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NORA Oilheat Technicians Manual

12-18 Limit Controls and Thermostats Figure 12-23: Hot water limit (aquastat), strap on type Figure 12-24: Hot water limit (aquastat), Immersion type Figure 12-25: Thermistor and it will break its contacts on rise in temperature. Reverse-acting controls make their contacts on a rise, and will break their contacts on a drop in temperature. The letter A after the Honeywell model number normally indicates a control that will open its circuit on a rise in temperature. The B control will normally close its circuit on a rise in temperature for use as a reverse control. Hot water limits may be of the strap-on type (Figure 12-23, or the immersion type, (Figure 12-24). Normally, the strap-on limit control is installed close to the boiler on the main supply riser. It should never be mounted on a pipe fitting such as an elbow or coupling. The strap-on type control is not as sensitive to temperature change as the immersion type controls and should not be used as a high limit control. Immersion controls should be installed in the tapped holes recommended by the boiler manufacturer. The temperature sensing element on hot water controls may be electronic thermistors, thermocouples, liquid filled elements, or helical bimetal elements. Liquid filled elements, or capillary sensing bulbs, are the most popular. Volatile liquid expands and contracts dramatically with changes in temperature. This expansion and contraction operates an internal diaphragm to open and close the switch. When installing the heat sensing bulb in the aquastat well, coat the bulb with the heat conductive compound, supplied with the control, to help transfer the heat from the well to the bulb. The immersion control equipped with a thermistor, (Figure 12-25), will respond faster to rapid temperature changes than the old immersion control of the bi-metal type or the capillary type. A thermistor is a heat sensitive device that increases or decreases resistance based on temperature. Reverse acting aquastat When the circulator is turned on by action of the room thermostat, the burner often starts simultaneously. When the circulator starts, it pushes hot water out of the boiler to the radiators, and an equal volume of cool return water from the radiators and system piping flows back to the boiler. This causes a drop in boiler water temperature. Starting the burner at the same time as the circulator helps the burner match its output to the heat content leaving the boiler. Also, in the event that the water temperature goes too low, the reverse-acting circulator control will function to stop the circulator until burner operation can restore effective boiler water temperature. Then the circulator limit will again close its contacts to turn the circulator on. Without the reverse acting aquastat, if a thermostat calls for heat while someone is taking a shower, the circulator comes on and sends all the heat in the boiler into the radiation and the boiler temperature drops below what is needed to produce hot water; the result is that the shower water temperature change can be quite noticeable. The reverse acting aquastat shuts the circulator off until the burner can build enough heat in the boiler to keep the tankless coil hot and heat the radiation. The second reason for a reverse acting aquastat is to keep the products of combustion from condensing in a high Chapter 12 Limit Controls/Thermostats


NORA Oilheat Technicians Manual
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