Incorrect Chimney Venting

Symptoms

• Incorrect chimney venting causes noticeable problems even when not using the wood stove or fireplace. Venting problems can cause cold drafts and bring unpleasant odors into the home. When starting a fire in a poorly vented wood heater, smoke blows back into the room at first, instead of flowing up the chimney.

  Problems can continue even when the stove holds a hot fire. Smoke blowing back into an airtight stove can cause small wood gas explosions or can puff out into the living area when the door opens for refueling. Stoves might work well on most days, but back draft badly when wind direction changes.

Location

• Whether a chimney is located inside the frame of the house or built against an outside wall, greatly affects the efficiency of the draft. A warm chimney vents air better than a cold chimney, since warm air rises. A chimney exposed to the elements starts out with a slow draft and improves as the fire heats the stack.

  When the fire dies out, cold temperatures can chill the entire chimney and cause a constant cold back draft. Chimneys located inside the house expose only a short upper section to temperature extremes. The warmth of the house creates positive air pressure and prevents back drafting.

Height

• Adding height to the chimney improves venting, but increases the construction cost and makes cleaning more difficult. Chimneys at least 3 feet taller than a flat roof should vent properly, and chimneys reaching 2 feet above a peaked roof also draft correctly. If any part of the building within 10 feet rises higher than the chimney, the chimney might back draft.

  Eddies of wind form around high parts of the building and push downward on the downwind side. Where other tall buildings or trees cause frequent eddies, chimney height might not solve the problem. Special vent caps can compensate for the eddy effect.

Construction

• Airtight stoves with higher BTU outputs require stove pipes of larger diameter than those designed for smaller heaters. To ensure a good draft, the diameter of the chimney flue must exceed the diameter of the correct stove pipe by 25 percent, according to the University of Missouri Extension.

  Fitting a heater to a narrower flue restricts the flow. Obstructions such as old bird nests cause similar problems, so always check the chimney before lighting the first fire of the season. Modern airtight homes also cause venting trouble by restricting air intake. Opening a window 1 inch may restore the draft and clear the air.