(Heating oil was 25¢ a gallon in 1964!) Additional benefits included more reliable burner performance, reduced burner emissions, and increased safety by tripping a boiler off line if the draft turned positive.Ĭheck out the first page of that article below. A burner which produces clean combustion only with high draft may cause smoke and soot any time the chimney is not producing high draft.Danbury, CT – In 1964, Preferred Instruments published an article in the Fuel Oil & Oil Heat magazine. During that time, draft controls were used primarily to control excess draft from tall chimneys and lower excess air to conserve fuel. Using a high draft setting to obtain enough combustion air for clean burning can cause trouble. A burner must produce good smoke-free combustion under low draft conditions. Be sure the burner does not depend on this air by setting the burner for smoke-free combustion with a low over-fire draft (.01 to. That is why proper draft must be obtained before the air adjustment is set.īecause there is little draft during a cold start-up, you cannot depend on the additional combustion air caused by draft. The burner may very likely smoke as a result of this change. You know what happens when the excess air is not properly adjusted. This is a reduction in draft of about 18% which will cause a reduction in the amount of air flowing into the combustion chamber. 01 inches of water, the total pressure becomes. If the combustion chamber draft drops to. 10 inches of water, the total force causing air to flow will be. If the combustion chamber has a draft of. To understand these problems, consider that the air pressure (positive draft) caused by a flame retention burner fan averages about. High draft during burner-off periods increases the standby heat losses up the chimney.Excessively high draft increases the air delivery of the burner fan and can increase air leakage into the heating plant, reducing CO2 and raising stack temperature, resulting in reduced operating efficiency.Too little draft can reduce the combustion air delivery of the burner and can cause smoke.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.
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