Insufficient Draft in Chimney

Wrong Size

• A chimney that’s too small for your combustion appliance won’t let the smoke out fast enough, while a chimney that’s too big won’t get hot enough to vent smoke by natural draft. Fireplace chimney flues should have a cross-sectional area of one-tenth the area of the fireplace opening. The chimney area for other combustion appliances should equal the area of the appliance’s flue outlet. If the chimney is too small, you must replace the chimney or the combustion appliance. A chimney that’s too big can be fitted with a liner to reduce the flue area. A chimney that’s too short may not provide enough draft. A chimney should be at least 2 feet higher than any part of the roof within 10 feet of the chimney.

Blockage and Leakage

• Bird nests, leaves and other debris can partially block a chimney flue. Creosote buildup from a wood burner can also reduce the chimney flue diameter. If you have a spark mesh cap, it can become clogged with combustion byproducts. The solution is to have a chimney sweep come in to clean out the chimney and chimney cap. Connecting multiple combustion appliances to the same flue can introduce cold air that effectively reduces the draft. Each combustion appliance should have its own separate flue. Also, make sure your chimney is in good repair and not leaking air through masonry or metal joints.

Negative Pressure

• A clean, tight, properly-sized chimney can still smoke into the house if indoor conditions create negative air pressure that sucks smoke backwards down the chimney. Negative pressure can occur in a house that is very tightly sealed, or that has exhaust fans or blowers that create a negative pressure zone. You can also suffer negative pressure on the first floor if leaky upper levels of a multistory house vent warm air outside. The simple solution is to open a window to allow fresh air to enter. A better solution is to provide your combustion appliance with a direct connection to the outside to take in fresh air.

Atmospheric Conditions

• Sometimes atmospheric conditions inhibit chimney draft. In very cold weather, a chimney can fill with dense super-cold air that can block rising smoke. The solution is to “prime” the draft by heating the chimney by burning balls of newspaper one at a time or lighting a can of jellied alcohol fuel at the base of the chimney until natural draft is restored. Atmospheric inversions can trap dense air at the chimney-top level, preventing smoke from rising normally. Winds can cause downdrafts that prevent smoke from rising. Commercial draft-inducing wind caps can help this problem, or you can simply wait for the winds to die down.
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