Chevrolet 1998 C/K Full-Size Pickup Owner's Manual page 165

Heating
The heater works best if you keep your windows closed
while using it. On cold days, use the HEATER or
VENT/HEAT setting with the temperature knob in the
red area.
Outside air will also enter the vehicle when the heater
the air conditioning fan is running, unless you have the
recirculation button pushed in. For more information o
the recirculation button, see "Air Conditioning" earlier
in this section.
If you use the engine coolant heater before starting your
engine in cold weather, 20°F (-8
O
C) or lower, your
heating system will produce warmer air faster, to heat
the passenger compartment in cold weather.
The use of an engine coolant heater also reduces the
time it takes for the engine to reach normal operating
temperature, and shortens the time it takes the heater to
reach full output. For more information, see "Engine
Coolant Heater" in the Index.
Ventilation System
For mild outside temperatures when little heating or
cooling is needed, use VENT to direct outside air
through your vehicle. Air will flow through the
instrument panel outlets.
Your vehicle's ventilation system supplies outside a i r to
the inside of your vehicle when it is moving. With the side
windows closed, air will flow into the front a i r inlet grilles,
through the vehicle, and out the a i r exhaust valves.
Your vehicle has air outlets in the center and on the sid
of your instrument panel. You can move the outlets from
side-to-side or up and down to direct the flow of air, or
close the outlets altogether. When you close an outlet, it
will increase the flow of air coming out of any outlets
that are open. These outlets do not completely shut off
airflow when in the closed position.