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NORA Oilheat Technicians Manual

Burner off-cycle heat loss is caused by air flowing through the heating unit when the burner is idle. The draft of the chimney creates negative pressure in the heat exchanger. It pulls cold air into the boiler or furnace at the burner air inlet and through other leaks in the unit. This air travels across the hot combustion chamber and flue passages where it is heated and carries the heat out of the house through the chimney. The size of this loss varies with burner design, chimney draft, the operating temperature of the unit and installation. It is an important cause of inefficiency, especially for older and oversized units. Figure 16-3 shows off-cycle loss. Off-cycle is also affected by the temperature of the boiler or furnace during the burner off-period. The higher the operating temperature, the greater the burner onperiod. Restriction of off-cycle airflow by the burner can reduce heat loss. Generally, older burners were designed with open combustion heads that provide very little restriction to off-cycle airflow. In contrast, high-speed flame retention head burners reduce off-cycle airflow and thereby reduce heat loss. This is a primary reason why oilheating systems have much lower offcycle losses than typical natural gas heating units. Natural gas heaters often use open “atmospheric” burners that do not restrict off-cycle airflow through the heating unit. Additionally, traditional gas units have large draft hoods that continuously remove heated air from the home. Oilburners push air into the heat exchanger while atmospheric gas burners depend upon draft to pull air in. This is why oilheat exchangers can be more restrictive than gas, making them more efficient. Low mass combustion chambers (including ceramic fibers) will store less heat than high-density firebrick materials, so they will have lower losses. Similarly, small low mass boilers or furnaces store less heat than their older heavier counterparts and will have lower off-cycle losses. Oversized heating units have longer off periods and off-cycle loss will be higher. A heating unit that is closely matched in size to the building’s heating requirements will provide the lowest off-cycle heat loss and highest efficiency. Proper heating system adjustment and maintenance also affects burner off-cycle heat loss. Three examples are: air leaks into the heating unit, temperature control settings of the boiler or furnace, and fuel firing rate. Air leaks Air leaks into the heat exchanger should be avoided whenever possible because they provide a path for off-cycle airflow. Initial start-up and annual servicing procedures should include sealing all such leaks before the final burner adjustment. Some common locations for air intrusion include the space between the burner air tube and the combustion chamber opening, the connection between the combustion chamber area and heat exchanger, the space between sections of cast iron boilers, heat flanges, and loose-fitting clean out and flame inspection doors. Eliminating these unnecessary air leaks will reduce off-cycle airflow and heat loss. Chapter 16 Energy Conservation Chapter 16—Energy Conservation 16-7


NORA Oilheat Technicians Manual
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