I've heard that fire creates a vacuum due to the consumption of oxygen. Obviously, combustion results in other gases being released (CO2 and H2O vapor). Some have said that there is no vacuum; others have said there is in fact a partial vacuum. I assume that it depends on the relative densities of the gases (i.e. how does the density of the oxygen + fuel compare to the density of the byproducts). But if a burning fire does not create a vacuum, what causes the expansion and contraction cycle which leads to the observation of smoke "puffing" through holes and cracks when a room has the potential for a backdraft fire? Is it just that the gases in the room are cooling and contracting, which draws in fresh air, leads to more combustion, and then produces heat and expansion with the cycle repeating? Or does combustion produce a partial vacuum in some instances?