Back drafts are caused when the house is under negative pressure.    A good sign the house is under negative pressure is when you open a door and it closes by itself behind you or if you open a window and a breeze blows in.  Depending on the appliances you have operating in the house, back draft conditions may exist and occupants  may be subject to carbon monoxide poisoning.   CO is produced when you have incomplete combustion, when the fuel being burned does not have enough combustion air to completely burn the fuel.  A common situation found during a home inspection is a furnace room being sealed up with dry wall without sufficient supply air.    This situation is fine if the appliances in the the room is direct vented.   That means the intake supply air and exhaust are piped directly to the exterior through the side walls.   The appliances of concern include hot water tank or mid efficiency furnace which uses natural draft to vent the combustion exhaust,   Natural  draft appliances require the house to be under positive pressure to work  adequately.

During a home inspection, experienced home inspectors look for the above conditions and note the potential hazard.  In addition, some subtle situation the home owner may not be aware of include the dryer acting as an exhaust, a kitchen exhaust fan, a attic or whole house fan exhausting directly to the exterior, a chimney flue left open, which all can contribute to the house being under negative pressure preventing natural draft appliances from working properly.

Gaps in the duct work can decrease efficiency of system by about 15%. This is a typical furnace upgrade from mid efficiency natural draft to high efficiency 2 pipe direct vent. The upgrade resulted in modifications that needed tape and patches to seal gaps.

Another subtle condition are the leaks in the ducts from the boot of the furnace to the jumper in the furnace room.   All the leaks add up to the equivalent of having return vent near the blower motor.   First, the return air should come from either the first floor or second floor but not in the basement as a rule of thumb.  There is typically enough leaks in the duct works around the furnace to decrease the efficiency of the system about approximately 15%.  The blower motor is strong enough to create a negative pressure environment in the furnace room and thus create back drafts in the natural draft vents.  With a return vent close by or at the blower motor, a negative pressure environment is also created.