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NORA Oilheat Technicians Manual

With either type, when the float is in a level position, it holds a single-pole singlethrow switch in a closed position. In the event the water level inside the boiler drops below the safe operating level, the float will also drop, thus opening the switch and breaking the hot line circuit to the burner. Probe type low-water cutoffs are becoming very common on most boilers, replacing the float types. These cutoffs may have timing devices to prevent nuisance shut downs should the boiler water surge. Probe type cutoffs send a low voltage charge through the water to ground on the boiler’s metal. Don’t switch to a probe control without first getting the boiler manufacturer’s recommendations as to where it should be installed. In Figure 12-19 (opposite page), the wiring diagram shows how the low-water cutoff is connected into the main heating plant circuit. The low-water cutoff is connected in the hot line, preceding the pressure control and in series with it and the primary control. Thus, the low-water cutoff may be called a low water line voltage safety device. Low-water cutoffs on steam boilers may be incorporated with, or wired to, electronic solenoid water valves called automatic water feeders. Chapter 12 Limit Controls/Thermostats Chapter 12—Limit Controls and Thermostats 12-17 Figure 12-20 Figure 12-21: Single function aquastat Figure 12-22: Dual acting aquastat Hot water limit controls Hot water limit controls, sometimes called aquastats, control the temperature of the water in the boiler, and the temperature of the domestic hot water. They are all just switches that function automatically. In a basic forced hot water system, these controls must perform three functions: 1. They provide high limit protection against excessive boiler water temperatures. 2. They are employed for the purpose of maintaining a pre-determined boiler temperature, especially in systems that use tankless heaters for domestic hot water. 3. They are used to keep circulators from lowering the boiler water temperature too low. Figure 12-21 shows a typical single function aquastat, the L4006 with two terminals and Figure 12-22 shows a dual acting L6006 with three terminals. The high limit protection control prevents the boiler water temperature from rising to unsafe levels that create steam. Low (operating) limit protection means that the boiler water is not allowed to drop below a certain temperature. In the event the boiler water temperature falls below the dial setting of the low limit control, the burner would be turned on. Hot water temperature controls are either directacting or reverse-acting. As explained earlier, a direct-acting control will make its contacts, completing an electric circuit, on a drop in temperature


NORA Oilheat Technicians Manual
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