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NORA Oilheat Technicians Manual

Chapter 12—Limit Controls and Thermostats 12-19 efficiency boiler. The new, high efficiency boilers extract so much heat from the products of combustion that they can lower their temperature below the dew point of the combustion gases. At this temperature, the water vapor turns to water. While it does this, it picks up the sulfuric oxides in the gases and creates sulfuric acid. This results in scale build-up on the heat exchanger. The reverse acting aquastat minimizes the scale build-up. When the boiler water temperature falls below the set point, the aquastat shuts off the circulator and the heat from the burner raises the boiler water temperature. This allows the temperature of the combustion gases to stay above the dew point. See the chapter on combustion theory (Chapter 7) for more detail. Immersion aquastats should be mounted as follows: 1. When mounting immersion aquastats, try to avoid using bushings on the well. Bushings may prevent the temperature sensing element from extending far enough into the boiler water to be in the direct path of the hot water. Locate the element in freely circulating water. 2. Handle the aquastat with care. Do not damage the sensing element. 3. The bulb of the immersion aquastat should bottom in its well. 4. Be certain that the sealing washer is in place (remote control). 5. Make certain the case is mounted level if a mercury switch is employed. The domestic hot water low limit (operating) control used for controlling the domestic hot water supply should be installed on the boiler as close as possible to the hot water generator on a tankless hot water heating unit. In the event a tank is used for hot water storage, it may be installed in the hot water tank. With most hot water producing boilers, this control must be wired in parallel with the thermostat. The reverse-acting circulator control should be installed in the boiler where the water returning from the circulator will surround the temperature sensing element. Figure 12-26 shows all three types of aquastats wired into a one zone circuit with the use of a switching relay. Switching relays Switching relays control a line voltage load with a low voltage thermostat. They are used Figure 12-26 Aquastat Wiring with One Zone Circuit and Switching Relay L1 L2 Chapter 12 Limit Controls/Thermostats T-Start Low Limit High Limit Reverse Acting Limit Circ. Motor Ignition Transformer Burner Motor 120 V


NORA Oilheat Technicians Manual
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