Chimney Design Considerations

A good chimney must provide containment for the heat and fire so that house structures cannot become overheated and they must safely and adequately convey cumbustion by-products outside the building without adversely affecting the combustion process itself.

Chimney height and flue cross-sectional area are the two most critical factors in designing an effective chimney system. The chimney height (the vertical distance between where air enters the system and the top where it leaves) creates a natural draft. This draft manifests itself as a negative pressure at the lower end (the fireplace draft control or opening) since the top end is exposed to ambient atmospheric pressure.

This negative pressure causes air to be sucked into the draft opening on the appliance. There will usually be a natural "chimney effect" even without a fire going since warm house air wants to rise and uses the chimney as the easiest way to rise out of the heated room.

Once a fire is started, the by-product gases produced are momentarily at about 2300°F (with a wood fire) and create an increased draft; enough to remove all of the by-product gases from the structure as well as some additional "excess air."

If the flue gases maintain their heat, they continue to want to rise in the chimney, maintaining the draft. If the chimney is on an outside wall exposed to sub-zero temperatures, the chimney gases will generally lose more heat and have a less effective draft. That is why an interior chimney generally tends to draw better than outside wall chimneys do. A way to accomplish the desired effect is to provide high-temperature insulation around the inner flue liner. Class-A Insulated (double wall) Chimney does this in the product. Thermix insulated concrete accomplishes it after the fact.

Necessary chimney height is determined by the interaction of a large number of variables. Climate, terrain, presence of nearby tall trees, location related to the building all can affect chimney performance. Building codes generally require 3 feet above the roof it goes through and two feet above any roof within ten horizontal feet. This is a generic solution to a complex problem which nearly always gives good results. With a high efficiency product such as the JUCA wood burning furnaces, which extracts so much heat from the smoke before it leaves the appliance, the resultant smoke is cooler. It is therefore less prone to rise in the chimney. It is therefore advisable to err on the side of a taller chimney if aesthetics allows flexibility here.

In the Northern United States, prevailing westerly winds dominate the winter. If a chimney is on the west side of a tall house which presents an obstacle to winds trying to pass by, some of that wind could try to use a short cut to get by the house. Instead of going around it, some might try going DOWN the chimney, into the house, and then leak out around windows and doors on the east side. Motorized kitchen and bathroom exhaust fans could accentuate the problem by removing air from the house, thereby requiring replacement air to come from somewhere (like down an open chimney.)

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Nearby trees could create turbulence and downwardly moving air downstream of themselves. If your house is there, you could get strong downdrafts at very irregular intervals.

Modern super-tight houses also represent a complication. Older houses had plenty of infiltration around doors and windows and even through the walls to supply a fireplace or woodstove. Modern building techniques have reduced infiltration (as an energy efficient effort) to the point that not enough air may leak into the house to replace the air that must go out the chimney. Bad draft is the result. It might even be necessary to open a window to allow the fire to draw properly. This situation would have been eliminated if outside combustion air had originally been provided for the fireplace. When smoke goes up the chimney, it has its own source of supply air via a direct intake duct from outdoors.

Chimney cross-sectional area is also important in proper chimney design. In general, the area of the flue should be no less than one tenth of the largest open area of the fireplace. This is to make sure that all by-product gases are able to get out of the fireplace under all conditions. Under many conditions, a much smaller chimney would suffice, but for unusual weather conditions during an intense fire, the proper size flue will likely be necessary.

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