Backdrafting is the flow of air down a flue or chimney and into a house caused by low indoor air pressure that can occur when using several fans or fireplaces and/or if the house is very tight.

 

Forces working to depressurize a home include those shown in the figure opposite: bathroom exhaust fans, kitchen range hoods, and clothes dryers. Other forces not shown in the diagram include the fireplaces, leaky return ducts near combustion equipment, leaky supply ducts outside the conditioned space, wind, and the chimney effect (warm air rising in a building tends to depressurize lower areas). If these forces are great enough, they can work to suck air and combustion products back down the chimney or flue and into the house, as shown by the red arrows in the diagram.