Abstract: A novel horizontal-vertical (H-V) direct vented gas heating unit comprises a box-shaped heating unit having seven walls which surround a combustion chamber. One of the seven walls comprises a diagonal wall connected between the back vertical wall and the top wall. An exhaust stack extends through the diagonal wall and is surrounded at its exit by a coaxial fresh air inlet adapter which connects to a fresh air passageway which extends down the back wall. An inner heat exchanger comprise passageways which extend along six of the seven walls outside the combustion chamber. The fresh air passageway on the back wall extends through the heat exchanger passageway in the back wall and enters a plenum having two exits, one of which is positioned opposite the upper level and the other opposite the lower level of a dual level burner.