Furnaces | 5 Min.

6 Reasons Why Your Furnace Blower Keep Turning On and Off Repeatedly.

Written by Nikita

furnace blower

Furnaces are built with a protection mechanism that avoids unsafe situations such as a fire or a gas leakage. Please note, that most of the issues with the furnace come from inappropriate furnace installation. Sometimes, your furnace may quickly turn off without providing enough heat to warm up your home. This is called short cycling, i.e., short run times and short off times when the furnace turns off and on very often. It means if you set your thermostat to 72 degrees and the furnace shuts down at 65 or 68, any temperature range before reaching that.

How Your Furnace Blower Normally Works

Before delving into the potential issues, let’s first understand how your furnace blower is supposed to operate in normal conditions. In a typical heating cycle, your furnace blower fan should come on approximately 1.5 to 3 minutes after your thermostat calls for heat. It should continue running until the heating cycle ends, which is typically a few minutes after the desired temperature is reached. However, when your furnace isn’t actively heating, your blower fan shouldn’t be running at all. If you notice that your blower is turning on and off during a heating cycle or after the call for heat stops, it indicates a problem that needs to be addressed.

Reason Why Your Furnace Is Turning On and Off Repeatedly

This issue is often referred to as “short cycling,” and it can have several causes. But the most common reason why your furnace is turning on and off repeatedly can be due to issues such as a clogged filter, thermostat problems, improper airflow, or a malfunctioning component.

Let’s have a look at all the major issues that can lead to the short cycling of the furnace:

1. Low airflow

Low airflow may be one of the top reasons for short furnace cycling and continuous turning on and off of the furnace. Different reasons cause low airflow:

2. Faulty Thermostat

The thermostat controls your complete heating system. Breakage or faulty thermostat may also cause short cycling of a furnace. Several reasons cause a thermostat to function poorly:

3. Fire Sensor

The flame sensor lets you know if there is any flame when the gas valve is open and allows natural gas flow. If there is no flame, the sensor shuts off the gas valve to prevent the gas from entering your home. But if the flame sensor is rusted or dirty, it may not work correctly and will shut down your system.

It is recommended to contact an expert, especially if your furnace turns off and on too quickly.

4. Blower Motor

The blower motor circulates the air throughout your house. In rare conditions, the furnace may run even if the blower motor is off. And if it happens, there will be no airflow over the heat exchangers, which may cause your furnace to go off.

To make sure that the blower motor is not the reason behind short cycling, hold your hand near the vents to make sure there is no air coming. If there is weak airflow or no flow, the blower motor may be the cause of your short furnace cycling.

5. Flue Pipe

The flue pipe, also known as the exhaust vent, is located on your roof and can also cause your furnace to short cycle. A damaged vent pipe or any blockage, such as a bird nest in the summer, causes your furnace to overheat by building up hot gas in it.

The flue sensor will shut down your Furnace to keep you safe from carbon monoxide or any hazardous gas on heating up. When your air furnace shuts down for anything small, such as a clogged air filter, it can be unpleasant; such safety features avoid harming your family and your house.

6. Over-sized Furnace

It will cycle on and off regularly if your furnace is too big for your home. This is not a dangerous condition but it can lead to higher utility bills due to the increased energy usage. In this choice, it is appropriate for you to change your furnace if you want to eliminate this issue. If you just got your furnace built up and are short cycling, it means that the technicians did not conduct a manual J heat load. Short cycling is not a factor that should be ignored.

To examine your device, contact a competent HVAC contractor as soon as you realize there’s a problem. If you don’t, you may be looking at substantial repairs or even replacing the whole system.

Solutions for Blower Cycling During a Heat Cycle

While most of the problems listed above require professional assistance, there are a few steps you can take on your own to try and solve the blower cycling issue. Before calling a professional, consider trying the following:

1. Check for a Dirty Air Filter

A clogged air filter can limit the amount of cool air passing over the heat exchanger, causing it to overheat and shut off. This triggers the blower to turn on and help cool down the heat exchanger. By replacing the dirty air filter with a clean one, you can ensure proper airflow and potentially resolve the blower cycling problem.

2. Clean the Air Handler Blower Fan

Similar to a clogged air filter, a dirty air handler blower fan can impede cold airflow over the heat exchangers, leading to blower on/off patterns. If you notice dirt on the blades of the air handler fan, try wiping them carefully with a dry towel or toothbrush to improve airflow.

If these DIY solutions do not resolve the issue and your blower continues to cycle on and off during a heat cycle, it’s crucial to consult a professional HVAC technician who can inspect your furnace and identify the underlying problem.

Possible Causes of Blower Turning On/Off When Furnace Isn’t Heating

If your blower is constantly turning on and off when the furnace is not actively heating, it’s likely due to a faulty fan limit switch. The fan limit switch is a control system that determines when the furnace blower assembly should turn on and off. It utilizes a temperature probe to detect temperature changes inside the heat exchanger. However, when the furnace burners are not in operation, temperatures inside the heat exchanger should remain relatively stable, and the blower fan should not be needed. If the blower continues to cycle on and off in this scenario, it suggests a defective fan limit switch that needs to be addressed by a professional HVAC technician.

Solutions for Blower Cycling When Furnace Isn’t Heating

When the blower is constantly turning on and off when the furnace isn’t heating, it’s best to leave the repair or replacement of the fan limit switch to a professional. The inner workings of the switch can be complex, and improper handling can lead to further damage. An HVAC technician will have the expertise to diagnose and resolve the issue effectively and safely.

Call Smile HVAC: If you reside in the Toronto area or its vicinity, you can rely on our HVAC-certified technicians for top-quality service when your furnace is cycling. Contact Smile HVAC for furnace maintenance in Toronto and GTA.


Why does my furnace blower fan keep turning on and off?

Your furnace blower fan may repeatedly cycle due to a clogged air filter, a malfunctioning thermostat, inadequate airflow, or issues with the furnace's limit switch. Each of these issues can disrupt the blower's operation.

Why does my blower motor keep cutting out?

A blower motor can cut out because of electrical problems, overheating, or worn-out components. It may also be due to a malfunctioning capacitor or issues with the motor's bearings.

Why does my furnace blower motor work sometimes?

Intermittent blower motor operation can result from loose wiring, a faulty control board, or a damaged motor. Insufficient maintenance or worn components may also lead to sporadic functioning.

Can a thermostat cause short cycling?

Yes, a malfunctioning or improperly configured thermostat can cause short cycling. If the thermostat is not calibrated correctly, it may signal the furnace to turn on and off too frequently.

How do you know if the blower motor is bad in the furnace?

Signs of a bad blower motor include reduced airflow, unusual noises, or the motor not starting at all. If you suspect a faulty blower motor, consult an HVAC technician for a thorough inspection and potential replacement.

How do you fix HVAC short cycling?

Fixing HVAC short cycling may involve addressing issues like a clogged filter, thermostat calibration, or problems with the limit switch. It's advisable to consult a professional technician for accurate diagnosis and repair.

What to do if the furnace fan keeps running?

If your furnace fan continuously runs, it may be due to a thermostat issue, a malfunctioning limit switch, or problems with the fan control. Consult an HVAC technician to diagnose and resolve the problem.

How long should the furnace blower run?

The ideal duration for the furnace blower to run depends on various factors, including the system's design and your heating needs. On average, a blower should run for around 15-20 minutes per cycle to ensure efficient heating and proper air circulation.