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Why a Furnace Blower Turns On and Off Repeatedly? (Plus Easy Fix Tips)

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Furnace Blower - LG Home Comfort
January 16, 2023

During the colder months, your furnace works hard to keep up with the demand for warm air in your home. But if you notice that your furnace blower turns on and off frequently, your unit most likely has a problem. Typically, the furnace should go through 2 to 4 heat cycles per hour. Each cycle should last up to 5 to 15 minutes, depending on your thermostat’s setting and how cold it is outside.

How Do Furnace Blowers Normally Work?

There are four types of a furnace: gas, oil, electric, and propane. Each type has different components that manage the process of heating your home. One of these components is the blower fan, which circulates air through your ductwork to provide heat throughout your entire house. It works as soon as a thermostat detects a drop in temperature inside the space and signals for more heat.

In normal operation, a furnace blower should constantly run until the heating cycle ends to keep up with demand. This helps regulate air flow and prevent overworking the furnace or costly energy bills due to running it too frequently.

A gas furnace is the most popular one, and the blower inside it works by pushing air from your home’s return ducts, over the heat exchanger (the part that gets hot), through the supply ducts, and into all of your home’s rooms.

Here’s how furnace blowers usually work:

  • First, it will draw air from the return ducts, and then the blower fan will move that air over a heat exchanger. This is where the gas burners are located, producing heat for your home.
  • After the air passes over the heat exchanger, it is sent out of the supply registers as hot air throughout your home to raise its temperature.

Why Does My Furnace Blower Keep on Turning On and Off?

If your furnace turns on and off too frequently, there could be several underlying causes. Some of these include:

  • A dirty air filter
  • An obstructed airflow from closed vents or return ducts
  • Faulty thermostat setting
  • A shorted limit switch
  • Blocked ductwork or a clogged furnace burner
  • Corroded or dirty flame sensor

Some of these issues can be fixed easily by the homeowner, and some may require a professional to come in and take a more in-depth look. Either way, it’s essential to address this issue before it leads to other potential problems in your home, such as frozen coils, cracked heat exchangers, or even carbon monoxide leaks.

How to Fix Furnace Blowers That Constantly Turn On and Off

You can take some steps to fix the problem without calling professionals. Here are some of them:

Check for a Dirty or Clogged Air Filter and Replace It if Needed

If your furnace’s air filter is clogged or dirty, it can reduce the amount of air flowing through the system. This lack of airflow will cause your furnace blower to work harder and cycle on and off more often than usual. To prevent this from happening, check your air filter at least once a month and replace it if needed.

Here’s how you can do it:

  • Check your furnace’s user manual or contact the manufacturer to find out what type of filter you need and where it’s located..
  • Once you locate it, remove the old filter and place a new one in its spot.
  • Make sure to check the size and shape of the filter as well to ensure it fits properly.

Moreover, ensure your home’s air vents are open and not blocked by any furniture or curtains. This ensures the airflow is unrestricted, minimizing the strain on your furnace’s blower motor.

Check Return Ducts and Registers for Obstructions

Return ducts are how air is drawn into a furnace system, while registers are outlets that distribute warm air throughout your home. If these become blocked due to furniture placement, they can restrict airflow and cause your furnace blower to cycle on and off frequently. To prevent this, ensure all return ducts and registers are clear of any obstruction.

Here’s a simple guide on how to do it:

  • Start by removing any furniture or items blocking your return ducts and registers.
  • Clear away dust and debris with a vacuum cleaner with a crevice attachment.
  • Once you’re done, reassemble the return vent and register covers as needed.

Check Your Thermostat Settings

It’s possible that your thermostat settings are not configured correctly, and this could be causing your furnace blower fan to turn on and off often. Adjusting the settings on your thermostat can help to prevent this from happening.

There are types of a thermostat, such as non-programmable, programmable, WiFi, and smart thermostats. The settings will vary depending on the type of thermostat you have.

  • For a non-programmable thermostat, ensure it is set to “Heat” mode and the temperature is equal to or above your desired comfort level.
  • You can customize the heating schedule to match your lifestyle for a programmable or WiFi thermostat.
  • If you own a smart thermostat, you can use its features, such as energy-saving modes, which allow it to sense when you’re away from home and adjust temperatures accordingly. This helps reduce energy costs while ensuring that your furnace’s blower cycles on and off at optimal times.

If you are unsure, you can call a professional to help you inspect and adjust the settings on your thermostat.

Inspect the Fan Limit Switch for Damage or Shorts

Limit switches control how often your furnace blower turns on and off. If these become damaged or shorted out due to age or wear, this can cause them to malfunction and lead to cycling issues. It may enter a lockout mode requiring you to reset the switch or the entire furnace system. This safety mechanism prevents further damage or carbon monoxide leaks in your home.

Depending on your furnace’s age and model, you can reset it yourself. For newer models, turn off the power for about 30 seconds and turn it back on again. If it doesn’t work, call a professional for help.

A bad fan limit switch limit also needs to be replaced, which you can do yourself. You need a multimeter to test the switch. Make sure that the new switch matches your old one. Here’s how to test it:

  • Shut off the furnace’s power and gas.
  • Locate the switch limit, unscrew it, and remove the wires from the terminals.
  • Connect the electrodes of your multimeter to the switch’s terminals, set the meter to “Ohms,” and check for any shorts or resistances. If everything looks good, you can install the new limit switch in its place. Be sure to follow safety guidelines when doing so.

Check Your Flame Sensor

Your furnace’s flame sensor detects the presence of a flame. It shuts off the gas valve if it doesn’t detect one to keep gas from entering your home. If this becomes dirty or corroded, it can cause short cycling and result in your furnace blower shutting off and on frequently.

You can clean the flame sensor yourself with a rag or brush and some rubbing alcohol, but if it doesn’t work, you may need to call a professional for help. They can inspect the sensor and replace it if necessary.

Conclusion

A furnace cycling on and off repeatedly can be caused by various reasons, from dirty filters and obstructions in ducts and registers to incorrect thermostat settings and faulty limit switches. By following the steps above, you can help prevent your furnace from cycling too frequently and save energy costs. If all else fails, call a professional for assistance. Regular maintenance on your furnace system will also help keep it running smoothly and reduce the chance of frequent cycling.