damper-slab02 Air exits into ashbox, just below firebox floor.
The base slab on this contraflow heater is 4″ thick precast vermiculite concrete, and is easy to cut.
This allows considerable leeway in routing out a channel on the underside of the base slab, if necessary, to align the outside air damper with the air supply hole in the house floor slab, left by the contractor.

damper-slab03Tight fit, no gaskets.
The axle is slightly off centre, to weight the pivoting plate for a positive closure.