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NORA Oilheat Technicians Manual

Center Pointer (Fan On) and newer version) has indicators for the fan on position and the fan off position as well as an indicator for the high limit setting. The function of the fan control is to operate the system blower when the air temperature is within the fan control dial settings. The fan control will permit the fan to operate when the air temperature in the furnace rises above the fan on setting as prescribed for the specific system or the manufacturer’s requirements. The fan control will also prevent blower operation in the event the air temperature is below the fan off setting of the fan control. This prevents cool air from being forced into the living area during cold weather. Many fan controls provide for manual operation to provide for summer air circulation. After the burner has been on for a short period, the element of the fan control will sense the desired amount of heat in the plenum or bonnet of the furnace, and start the blower. The blower will then run as long as there is heat, which can be for some time after the burner has stopped. Scaleplate Limit terminals and fan terminals (old and new) connect the line voltage wires of the heating system electrical circuit. As shown in Figure 12-10 on following page. The heat sensing element, or bimetallic element, expands and contracts with a change in furnace temperature. Since the element is helical in shape, it turns with a circular motion, either clockwise or counterclockwise, depending upon whether the furnace air is being heated or it is cooling off. The operation of the fan switch control as shown in Figure 12-10 is as follows: 1. If the furnace temperature is the same as the room temperature and the fan on indicator is set at 140ºF, the fan off indicator set at 110ºF and the thermostat then calls for heat, the burner is turned on. 2. As the burner produces heat, the air in the furnace begins to rise in temperature and the helical element reacts to this change in temperature. In its attempt to expand, the bimetal causes the scale plate to turn in a clockwise direction. Once the scale plate Chapter 12 Limit Controls/Thermostats Figure 12-9: High limit Chapter 12—Limit Controls and Thermostats 12-9 Left Hand Pointer (Fan Off) Right Hand Pointer (Limit Opens) Safety Stop Fan Off Set Point Lever Fan On Set Point Lever To Release Stop, Press Stiff Wire In Hole Adjustable Limit Stop Limit Set Point Lever (Factory-Set at 200°F (93C)


NORA Oilheat Technicians Manual
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