doesn’t, it’s trying to tell you something.

Understanding how the limit switch and blower interact helps you recognize early warning signs before they turn into expensive repairs or safety hazards. Addressing the issue early protects not just your equipment but also your comfort and peace of mind during the colder months.