Air Recirculation Button: When to Use It

When to Use the Air Recirculation Button in Your Car—and When You Shouldn’t

If you’re like me, you’ve probably noticed that little button on

The lighted button shows the HVAC system is in the Fresh-air flow through mode

your car’s climate control panel with the symbol of a car and an arrow looping inside it. That’s your car’s air recirculation button, and it’s an underrated feature that can significantly improve your driving comfort and air quality—when used correctly. Let me walk you through exactly when to press that button and when to leave it alone.

What is the Fresh Air Flow-through Mode, And Why Is It Always On?

In the Fresh Air Flow-Through mode, the HVAC system opens a flap door to allow outside air to enter the cabin. When set to the Fresh Air Flow-Through mode, the air exiting the HVAC vents is 10% to 15% outside air and 85%-90% recirculated air. In other words, the cabin always gets some outside air, even with the HVAC fan turned to the OFF position.

Why Constant Fresh Air Flow-through Circulation Is So Important in an Auto HVAC System

Car cabins need fresh air flow-through to purge stale air and to reduce CO₂ accumulation from driver and passenger exhalation. Automotive HVAC engineers are keenly aware of the effects of CO₂ buildup in the cabin. It’s a far more serious consideration than most drivers imagine.

“According to a study (CFD simulations of effects of recirculation mode and fresh air mode on vehicle cabin indoor air quality) published in Science Direct, the concentration of atmospheric CO₂ is typically around 400 ppm. However, vehicle passengers exhale CO₂ at concentrations as high as 38,000 to 56,000 ppm. The Lee and Zhu (2014) study found that the CO2 concentration in passenger car cabins typically ranges from 630 to 2500 ppm. Without fresh air flow-through ventilation, even normal breathing by the passengers results in a significant increase in the CO₂ concentration within the cabin. Shu et al. (2015) found that the CO2 concentration in taxis in Los Angeles ranged from 700 to 1600 ppm. The Zhu et al. (2007) study showed that for a 3-passenger car using the recirculation mode, the CO2 concentration increased to 4500 ppm in just 10 min due to the relatively small size of the vehicle cabin.

An increasing number of studies have shown that CO₂ can have serious deleterious effects on cognitive function and decision-making, even at low-to-moderate concentration levels and short-term exposures. Katarzyna (2011) showed that the continuous use of the recirculation mode in the cabin of a passenger car led to CO₂ concentrations of around 4500 ppm, and thus posed a significant risk to driving safety. Satish et al. (2012) reported that subjects exposed to 2500 ppm CO₂ for 2.5 hours showed a statistically significant reduction in decision-making ability. In general, the results in (Kajtar et al., 2003, 2006; Katarzyna, 2011; Satish et al., 2012) suggest that CO₂ concentrations in excess of 2000 ppm may lead to a significant reduction in driver awareness and judgment ability. Chang et al. (2018) showed that to keep the CO₂ concentration within a vehicle cabin at a level of less than 2000 ppm, the cabin had to be supplied with fresh air at a rate of 4 l/s for each occupant.”

What Does the Air Recirculation Button Do?

First, let’s understand how the air recirculation button works. When you activate the recirculate mode, your car’s HVAC system closes the outside fresh air intake, causing the system to recirculate the air already inside your cabin. This keeps outside air—and whatever it’s carrying—from entering your vehicle. Instead, your system continuously cycles and conditions the air within your car.

The Advantages of Using the Air Recirculation Button

1. On Hot Days It Cools Down the Cabin Faster— When you first turn on your air conditioning, the system works hard to cool the super-hot air inside your car. Switching to recirculate mode allows the system to cool the already-cooled air in your vehicle further, allowing your A/C to work more efficiently. You’ll cool down much faster and reduce the strain on your vehicle’s air conditioning system.

2. It Removes More Pollen and Airborne Particulate Matter— The recirculate mode causes the cabin air to run through the cabin air filter (if equipped) more times to remove exhaust fumes, industrial pollution, and even the smell of that truck belching black smoke can seep into your cabin if you’re pulling in outside air. Recirculating the air keeps those pollutants out, giving you cleaner air to breathe.

For those of us who suffer from seasonal allergies, the last thing we want is pollen, dust, or other allergens wafting into the car. By switching to recirculate mode, you minimize the inflow of outside air—and with it, the allergens that make you sneeze, itch, or worse. Combine this with a clean cabin air filter, and you’ll feel like you’re driving in an allergen-free bubble.

3. On Cold Days, It Heats The Cabin Faster— In winter, I use recirculate mode to keep the warm air generated by the heater inside the cabin. Since the outside air is often freezing cold, allowing it into the vehicle can counteract the work your heating system is doing. Recirculating the air helps the heater maintain a consistent, comfortable temperature.

When Not to Use Air Recirculation Button

While recirculate mode can be a game-changer, there are times when it’s best to avoid using it.

1. In Cold Weather, the Air Recirculation Mode Makes It Harder To Defog Your Windows— Have you ever noticed your windows fogging up when the button is in the recirculate mode, especially on a chilly or rainy day? That’s because recirculation mode traps moisture inside the cabin, and without fresh air to balance it out, condensation builds up on your windows.

If your windows start fogging, turn off recirculate mode and let in outside air or crack open a window. Better yet, switch on your defrost setting. This directs air to the windshield and side windows, helping to clear them up quickly.

2. On Long Drives, the Air Recirculation Mode Creates an Uncomfortable Cabin Environment— Here’s something I’ve learned the hard way: using recirculate mode for extended periods can make the cabin feel stuffy and stale. Without fresh air coming in, oxygen levels can drop slightly, which might leave you feeling drowsy or uncomfortable.

If I’m on a road trip or a long commute, I switch to fresh air mode periodically—usually every 20-30 minutes—to bring in some outside air and maintain a good balance. It’s especially important if you’re driving with a full car, as more people mean more CO2 concentration buildup inside the cabin.

3. When Driving in Clean, Comfortable Conditions, Take Advantage of the Fresh Air— If the weather is mild, the air quality is good, and your car is at a comfortable temperature, there’s no need to use recirculate mode. Letting in fresh air keeps things natural and can even improve your mental alertness.

Pro Tips for Using Recirculate Mode

• Maintain Your Cabin Air Filter— If you’re going to rely on the air recirculate mode to keep your cabin air clean, make sure your cabin air filter clean. A dirty filter can reduce airflow and circulate dust and debris within your car. I recommend checking and replacing it every 12,000 to 15,000 miles or as specified in your owner’s manual.

• Keep an Eye on Window Fogging— Even if you’re a die-hard recirculate mode fan, don’t ignore the signs of excess humidity in your car. A quick switch to fresh air mode or defrost can prevent foggy windows from becoming a safety hazard.

• Use It Strategically— Recirculate mode isn’t an “always on” setting. Use it strategically based on your conditions, like during traffic jams, extreme weather, or allergy season.

©, 2024 Rick Muscoplat