Top-Vent Counterflow Models require less than one square foot of floor space. They may be recessed into the stud space or surface mounted and vented vertically through the ceiling and roof. Direct-Vent Counterflow The forced-air circulation comfort heating system uses only outside air for combustion, as the burners are completely sealed within the combustion chamber. Combustion products are discharged outdoors through the wall vent. This eliminates the expense of a vent. All vent caps include a built-in “Bird Guard.” All vent material is included. Ideal for garages.