GMC 2001 C3 Owner's Manual page 166

FLOOR: This setting directs most of the warmed
air through the floor outlets and center floor console
outlets, as well as some air through the windshield
defroster outlets and side window defroster outlets. If air
from the center floor console outlets is not desired, turn
the vents off at the console.
BLEND: This setting delivers air through the
floor outlets, center floor console outlets, the windshield
defroster outlets and the side window defroster outlets.
The air flow can be divided between floor and defrost
depending upon where the knob is placed between the
settings. When in blend mode, the air conditioning
compressor may run to assist in dehumidifying the air
and limit window fogging.
DEFROST: This setting directs most of the air
through the windshield defroster outlets with airflow
also delivered from the side window and the floor
outlets. When in defrost mode, the air conditioning
compressor may run to assist in dehumidifying the air
and limit window fogging.
OUTSIDE AIR: This setting should be used
for normal system operation and directs the system to
use outside air. Using outside air will limit stale air
odors and help to prevent excessively humid interior air
(from wet boots or clothing or maximum passenger
loads) from being redirected onto windows.
Pressing the outside air button will cancel the
recirculation button.
RECIRCULATION: This setting limits
the amount of outside air entering the vehicle to a
very small amount. This will be helpful to limit
odors entering the vehicle and will assist in reaching
comfort in very hot temperature conditions. Pressing
recirculation will cause the system to recirculate interior
air continuously and may cause the vehicle windows
to fog if the passenger compartment floor is
excessively wet.
Pressing the recirculation button will cancel outside air
operation. To return to normal system operation and to
prevent stale passenger compartment air, be sure to
return to the outside air position.
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