Balancing air device for a heating unit

A balancing air device for a heating unit in a building having a flue for the passage of waste combustion gases from a combustion chamber in the heating unit and into a stack capable of having a draft flowing therein, comprising, baffle means in the flue for partially constricting flow therein and to provide sufficient flow therethrough when the heating unit is operating, an open valveless fresh outside air line for supplying air from outside the building, an open valveless combustion air line connected between said outside air line and the combustion chamber for supplying air to the combustion chamber, and an open valveless air return line connected directly between said outside air line and the stack for returning air supplied from outside the building into the stack when the heating unit is not operating, with the relative cross-sectional flow area of said outside air line being 3, the relative cross-sectional flow area of said combustion air line and said return air line being from 1 to 3 and from 1 to 3 respectively, so that, when the heating unit is not operating, no air is supplied to the heating unit through the combustion air line and all the air is supplied through the air return line to satisfy a draft in the stack, said return line being connected into the stack through a common opening with the flue of the heating unit.