In residential refrigeration units, a thermostat controls the temperature by connecting and disconnecting the power to the compressor. This type of cold control reacts to the temperature changes.
There is a correlation between the pressures created in the sealed system and the temperatures produced by the vaporizing refrigerant. Commercial units take advantage of this relationship by employing pressure controls that govern the operation of the compressor to regulate the temperature in the unit by controlling the pressures inside the sealed system. Serving a two-fold purpose, they regulate the temperature and, at the same time, protect the system from pressures that become too high or too low. They do this by disconnecting and reconnecting the power to the compressor motor. There are three types of pressure controls utilized in commercial refrigeration. They regulate either the high or low pressures or act as a safety device for the oil pressure in the compressor. Some units employ a low-pressure control, some employ a high-pressure, and some employ (including the oil-pressure control) all three. All of the pressure controls are safety devices.