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NORA Oilheat Technicians Manual

Chapter 7—Combustion 7-17 together and check all joints for possible leaks. If you suspect air leaks, but cannot find them, temporarily set the draft regulator to give maximum draft and run a candle or lighted match around the joints to see where the smoke is drawn in. Repair the leaks and set the over-fire draft to -.01" to -.02". Also, look for locations where light from the flame can be observed through cracks. Smoke After you have determined that draft conditions are correct, the next step is to adjust the burner to create a flame that will not produce smoke. Keeping smoke to a minimum is a must if the heating system is to operate at peak efficiency without further attention during the entire heating season. We measure smoke concentrations by taking a sample of the flue gases with a smoke tester and smoke scale, which identify smoke numbers from zero (no Figure 7-17: Smoke vs. percent CO2 curve Chapter 7 Combustion Figure 7-18: Smoke scale Percent CO2 Smoke Spot Number smoke) to a number 9 (heavy smoke). Any smoke numbers above a number 2 cause rapid soot accumulations on heat exchanger surfaces. This causes flue gas temperatures to rise and efficiency to drop, Figure 7-16. Excess combustion air is needed to reduce smoke concentrations to a minimum acceptable number (less than a number 1). As we have discussed, this excess air must be held to a minimum in order to reach peak efficiency. Thus, correct burner adjustment is the proper balance between smoke and combustion air. To reach high efficiency, we must adjust burners for Zero Smoke and low excess air while maintaining a high level of CO2 or a low level of O2, Figure 7-17. The typical smoke tester draws a total of 2,200 cubic centimeters (ten full strokes of the sampling pump illustrated) of flue products through a standard grade filter paper. The color of the resulting smoke stain on the filter paper is matched to the closest shaded spot on the


NORA Oilheat Technicians Manual
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