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If there were universal laws to describe ventilation, the first might be: Air out equals air in. This means that all of the air exhausted from fans and chimneys in a house is immediately replaced — somehow. Either it is forced into the house by a fan, or the indoor air pressures drop until outdoor pressures are strong enough to push replacement air through leaks and holes in the structure. This works fine up to a point. But when indoor negative pressures overcome the natural buoyancy of warm gas in chimney flues, they can reverse the upward flow of combustion gases and draw them back down into the house. When this happens, you have backdrafting: the pressure-induced