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This large variation cannot be tolerated for the following reasons: The high draft is over 12 times more than the low draft. You can see that the draft produced by this chimney could be expected to vary from. To indicate the effect of these changes, the information in the chart on page 2 was determined for a 20 foot high chimney. After the heating unit has operated for a while, the gases and the chimney surface will be warmer, increasing draft. When the heating unit starts up, the chimney will be filled with cool gases. Since the outside temperature and flue gas temperature can change, the draft will not be constant. The weight per unit volume of the air outside the home - the colder the outside air, the greater the draft.The weight per unit volume of the hot combustion products - the hotter the gases, the greater the draft.Chimney height - the higher the chimney, the greater the draft.There are three factors which control chimney draft: “Induced draft” blowers can be used in the stack to supplement natural draft where necessary. The vacuum is then created throughout this column of hot gases.Ĭurrential Draft occurs when high winds or air currents across the top of a chimney create a suction in the stack and draw gases up. The rising is contained and increased by enclosing the gases in a tall chimney. Since hot combustion gases weigh less per volume than room air or outdoor air, they tend to rise. A given volume of hot gas will weigh less than an equal volume of the same gas at a cool temperature. Just as a mercury barometer is used to measure atmospheric pressure in inches of mercury, a draft gauge is used to measure draft intensity (pressure) in inches of water. Draft intensity is measured in “inches of water”. Draft volume specifies the cubic feet of gas that a chimney can handle in a given time. The amount of vacuum is called draft intensity. In the oil heating industry, “draft” describes the vacuum, or suction, which exists inside most heating systems.