With the introduction of the Advance Design Cab in 1947, a new dealer installed heater also became available. When used together this new body was referred to as The Cab That Breathes.
All U.S. manufactured Advance Design Cabs had a row of louvers on the right outer cowl which allowed air to enter the cab interior with the help of the new optional fresh air heater. It was a great improvement over prior years!
As air was brought in from the outside it passed through the heater core. This warmed air usually held less humidity than air inside the cab due to the breath of passengers. The fresh, dryer, air helped prevent the inside of the windows from fogging during very cold days, thus GM said the cab breathed.
This excellent heating system was made even better in 1953 with the introduction of the revised airflow heater. By pulling a knob, at the left of the steering column, a door inside the heater would cover the outside air intake. Now the heater was of the recirculator design. It pulled air from inside the cab so that inside air was re-warmed. Yes, humidity did accumulate quicker and windows could fog but heating the air inside was faster on a cold morning.
Note these pictures of the improved 1953-55 fresh air heater. The driver operated cable was pulled and a door inside the heater shut off outside air. At the same time, a one inch wide door on the case opened to allow recirculation of air to occur. A non-related lever on the case directs the heated air to the floor or above through the defroster ducts.
This ingenious idea by GM engineers now allowed the driver to have a cab that breathed or one that did not.
Hand operated defroster lever, cable for intake door, and the water flow adjusting knob (above)
The knob at the left of the steering column for pulling the cable (above)
The main heater attached to the right inner cowl panel (above)
Side view of heater with water control rod in foreground (above)
A close up of the one inch intake cable operated door on the heater case (above)