Page 324

NORA Oilheat Technicians Manual

flue gases. Condensing equipment, which deliberately condenses the water in the combustion gases, is available in some heating appliance lines. These units are designed to handle the water and the acids that are created in this process. To prevent condensation in conventional heating units, exhaust gas temperatures need to be 350°F or more. The 350° exhaust gases also produce the chimney draft that is required by conventional systems for normal flue gas venting. “Sensible heat loss” depends on the temperature of exhaust gases and their total volume. Increased excess air increases the volume of combustion gases, which also increases the velocity of these gases through the heat exchanger. The faster the gases move through the heat exchanger, the less heat can be extracted. This raises stack temperatures and lowers efficiency. Lowering excess combustion air and/or flue gas temperature can reduce sensible flue heat loss. New oil-powered appliances operate very efficiently, with flue heat losses ranging from 11 to 15%. This is close to the highest value possible for non-condensing systems. Remember that a net stack temperature of about 350°F or more is required to avoid water condensation and to maintain adequate chimney draft. Sidewall vented units can operate with somewhat lower temperatures. Off-cycle heat loss Burners in home heating systems do not operate continuously, but cycle on and off. A typical burner will run between 15 to 20% of the time and remain idle for the remaining 80 to 85% of the time. Chapter 16 Energy Conservation FLUE HEAT LOSS DURING BURNER OPERATION (Percent of Total Heat Content of Fuel Oil) Average Typical Range Comb. Eff. Old Oil Heating Units 28 20 - 35 72% New Oil Heating Units 13 11 - 19 86% Minimum for Non-Condensing 10 — 89% Figure 16-3: Off-cycle heat loss Chimney Draft Pulls Heated Air at 145 - 220°F Hot Surfaces in Boiler or Furnace Cold Air in at 50 - 70°F 16-6 Energy Conservation


NORA Oilheat Technicians Manual
To see the actual publication please follow the link above