No, the small lever with the white knob pushes the air from cab discharge to the defroster ducts. It is just a directional flap on that small lever. My reference is more about the effect you gain with this heater with that door, it does dump a lot of heat.
I understand that, but what I am trying to convey is that pulling the cable knob which opens the narrow door, while at the same time closes off air from the outside of the truck converts the heater to a recirculating heater. That certainly would, as you say, dump a lot of heat since there will not be any outside air mixed in.
Carl
