Pontiac 1955 Owner's Manual page 21

once it is adjusted to produce a comfortable temperature in the car.
Even though the car has been standing in the cold it is not necessary
to move the "TEMP" lever, because this setting will produce as much
heat as "HIGH" while bringing the air temperature up to the setting.
Air Control—The air that is heated and circulated through the car
is outside air drawn into the car; the Venti-Heat System does not
recirculate stale air. The "AIR" control (lever on right of steering
column) regulates the entry of outside air through the underseat
unit and defroster unit. Moving the lever to the "OFF" position
closes the air valve. Moving the lever to either the "NORMAL"
or "DE-ICE" position opens the air valve.
The "AIR" control lever when moved from the "OFF" position towards
"NORMAL" allows an increasing amount of unheated air to be dis
charged through the defroster nozzles until the "NORMAL" position
is reached. As the control lever is moved from "NORMAL" toward
"DE-ICE", heated air is mixed with the cool air in increasing amounts
until at the "DE-ICE" position all air discharged by the defroster
nozzles is heated. As the "AIR" control is moved from the "OFF"
position, the valve controlling the air flow through the heater is opened
fully and remains this way even when the control is in the "DE-ICE"
position.
De-Icing Windshield—Moving the "AIR" control lever to the
"DE-ICE" position forces heated outside air through the windshield
defroster and defroster ducts to clear the windshield of ice. After
de-icing is completed the lever can be moved toward the "NORMAL"
position if desired.
[18]