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NORA Oilheat Technicians Manual

Oilburners need steady draft for proper operation. Insufficient or variable draft can cause operational problems. For example, a fire that pulsates or a rumble in the appliance may result from insufficient draft. Chimney venting creates natural draft There are two types of natural draft created in the chimney— thermal and currential. Thermal draft is created when Figure 6-2: Typical air flow for combustion and flue gases the air in the chimney is hotter, and therefore lighter (less dense), than the air outside. As the lighter air moves up the chimney, (See Figure 6-2), more air moves in from the surrounding room to replace it. Currential draft is caused by the suction created as wind rushes over the chimney top, creating a negative pressure in the chimney. Because wind is variable, currential draft is unpredictable and must be controlled. Occasionally wind will blow down the chimney causing a ‘down draft.’ A variable draft is created by a pressure difference between the top and bottom of the chimney. What affects draft Draft is created by a pressure difference between the top and bottom of the chimney. The draft produced by a chimney is variable, not constant. The temperature of the outside air, the temperature of the flue gases, the barometric pressure, and humidity of the air all affect draft. When the burner is first fired, the chimney is full of cool air and there is little 6-4 Draft and Venting or no thermal draft. As the chimney and gases warm, the draft will strengthen. As the outside air cools, the temperature difference increases, and draft increases. Other conditions that affect draft include wind velocity across or into the top of the chimney and flow restrictions in the chimney, flue pipe, or heat exchanger. It is important that the chimney be properly constructed, clean, and have no air leaks through cracks and gaps. The placement of the chimney and its construction can also affect draft. A chimney operates best when it is warm and dry. Therefore, a chimney with one or more of its walls outside the building does not work as well as an inside chimney. The outside chimney heats up slowly and cools off rapidly. Additionally, the chimney must extend at least two feet above nearby objects, such as the roof peak, trees, and other buildings within 10 feet. Overhanging trees or high buildings can affect the draft and can cause wind Chapter 6 Draft and Venting


NORA Oilheat Technicians Manual
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