A central-heating system or air-conditioning system if operated in conjunction with a room thermostat is said to be automatically controlled, whilst if it is operating without the thermostat it is said to be manually controlled. These systems are shown diagrammatically in figures 1.1a and b, respectively. In the case of figure 1.1b, if a change should occur in the outside air temperature, a change in room temperature would result and manual intervention would be needed to correct the room temperature. For the automatically controlled system of figure 1.1a the effect of a change in outside air temperature on the room temperature would be corrected without manual intervention.
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