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Sunday, January 29, 2012

What Draws the Smoke Up the Flue?



Sitting in front of this fire prompted the question, "What causes the smoke to rise up through the chimney?" Obviously, hot air rises but a bit of research provided the full story. The following excerpt comes from How Your Chimney Really Works within the Chimney Safety Institute of America website:

Draft and Flow
Many experienced chimney professionals use the similarities between water and air to explain the way your chimney works. Although most people don't realize it, the air moving up your chimney works under the same set of physical principles as water flowing in a hose or pipe.When a fireplace chimney is full of hot air, it actually pulls air through the firebox. This pulling effect is called draft and it corresponds to the amount of pressure in a water hose - the only difference is that the air pressure is negative and the water pressure is positive (think of using a straw to drink with instead of to blow bubbles). Thus, a chimney is called a negative pressure system. Increasing the draft in your chimney is like opening the faucet wider on the hose. The simplest way to increase the draft in your chimney is to burn the fire hotter - hotter air is lighter, so it has more pull.

Another way to get more draft is to increase the height of your chimney - except when the chimney is already so tall that frictional forces negate the effect of the extra height. Given the same amount of pressure, a larger pipe can obviously carry a greater volume of water than a smaller one. The same is true for chimneys - with the same amount of draft (pressure), a larger flue will exhaust more smoke from your fireplace than a smaller one. But just as a water hose can be kinked or plugged, the airflow in your chimney can have a restriction that slows down the smoke flowing up the chimney. Some of the reasons for poor flow in a chimney are: excessive creosote deposits; closed or plugged dampers; improper construction; structural damage or even a dirty chimney cap. In fact, having a plugged-up chimney cap at the end of your chimney is just like having a closed nozzle at the end of a hose - you just can't get any airflow through the chimney.


There's also a good section in the Chimney Safety Institute of America website on How to Select Firewood.