The warm air flowing up the chimney must be replaced in the living space with air from other sources. This commonly comes from air leaking in through doors and windows. In a tightly sealed home, or if exhaust fans vent air from the living space, the low air pressure in the home prevents the flow of exhaust gases up the chimney. When the gasses flow downward into the home, a downdraft exists. Open a window near the stove or fireplace to allow additional air into the home.
Chimneys use the natural phenomenon of hot air rising to create a draft. This column of warm air in the chimney pulls the exhaust gases away from the stove, furnace, fireplace or appliance and out of the home living space. When something disrupts this natural air flow, a downdraft occurs. This can carry exhaust gases that should be vented outside the home into the living space. This can occur for a variety of reasons, all of which create a dangerous situation in the home.
Winds blowing over the top of the chimney help draw exhaust gasses up and out of the chimney. If something disrupts the wind movement and creates downward momentum, this can force air down the chimney. Situations such as a tree next to the home can create the downward wind. Sometimes adding height to the chimney prevents this type of downdraft.
The draft in the chimney occurs when the chimney fills with naturally buoyant warm air. When you first light the fireplace or stove, the cold chimney can't draw the exhaust up and out of the home. Create some heat by burning newspaper in the flue opening, if possible.
A plugged chimney causes smoke and exhaust gas to enter the home similar to a downdraft. Check to make sure the chimney and flue are clear, if addressing the downdraft issues does not solve the problem. Clearing a clogged chimney may require the services of a chimney sweep professional.