burner at basement air temperature (usually about 63°F). The exhaust gases at the breach are normally between 350° and 600°F. The heat in the gases that go up the chimney is heat loss. Some loss is unavoidable, but heat loss can be reduced by better burner air adjustment, clean heat exchanger surfaces, or equipment replacement. New equipment uses much less combustion air and some units do not need a draft regulator, hence less heat is lost up the flue. Water Vapor Loss The water vapor (latent heat) loss is covered in Chapter 1. It is fixed and cannot be altered except through use of a condensing furnace or boiler covered in Chapter 4. The only way to reclaim this energy is to lower the combustion gas temperature to a point where the water would condense (turn from steam into water) and the latent heat would be released. This can only be done in specially designed boilers or funaces. Excess Air (Sensible) Heat Loss Excess air or sensible heat loss (also called chimney loss) is the amount of energy that leaves the building along with the combustion gases. The chimney must be heated in order for it to operate. The larger the temperature differences between the combustion gases and the outdoor temperature–the stronger the draft. The combustion gases that must be removed from the building include: carbon monoxide, smoke, sulfur oxides, nitrous oxides, steam and carbon dioxide. The size of this heat loss varies from unit to unit and is determined by the volume of gases that must be removed. The more gases–the higher the stack temperature and the lower the efficiency is. Off-cycle Heat (Idle) Losses Off-cycle heat losses, also called idle losses, occur when the burner is not running. Oil burners do not operate continuously; they cycle on and off in response to demand. Because most boilers and furnaces are oversized, the typical residential or light commercial burner operates between 15% and 20% of the time during the heating season—less than 900 hours a year. 12 National Oilheat Research Alliance Idle losses are caused because chimneys continue to draft even when the burner is not operating. Whenever the air inside the chimney is hotter than the air outdoors, the chimney will create negative draft. When the burner is off, this draft will pull warm room air into the burner air intake, across the hot heat exchanger and up the chimney. It will also draw warm from the room into the draft regulator and up the chimney. If the appliance has any leaks, the chimney will also pull air through those leaks and up the chimney. The air that goes through the heat exchanger is a particular problem. The room temperature air will be heated as it goes through the heat exchanger. Remember, heat goes from hot to cold and in the winter the heat exchanger will be warmer than the room it is in, Figure 3. This heated air flowing up the chimney will warm the chimney and continue the cycle of losses. The best way to demonstrate this loss is to place a thermometer in the flue pipe after the unit shuts down. Watch how long it takes for Figure 3 Idle Loss Hot Surfaces in Boiler or Furnace Chimney Draft Pulls Heated Air at 145 - 220°F Cold Air in at 50 - 70°F
NORA Advanced Efficiency
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