A fresh air heating system includes air intake and discharge chambers, a direct-fired gas burner mounted in a burner chamber in fluid communication with intake and discharge chambers therebetween, and a bypass opening between the air intake and discharge chambers that is selectively closed by a hatch cover having a panel and a peripheral wall extending from the panel. The hatch cover is movable between a first position wherein the hatch cover closes the bypass opening and the panel is positioned substantially horizontal and a second position wherein the hatch cover is lifted away from the bypass opening so as to minimize variations of air pressure differential across the bypass opening; the hatch cover being configured to lift away from the bypass opening when there is a positive variation of pressure between the air intake and discharge chambers, wherein the peripheral wall allows minimizing aerodynamic lift and the inherent reduction of static pressure under the panel when the hatch cover is lifted away from the bypass opening.