Burners in conventional gas furnaces are
. This is sometimes referred to as an
. This means that we are not blowing any air into the combustion chamber to create an artificial draft condition (this is
), nor are we pulling air through the combustion chamber on exhaust side, again creating an artificial draft condition (this is
The amount of air seen by the burner is a function of the draft condition, which is largely determined by the chimney, and influenced by the convective heat from the burner. The chimney draft can change based on the temperature differences between indoors and out, and on the exterior wind strength and direction.