National Oilheat Research Alliance 13 the temperature to drop to room temperature. The whole time it stays high, energy is being taken from the heating system and the building. Restriction of air flow through 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, the restricted air passes in high-speed flame retention head burners reduce off-cycle airflow, thereby reducing heat loss. Oil vent dampers are a way to reduce off-cycle losses on older boilers and furnaces. They are a motorized damper installed in the vent pipe between the draft regulator and the chimney. They must be installed with a blocked vent safety switch to keep the burner from running if the damper fails to open on a call for heat. Most new equipment uses low mass ceramic fiber combustion chambers that store less heat than high-density firebrick materials used in older equipment. Thus, they will have lower Off-Cycle Heat Loss The main factors contributing to off-cycle heat loss are: • Burner design (Retention head burners allow less off-cycle air flow.) • Heat exchanger design (The tighter the heat exchanger passes and the higher the draft drop of the exchanger– the more it resists air flow.) • Heat storage capacity of the appliance (High mass units retain heat longer and have greater off-cycle losses.) • Chimney height and construction materials • The operating temperature of the boiler (water temperature) • Air leaks in the appliance • Frequency and length of off and on- cycles (the shorter the cycles, the greater the losses). off-cycle losses. Small low mass boilers store less heat than their older heavier counterparts and will have lower off-cycle losses. Heating system sizing is another important factor that affects on-off cycling. Oversized heating units will have longer off periods and off-cycle losses will increase. 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. Jacket Heat Loss Jacket heat loss is the heat that is lost through the jacket or casing of the appliance. It reduces the amount of heat delivered to the building and lowers system efficiency, Figure 4. Jacket losses are affected by the location of the appliance. If the unit is located in a closet off the kitchen, then most of the heat goes into the house; however, some of that extra jacket heat may go up the chimney as off-cycle losses. Outdoor units must be very well insulated to reduce jacket loss to acceptable levels. Figure 4 Jacket Loss Heat Loss Heat Loss Heat Loss Heat Loss Heat Loss Dry Base Boiler Illustrated
NORA Advanced Efficiency
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