Physics of the Flue

• In the home, air enters the fireplace opening and is drawn up through the fuel by the heated air above it. As the air passes through the fire, it is heated and rises. Oxygen feeds the fire and produces combustion gases. Ash and sparks that rise on the heated air fall back to the grate or lodge on the shelf that houses the damper, the flat piece of iron that closes off the flue when it is not in use. Lighter-than-air gases such as carbon monoxide aid this buoyancy. Super-hot gases rise, cooling slowly as they go, until at last they tumble into the outdoor air at the top of the chimney.
  
  

Flue Height

• A chimney cap extends the height of the flue.Jupiterimages/Photos.com/Getty Images
  
  Depending on the direction of the wind, the length of the flue, and the chimney’s placement on the roof and its proximity to an exterior wall, heated air may cool before it reaches the top of the flue. Air currents above the house create a vacuum at the top of the flue that pulls the air from the flue, but if the end of the flue is below the roofline or nearby trees, wind and air turbulence may blow down into the flue, starting a downdraft. The problem often can be solved by extending the flue, by adding either bricks or a chimney cap, and by trimming surrounding trees.
  
  

Blockage

• The flow of air in the flue results from the combination of buoyancy caused by heat and the continued movement, or "draw," of air, aided by inertia and the movement of air at the top of the chimney. Not much is required to disrupt the equilibrium of this system. A bird’s nest on the smoke shelf or leaves blown in by a storm may not block the flue completely, but they would most likely disrupt the capacity of the flue to draw, perhaps to the point of forcing a downdraft. Clearing blockages by sweeping the chimney improves airflow.
  
  

Depressurization

• Tightly sealed buildings may retard air flow.Comstock/Comstock/Getty Images
  
  Fireplaces that work efficiently may develop downdrafts when houses are newly insulated or leaky windows are replaced. The balance of pressures may be thrown off by a fan on a furnace air handler, a heat pump or another vented appliance. Attic leaks may draw off warm air and depressurize lower floors. Even bathroom exhaust fans can disrupt the flow of air through the house. Sealing any leaks in upper areas and increasing the inflow of outside air in lower levels of the house may help to offset the depressurization that encourages downdrafts.