The thermostat (or temperature control) stops and starts the compressor in response to room temperature requirements. Each thermostat has a charged power element containing either a volatile liquid or an active vapor charge. The temperature-sensitive part of this element (thermostat feeler bulb) is located in the return air stream. As the return air temperature rises, the pressure of the liquid or vapor inside the bulb increases, which closes the electrical contacts and starts the compressor. As the return air temperature drops, the reduced temperature of the feeler bulb causes the contacts to open and stops the compressor.
The advent of transistors and the semiconductor chips or integrated circuits has produced a more accurate method of monitoring and adjusting temperatures within a system. The microprocessor makes use of the semiconductor and chip’s abilities to compare temperatures and to program on and off cycles, as well as monitor the duration of each cycle. This leads to more accurate temperature control.
Figure 5-25 shows a microprocessor-based thermostat. As you can see from the front of the control panel, you can adjust the program to do many things and, in the process, save energy, whether it is operating the furnace for heat or the air-conditioning unit for cooling. These units usually come with a battery so that the memory can retain whatever is programmed into it. The battery is also a backup for the clock so that the program is retained even if the line power is interrupted.