The blower shown to the left forces primary combustion air into the main combustion chamber via cast iron louvers that are located between the firebrick that protects the firebox walls.
The opening shown here is covered with a damper during the installation of the furnace. The damper is closed by gravity when the draft blower is not on.

This view (right) shows the inside of the firebox of a residential Lynndale furnace. The arrows show the flow of primary combustion air, entering through the cast iron louvers then exiting underneath the baffle that separtates the primary and secondary combustion chambers.
The side view of a fully assembled Lynndale System shown below illustrates the complete path of air moving through the furnace, and the location of the main blower.

Side View

This view shows the size of the primary combustion chamber relative to the secondary chamber in the rear portion of the firebox.
This bypass damper is shown in the upper area of the baffle that separates the two areas.

Note also the ash removal tray beneath the cast iron shaker grates.