Troubleshooter: What is AC Recirculation and how do you use it?

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One of the most misused features on any vehicle has to be its HVAC (heating, ventilation, and air conditioning) recirculation system. Those little switches marked with either a MAX AC label or a pictogram of a vehicle with an arrow inside have been causing confusion ever since Packard put the first air-conditioning system in a non-luxury car in 1939. During those sweltering summer days and extremely frosty winter mornings, knowing how to use this feature can pay off in more ways than one.

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Troubleshooter: What is AC Recirculation and how do you use it? Back to video

When selected on hot days (with the temp setting set to cool) the RECIRC or MAX AC button does two things: it closes the fresh-air intake of the heater/AC system, and it sets the AC compressor to full-time operation with no cycling on or off. Many, but not all vehicles will see the blower fan turned up to its highest speed as well. The benefit of this is a quicker cool-down of the interior by closing itself to outside hot air.

Using this on cold winter days will create heavy frosting or fogging of the inside of all the glass surfaces of a vehicle as the system can’t reduce the humidity without some fresh outside air, no matter how hot you set the temp controls nor how high you turn up the blower fan.

Getting a car cooled down quickly on a hot day is another matter. Driving with the windows down can push just as much hot air in as it removes, so you’re best to do a few door swings before getting in. Lower a door window on the opposite side of your auto, then swing your door open and closed a few times (with its window closed) to fan out a good portion of the hot air. Once the vehicle is running, turn on the AC and when you start moving, switch it to MAX or RECIRC setting and close the windows tight. Within a few minutes the cabin temps should be comfortable enough to return the controls to regular AC and adjust the fan speed to a lower setting.

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This will let the AC compressor cycle off and on, reducing the load on the engine and fuel consumption rates. For optimal operation, vehicle AC systems require engine speed and air flow through the heat exchanging condenser among other things. This is why you’ll often note warmer air in slow or stop and start traffic. Cabin heat and cooling also see better performance when the blower fan speeds are kept lower than the fastest setting. This gives the air more contact time with either the heater core in winter or the AC evaporator coil in summer producing warmer or cooler vent temps.

On the topic of vents, don’t expect the outer dash vents to deliver the same cooling temps as the centre ones. These ducts are usually located just under the dash’s upper cover, so the sun’s heat can easily warm these longer channels on vehicles with wide dash tops and windshields.