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NORA Advanced Efficiency

Sealing these air leaks (Figure 2) with furnace cement will reduce off-cycle airflow and heat loss. To test for air leaks into the heat exchanger, compare the excess air levels over the fire and at the breech. They should be the same. If there is more excess air at the breech, this indicates there are heat exchanger leaks. Figure 2 Air Leaks Flue Gas Temperature The flue gas temperature (net stack temperature) is directly related to combustion efficiency. As the flue gas temperature rises, more heat goes up the chimney and the furnace, boiler, or water heater captures less heat. The lower the flue gas temperature–the higher the efficiency. Typical net flue gas temperatures for modern heating equipment are between 350°F and 450°F. It is good practice to be sure the net stack temperature at the breech is about 350°F to insure that the flue gas temperature at the top of the chimney is above 200°F (the point where sulfuric acid starts to condense). In most installations, a 350°F temperature should keep the sulfuric acid in the flue gases from condensing in the chimney. As sulfur levels in the fuel are reduced, this is less of a concern. If the net flue gas temperature at the breech is above 450°F, too much heat is going up the chimney and efficiency is lowered. Check for the following conditions and take corrective action as needed. 20 National Oilheat Research Alliance Soot and Scale Accumulation on the heat transfer surfaces—Soot and scale are great insulators and will prevent the heat exchanger from working efficiently. Brush and vacuumclean these surfaces and then determine the cause of the soot. Too high a firing rate—The heat exchanger does not have time to extract the useful heat because there is too much heat being produced in too short a time period. Overfiring causes the burner to short-cycle, causing many problems. A common cause of overfiring is increasing pump pressure without reducing the nozzle size. Increasing pump pressure increases firing rates. When raising the pump pressure, downsize the nozzle. Excessive draft through the heating unit— If the draft is set too high, excessive amounts of air enter the heating unit through the burner and secondary air leaks. These leaks lower the combustion temperature, which reduces the rate of heat transfer. Because there are more gases flowing through the heat exchanger, they must also flow more quickly and the heat exchanger has less time to remove the useful heat. The result is an increase in the net stack temperature and lower efficiency. Adjust the draft regulator and seal secondary air leaks. Outdated heating unit—Older boilers, water heaters and furnaces designed to operate with low static pressure burners cannot effectively transfer heat from the combustion gases to the water or air. The result is higher flue gas temperatures and reduced efficiency. Outdated equipment gives both low carbon dioxide readings and high flue gas temperatures. The effect on efficiency is devastating. Common fixes, such as bricks, baffles and turbulators in the heat exchanger, will slow down the flow, but replacing the old, inefficient boiler or furnace is a better choice. The Periodic Preventative Maintenance Tune-Up Periodic cleaning and adjustment of all


NORA Advanced Efficiency
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