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NORA Oilheat Technicians Manual

has reached the fan on position, 140ºF, the fan switch will close its contacts and the fan will be turned on, forcing warm air into the living area. 3. The burner will continue to operate until the room thermostat is satisfied, at which time it will turn off. However, the fan will continue to operate until the air temperature has dropped below the minimum setting or the fan off indicator setting. Once this point has been reached, the fan control will then “open” its electrical contacts, and the fan will stop. The heat exchanger of the furnace is still hot, although the burner, controlled by the thermostat, is not running; warm air currents will continue to rise to the bonnet. Under certain conditions this heat may be sufficient to again elevate the temperature to 140ºF, and the fan switch will again operate the fan until all of this heated air is delivered to the living area. In the event that the fan fails to operate, the air filters are clogged, or a blower belt is broken; the temperature in the furnace would continue to rise, going beyond the fan on position and ultimately reaching the high limit indicator setting of 200ºF as shown in Figure 12-10. Upon reaching this point, the limit control would open its electrical contact and prevent line voltage from reaching the primary control. This in turn would prevent line voltage from reaching the oilburner motor and ignition transformer thus causing the oilburner to go off. The high limit control would continue to hold its electrical switch contacts open until the air temperature in the bonnet had dropped below the 200ºF mark minus the differential of the switch. (Normally 25º). The helical element would at the same time cool, rotating in a counterclockwise direction, causing the limit indicator to also rotate in a counterclock- Chapter 12 Limit Controls/Thermostats Figure 12-10: Limit and fan terminals 12-10 Limit Controls and Thermostats


NORA Oilheat Technicians Manual
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