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NORA Oilheat Technicians Manual

If the water temperature is below 160°F, the double-throw switch will close the hot line to the burner by moving its contact up to terminal B, allowing power to pass to B1 and the primary control. The thermostat terminals, T and T of the primary, are now closed so the burner starts and will continue to operate until the water temperature reaches 180°F. Then the double-throw switch will transfer its contact from B to W and the burner stops. The boiler water temperature has now been restored to effectively provide domestic hot water. When the thermostat again calls for heat, the circulator can operate. The burner can be energized in two ways: first by an action of the thermostat and second by action of the SPDT switch even when the thermostat is not calling for heat. Figure 12-32 (previous page) also shows terminals ZC and ZR. These terminals are used when circulated zones are added to the system. When a call for heat energizes the zone relay, the use of these two terminals enables the control to act as though the main zone circulator was called. The control will start the burner and drop out the circulator if the boiler water temperature drops below the low limit/differential set point. This ensures consistent tankless coil performance. Caution Be careful when wiring hot water heating systems with multiple zones not to hook one set of powered terminals to another. The thermostat circuit (usually TT) is powered by the transformer in that control. Never hook wires from TT of one control to the TT of another control or switching relay. This will apply 48 VAC to the circuit and burn out the controls. 12-24 Limit Controls and Thermostats Since there may be differences in control models as well as control manufacturers, you are urged to study the operation of the various makes of controllers covered in the data and instruction sheets supplied by the manufacturer of the control. Testing limit controls High limit controls 1. With the burner operating and the room thermostat calling for heat, move the limit indicator to the low end of its scale. 2. Allow the burner to operate so that either the temperature or the pressure of the furnace or boiler rises to the limit setting. The burner should shut down. If it does not, then the limit control is either improperly installed or defective. 3. If it does shut down, then set the indicator at the desired temperature or pressure. Circulator controls 1. Set the thermostat to call for heat. 2. In the event the boiler water temperature is above the circulator switch (reverse acting aquastat) setting, the circulator relay should close its contacts and the circulator should operate. 3. Next, turn the thermostat down below room temperature. This should cause the circulator relay to open its contacts and stop the circulator. 4.If the boiler water temperature is below the circulator switch when the thermostat is turned up and calls for heat, the circulator should not operate until the burner has heated the boiler water to the setting of the circulator control. In the event the circulator does not operate as outlined above, then the circulator control is either improperly installed or faulty. Chapter 12 Limit Controls/Thermostats


NORA Oilheat Technicians Manual
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