What Causes Chimney Draft Blockage?

Most household chimneys are prone to developing blockage in the form of an obstructive chimney draft. This could be due to lack of general maintenance or some operational issues. Some of the most common causes of a chimney draft blockage include:

Cause 1 - Insufficient Chimney Height

Chimneys usually draw some amount of air, even when there is no fire lighted below. This phenomenon is called ambient updraft. It occurs because the top of the flue extends upward several feet, into a lower density atmosphere than exists at the bottom. This gradual draft of air often causes blockage among household chimneys, irrespective of their overall condition or age.

Cause 2 - Flue Blockage

If the size of chimney is appropriate and it still fails to have sufficient draft for exhausting fumes, the stovepipe or the chimney flue can be responsible for causing the blockage. Fallen bricks, bird nests and leaves can often block the chimney flue. Even a small amount soot or creosote build-up may reduce the flue diameter, interfering with proper draft.

Cause 3 - Chimney Resistance

Once you have determined that the chimney is clean, make sure that sufficient combustion air is being provided to the fire. The chimney pulls air through a fireplace or woodstove. As a result, negative air pressure can be created, which fights against the chimney draft and can actually draw smoke backwards, down the chimney flue.

Cause 4 - Air Intrusion into Chimney Flue

Each wood-burning stove or fireplace requires its own flue. If some other appliance is venting into the same flue, air can leak in through the second appliance. This leakage can also happen through cracks in the connector pipe or the thimble fittings. Similarly, excessive updraft of air can also cause a problem. Ensure that the airtight woodstove is not burning excessively. For this, the door and glass gaskets shouldn’t have an air-leak.