Why Does My HVAC Fan Turn On by Itself?

When your HVAC parts aren’t working in unison, you need a solution on the double!

Although located in a southern state, Lubbock winter temperatures dip to near-freezing 26-degree lows that have you yearning for heat and comfort. When the furnace powers on, you may breathe a sigh of relief, but what if it’s only your fan that’s working? You may ask yourself, “Why does my HVAC fan turn on by itself?” or wonder why it toggles on and off during cycles, and we have the answers.

On The Double! Heating & Cooling, the #1 HVAC team in Lubbock and the surrounding Texas areas, has spent over a decade helping residents with these and other heating and cooling concerns. Within that time, we’ve uncovered the common causes for these issues and devised solutions that alleviate them. Below, our team breaks them down and explains when to call for reliable HVAC services in Lubbock

How Do Cycles Usually Work?

Your heating and cooling system uses a thermostat to keep track of your current indoor temperature. The thermostat sends signals to your unit to turn on when there’s a rise or fall in temperature during the summer or winter, respectively. Throughout each cycle, the thermostat monitors the indoor temperature; when it becomes comfortable, it tells the system to power down. 

Each cycle 5 five to 15 minutes, depending on the temperature gap between the indoor temperature and your ideal thermostat setting, and you may notice two to four cycles each hour. 

Common Behavior from the Fan

The blower fan, which helps evenly distribute air through air ducts and vents, may not power on until between 1.5 and 3 minutes after the cycle begins. It may also remain on a few minutes longer than the rest of the system. But if it stays on too long, powers on again after the cycle ends, or toggles on and off during the cycle, call for a specialist. 

Reasons the Fan Toggles On and Off During Cycles

The fan should remain on during the entire cycle, so if you notice it taking breaks when it should be working, you may have dirty air filters that reduce airflow into the system. When less air passes over the heat exchanger, the exchanger overheats and turns off. When it cools down, the blower turns off, too, refusing to power back up until the exchanger does, so it’s key to change or clean your filters seasonally. 

Dirty or clogged air ducts and closed vents create a similar issue, limiting the air that enters and leaves your HVAC unit. While wiping down your vents and air ducts every three to four months reduces contaminants, call for annual duct cleaning that’ll leave the chutes spotless.

Other common causes for fluctuating fan operations include:

  • A dirty blower fan
  • Clogged burners
  • A faulty thermostat
  • Extreme temperatures in the heat exchanger 
  • Malfunctioning limit switch
  • An oversized unit

Reasons the Fan Powers on After a Cycle

However, if you’re wondering, “Why does my HVAC fan turn on by itself after the cycle?” we can narrow down the potential causes above. In most cases, a malfunctioning limit switch is the culprit since it’s the limit switch that sends signals to the blower fan to turn on and off. 

Like the thermostat, this control system monitors temperatures but in the heat exchanger rather than in your household rooms. When the heat exchanger heats up, the limit switch tells the fan to turn on to pull cool air in for heating and rerelease the warm air through vents. Therefore, the fan should turn on only when the heat exchanger is on, and the heat exchanger should work only when the unit cycles. 

So, if your fan is running when your unit is not cycling, false readings from the temperature probe or manual override mode may be the issue. Before calling a professional technician, look for the limit switch under the furnace hood. If the small white knob appears pushed in, the fan will run continuously, so hit the reset button that should send it back into auto mode. 

For a Five-Star Team on the Double!

From faulty thermostat settings and temperature fluctuations to leaky ductwork and an oversized unit, several things can cause blower fans to act up. Rather than live with this and other common HVAC problems, jeopardizing your comfort, unit health, and energy costs, count on our family-owned company. 

We’ll answer, “Why does my HVAC fan turn on by itself?” and more. Call On The Double! Heating & Cooling in Lubbock, TX, at 806-203-6512, and take advantage of our 24/7 assistance today!

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