GMC 2007 Acadia Owner's Manual page 209

The climate control system uses the information
from these sensors to maintain your comfort
setting by adjusting the outlet temperature, fan
speed, and the air delivery mode. The system may
also supply cooler air to the side of the vehicle
facing the sun. The recirculation mode will also
be used as needed to maintain cool outlet
temperatures.
Defogging and Defrosting
Fog on the inside of the windows is a result of
high humidity (moisture) condensing on the
cool window glass. This can be minimized if the
climate control system is used properly. There
are two modes to clear fog or frost from your
windshield.
Use the mode up and down arrows to select
the defog mode. Use the defrost button to
select the defrost mode.
-
(Defog): The defog mode is used to clear
the windows of fog or moisture and warm
the passengers. This mode directs air to the
windshield, floor outlets, and side window vents.
When you select this mode, the system turns
off recirculation and runs the air conditioning
compressor unless the outside temperature is
close to freezing. The recirculation mode cannot
be selected while in the defog mode. Do not
drive the vehicle until all the windows are clear.
0
(Defrost): Press this button to remove
fog or frost from the windshield more quickly.
This mode directs most of the air to the
windshield and side window vents and some
to the floor vents. In this mode, the system
automatically forces outside air into your vehicle.
The recirculation mode cannot be selected
while in the defrost mode. The air conditioning
compressor runs automatically in this setting,
unless the outside temperature is close to freezing.
Do not drive the vehicle until all the windows
are clear.
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