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NORA Oilheat Technicians Manual

Low voltage and line voltage thermostats In some older systems, line voltage thermostats were used to directly control the circulator without the use of a switching relay. Line voltage thermostats are not as sensitive as low voltage types and this often leads to wide fluctuations in the room temperature. If open blade contacts are used in the line voltage controller, the contacts will eventually burn, and pitting of the contacts is the result. At this point we can lose control of the room temperature. Line voltage thermostats are mostly used in commercial and industrial applications. Heat anticipating principle The differential of a thermostat is the number of degrees temperature change that are required to cause its bimetal or bellows to move the required distance to close or open its electrical contacts. The number of degrees difference between the opening and closing of a thermostat is called the mechanical differential. For example, if a thermostat opened at 70°F, and its contacts closed at 68°F, its mechanical differential would be 2°. Manufacturers incorporate an anticipating Chapter 12 Limit Controls/Thermostats 12-6 Limit Controls and Thermostats heater to increase the sensitivity of the thermostat. It reduces the mechanical differential. The heat anticipator is a small electrical resistance heater that fools the thermostat into thinking it is warmer in the room than it actually is. The heater is wired so that electric current flows through it when the thermostat calls for heat. The anticipator heater creates heat within the thermostat near the bimetallic element. This causes the thermostat to break its contacts prior to the room air reaching the temperature of the dial setting, so the burner is turned off slightly ahead of the time that the room air temperature increases to the dial setting of the thermostat. The blower in a warm air system continues to operate, bringing the room air temperature up to the dial setting. With a hot water or steam system the heat in the radiators or baseboard will raise the room temperature after the thermostat shuts off the burner and circulator. The anticipating heater must be adjusted to match the current that is supplied to the thermostat. We must adjust for the number of amps supplied to the heater, because the greater the number of amps, the quicker the heater will heat up. Current flow in this 24 volt circuit generally varies from 0.05 amperes to 0.6 amperes, depending on the make and model of control. When setting the heat anticipator, consider the length of the wire and other resistances in the circuit. The current from the control to the thermostat heater circuit should be measured with an amperage meter and the anticipator set to the amps in the low voltage circuit. If an ammeter is not available, set the anticipator to the amp rating found inside the cover of the control to which the thermostat is directly connected. Figure 12-5 shows the location of the heat anticipator in a heating and cooling thermostat. Notice the anticipator in the R Figure 12-5: T-87F heat anticipator


NORA Oilheat Technicians Manual
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