It’s another cold winter day, and you’re looking forward to lighting the fireplace. You have done this many times before, so you’re not expecting any problems. A few minutes later, smoke and soot are suddenly pouring out of the fireplace, and you’re wondering why. Fireplace backdrafts and downdrafts are more common than many homeowners realize. They are usually easy to fix once you have pinpointed the potential cause.
One of the most common causes of a backdraft is forgetting to open the damper when lighting the fireplace. If you start a fire and the damper is closed, the rising heat and exhaust will not be able to exit the chimney resulting in a backdraft that could cause smoke, soot, and other debris to be pushed out of the fireplace into the living space. Always ensure the damper is fully open before lighting the fireplace.
A flue obstruction can also cause a fireplace backdraft. Sometimes small animals such as birds, squirrels, raccoons, and other pests enter an open flue restricting the venting of smoke and exhaust. In other cases, leaves, twigs, and other debris clog the vent forcing the fumes back down the flue. Installing a chimney cap will help prevent flue obstructions and downdrafts. A chimney cap with a mesh screen will also keep curious critters from getting trapped in the flue. Periodically check to ensure the chimney cap is secure and replace when damaged.
When the accumulation of soot and creosote in the chimney restricts the airflow to the point where cold outside air is being drawn in faster than the smoke and fumes can vent, the draft reverses, causing a fireplace backdraft. Just a half-inch of sooty build-up in the flue is sufficient to restrict up to 20% of the airflow. Cleaning the chimney regularly can help minimize backdrafts from occurring.
Those chilly days can make the chimney very cold when you light the fireplace, especially if the fireplace is on an exterior wall or in the basement. When lighting a fireplace in a cold flue, there isn’t sufficient heat to lift the smoke up the chimney, resulting in a fireplace downdraft. Warming the flue before lighting a cold fireplace can prevent downdrafts. You can warm the flue by burning a piece of cardboard or rolled-up newspaper and holding it inside the firebox under the flue opening until you feel the draft reversing with the heat rising the chimney.
If you store firewood outside or in a shed where it can get damp, the extra moisture will create more smoke and creosote with less heat. Without sufficient heat, there is an increased risk of a downdraft. To minimize downdrafts, burn firewood that has been seasoned for six months or more. Store the wood in a dry place to prevent it from getting wet in the rain or snow.
Scheduling an annual chimney inspection and cleaning can help prevent most fireplace backdrafts/downdrafts. Also, the Chimney Safety Institute of America (CSIA) states that 1/8” or more of creosote is a fire hazard and should be removed before lighting the fireplace. A certified chimney specialist will perform a visual inspection of the entire chimney system. The technician will note any obstructions, creosote, sooty build-up, damaged chimney cap, or other issues that will need to be corrected to prevent the occurrence of a fireplace backdraft or
other safety hazard.