Our "Direct-Vent" wall furnace requires installation on an exterior (outside) wall and must vent horizontally through the exterior/outside wall only. The direct-vent type of furnace draws outside air in from the vent tubes and returns the combustibles back through the vent to the outside. Direct-Vent furnaces work great in garages and bedrooms. Direct-Vent Gravity Models include #1403621, #1403622, #2203621, #2203622, #3003621 and #3003622. Direct-Vent Forsaire Counterflow Models include #4007732, #4007731, #6257732, #6257731, #4007332, #4007331, #5507332 and #5507331.