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“Chimneys 101"  How Chimneys Work


The purpose of any residential chimney is to safely remove the products of combustion from your house, such as smoke, carbon dioxide, carbon monoxide and water vapor. Chimney troubleshooting begins with a good understanding of how chimneys actually work. Two basic principles must work together to successfully vent the exhaust gases from an appliance (i.e. fireplace, furnace, boiler, water heater, etc.), so that the smoke or fumes go out the chimney top and not into your home. These two huge factors are draft and flow.


Draft is the desire of a gas to move through a venting system. Smoke always flows in the direction of the least pressure. If you think of your chimney as a soda straw in a drink, then when you decrease the pressure at the top of the straw, the fluid will move up and into your mouth. The same laws of physics apply to the smoky exhaust fumes in a chimney. Technically the pressure difference, between the interior and the exterior of a chimney, is called draft. Nothing has to be moving to have draft, because draft is the desire to move. You can push against a brick wall that doesn’t move, even though you want it to move. That pressure in a chimney is called draft.


Flow is the rate that gases actually move through a chimney. The measure of the molecules of exhaust gases that move through the venting system, per unit time, is called flow. Think of a multi-lane superhighway loaded with cars that are moving very well. The traffic flow is good. The same analogy applies to a chimney that works well.  If the exhaust products are moving up a chimney well, then you have good flow.  You can have a chimney with a poor flow, but a good draft.


Finally, think of a garden hose turned on at the house, but the nozzle is still closed. The desire for the water to move through the nozzle (pressure) is strong, even though the flow is zero. If you open the nozzle, then the pressure drops a little and the flow of water is good. In a chimney, the pressure to move is called draft and the flow is the rate of movement of the exhaust gases through the system.

Draft and flow affect each other and are influenced by dozens of other external factors that vary in importance from house-to-house. Understanding and correctly interpreting the relative effects that these factors have on a specific chimney will determine the proper diagnosis, which is critical to the success of any troubleshooting venture.

The whole evaluation process involved to properly diagnose a chimney problem is as much an art as a science. Many years of experience, seasoned with intuition and the laws of physics, often produce good results. The cures are as varied as the causes of the chimney troubles and that’s why hiring a certified, veteran, chimney professional with a good track record is often the most logical move in correcting a poorly functioning chimney.