Outside air enters the house through the vent. During downdrafting, hot combustion gases spill into the house instead of flowing up the vent. Anything that moves air out of a house and depressurizes the house can cause downdrafting. This includes exhaust fans, any vented heating appliance, fireplaces, even holes in the ceiling. Vents in tight homes are especially susceptible to downdrafting.
Warm air is lighter than cold air and normally rises.
A chimney relies on this buoyant effect to carry combustion gases out of the house. Unfortunately, buoyant forces are weak and can easily be overcome by other air movements, such as those created by bathroom exhaust fans, clothes dryers, and air leaks in the house. Also high winds around the outside of the building or a chimney which is too short can create a downdraft. Downdrafting usually disappears as the appliance warms up. It can, however, last for several hours.