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: