Why Does My Fireplace or Woodstove Smoke?

This is probably the most common question that homeowners ask the chimney sweeps on the CP Forum, an Internet discussion list for chimney professionals. It's also one of the questions I most frequently heard from my customers as an in-the-field chimney sweep.

The updraft or pull in a flue (passageway for smoke or combustion gases) is determined by the difference in temperature inside the flue and the temperature outside the chimney, along with other factors including the height of the flue and the resistance to airflow within it.

A column of hot gases in a flue weighs less than an equivalent column of cold air outside, so the pressure inside a warm flue is less than the pressure outside. This small difference in pressure creates draft.

If a chimney system fails to adequately pull the byproducts of combustion away from the living space, it is usually a result of one or more of the following problems. Many of these same principles also apply to the venting of other appliances such as water heaters.

Chimney smokes: chimney obstruction in the wood stove or fireplace

The smoke path might be blocked by something. Make sure the damper is open when there is a fire going.

Chimneys that are used frequently are commonly blocked or narrowed by soot or creosote. I was able to solve the majority of my customers' fireplace or wood stove smoking complaints with a thorough sweeping of the entire system from chimney cap to firebox.

If the system is little used, or if this is the first use of the season, a bird or animal may have built a nest in it.

Another frequent cause of blockage is deterioration of mortar or other components used in building the chimney, which sometimes flake off and accumulate in the turns of the smoke passageway.

Chimney smokes: cold flue

Smoke and combustion gases cool rapidly in a chimney exposed to the elements. Ideally, chimneys should be built within the house envelope to keep them warm.

Cooler temperatures in the flue may cause the fire to burn sluggishly. Smoke may spill out of the front of a fireplace or the door of a wood stove especially when the fire is dying out.

If smoke refuses to go up the flue when the fire is first lit, a column of cold air may be blocking the flow. A wood burner's trick is to point an electric hair dryer up the flue at the top of the firebox to start the flow of warm air. Be careful not to blow soot and ashes around!

Chimney smokes: house depressurization

Fireplaces and wood stoves located in basements pose a particular problem if a chimney is located on the exterior of the house. Since heat rises, the warm air in a house presses on the upper portions of the dwelling and the air pressure at bottom of a house is lower than elsewhere in the house. If the air pressure in the area of the fireplace or wood stove is less than the air pressure outside, smoke and combustion gases may be drawn into the house.

Appliances such as kitchen and bathroom exhaust fans and furnaces may reduce the air pressure in the house, particularly in houses weatherized to modern standards. Such houses have much less air flowing through them than was typical of older homes. Unless there is a source for the exhausted air to be replaced, the air pressure in that part of the house may be reduced to the extent that smoke and combustion gases are pulled into the house instead of drafted outside.

Chimney smokes: chimney design problems

Problems with the design of the flue and combustion system are common. Many chimneys are too short. The chimney must be tall enough to induce draft.

It must also be high enough so that the top of the chimney is clear of air turbulence caused by the roof of the house, nearby structures or trees. Wind induced downdraft problems are common when a chimney is located downwind from an object that causes air turbulence.

Combustion systems operate best when the size of the flue is in correct ratio to the amount of air that flows through it. Many fireplace chimneys are actually too large inside to operate well.

Smooth airflow through the system is also important. Excessive angles in the path of the smoke and combustion gases cause turbulence, which reduces the flow.

Many chimney sweeps are trained and equipped to help you diagnose and correct these problems. Keep your wood burning heating system in good working order with regular maintenance by a qualified professional chimney sweep.

If you don't already have a good chimney sweep, ask around in your community for a company with a good reputation. Sweeps also demonstrate their competency through certification programs offered by the Chimney Safety Institute of America (CSIA), the Wood Education Technology Transfer (WETT) program in Canada, and some state chimney sweep organizations. A listing of chimney sweeps is available at www.chimneys.com.

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