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NORA Oilheat Technicians Manual

Chapter 7 Combustion deterioration, liner failure, and rusting of the appliance. Take a gross stack temperature reading, subtract the temperature of air entering the burner (ambient air temperature) from the gross stack temperature to get the net stack temperature needed for calculating efficiency. percentage of excess air in the flue gases (CO2 or O2) and the net stack temperature is the combustion efficiency. Causes of high stack temperature • Soot Deposits: The insulating effect of soot deposits prevents good heat transfer through the heat exchanger. Inspect the heat exchanger, clean if necessary, and adjust the burner to a zero to trace smoke. • Excess Air: Excess air cools the combustion gases and increases their volume. This results in lower heat exchanger poor air-fuel mixing, poor burner adjustment and air leaks into the heat exchanger. • Overfiring: Firing a heating system at a higher GPH (gallons per hour) than it was designed for causes high rates of gas flow through the heat exchanger and results in high stack temperature. Dew point temperature The dew point temperature is the temperature below which the water vapor Draft is the flow of air and (more air than needed to stop smoke), combustion gases through the burner, heating unit, and venting system. Draft is required to remove the flue gases from the heat exchanger. The relationship between the efficiency. Excess air can be due to contained in the flue gas turns into a liquid. This change is often referred to as condensation. Below the dew point temperature, moisture exists; above the dew point temperature, vapor exists. If the 7-14 Combustion chimney or venting material falls below the dew point temperature, condensation in the flue will occur. To prevent condensation, the stack temperature should range from 270°F to 370°F above the ambient air temperature for non-condensing appliances. (With new high efficiency equipment that does not have a draft regulator, combustion gases can be on the low end of this range; if there is a draft regulator, they should be closer to the high end.) With the new condensing appliances, the stack temperature will be close to the return air or water temperature from the heating system and usually below 125°F. The lower the heating system return air or water temperature, the higher the efficiency will be on a condensing appliance. Using the dial type stack thermometer Slide the holding clip out to the end of the thermometer stem. Insert the small tab into the top of sampling hole in the stack and push the thermometer in far enough so the tip is in the center of the flue pipe. Operate the burner until the thermometer reading is rising no faster than 3 degrees per minute and record the reading. If the stack temperature begins to approach 1,000 degrees, remove the thermometer, because readings above this temperature will damage it. Draft Draft is the flow of air and combustion gases through the burner, heating unit, and venting system. Draft is required to remove the flue gases from the heat exchanger. If draft is too low, then the combustion


NORA Oilheat Technicians Manual
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