Page 268

NORA Oilheat Technicians Manual

the room thermostat, and are preferred because many new boilers have a boiler water capacity of only a few gallons. Since space and accuracy are also factors, these combination controls are almost always now integral packages. Triple acting aquastat relays Dual and triple function aquastat controllers incorporate all of the various limit functions. Only one boiler water immersion element is required, simplifying installation. Figure 12-32 is a schematic diagram of a triple acting controller, the Honeywell L8124A. This controller incorporates a line voltage circuit, a low voltage circuit, and a switching relay, and includes all three limit controls: low, high, and reverse. Assume that the low limit is set to open at 170°F, the high limit is set to open at 200°F, and the differential is set at 20°F. The system may be analyzed as follows: When the thermostat calls for heat, it closes its switch, energizing the low voltage circuit through the secondary (24 volt) coil of the step-down transformer. The IK solenoid electromagnet is energized, pulling in the relay clapper, making contact IK1, closing the line voltage circuit to start the circulator that also closes contact IK2 in the line voltage circuit to start the oilburner at the same time. The circulator limit switch in the circulator hot line must be closed to allow the circulator to operate. It will be closed if the boiler water temperature is above 180°F. If this switch is open, only the burner will operate, which will cause the boiler water temperature to increase to 180°F and then the circulator limit switch 12-22 Limit Controls and Thermostats will close and the circulator will start. The reason for this is that the low limit has a built in differential of ten degrees. The differential is added to the set point, less the differential. So, you subtract 10 degrees from the 170°F set point to get 160°F and then add 20°F to this number. In the summer, the burner will operate between 160°F and 180°F working off of the 20° differential. The high limit switch is located in the burner hot line and it must be closed to allow the burner to operate. It will always remain closed unless the boiler water temperature is higher than 200°F. This switch will again close the circuit when the water temperature drops below 190°F based on its 10° fixed differential. When the thermostat is satisfied, the circulator will stop and the burner may also stop, or it may continue to operate for a short period if boiler water temperature is below 160°F. This is the control operation during a normal heating cycle, from thermostat on to thermostat off. Now when a hot water boiler also provides domestic hot water, the low (operating) limit functions, to cause the burner to run and heat the boiler water year round to provide adequate domestic hot water through the tankless or the tank type coil installed in the boiler. The low limit switch closes to run the burner when the boiler water temperature drops below 160°F. It is desirable to open the circulator switch at the same time so that the circulator cannot operate until proper water temperature is again restored. The control manufacturer has incorporated a Single Pole Double Throw (SPDT) switch installed in the hot line to both the circulator and the burner for this purpose. Chapter 12 Limit Controls/Thermostats


NORA Oilheat Technicians Manual
To see the actual publication please follow the link above