In the typical dual-pressure system, one pressure regulator reduces tank pressure to between 13 and 15 psig, and the other to between 20 and 25 psig. The lower pressure is normally supplied to the controller-a summer/winter thermostat, for example-when the demand calls for cooling (summer cycle). The higher pressure is supplied to the thermostat when the demand calls for heating (winter cycle). In day/night applications, the lower pressure is usually for day operation, and the higher for night operation.
The compressor must be sized properly so that it does not operate continuously. Normally, compressors should not run more than one-third of the time. This extends the compressor life and allows sufficient cooling of the compressed air in the air storage tank to permit condensation of water and oil vapors. These contaminants are removed by an automatic drain trap piped to the bottom of the tank.
Excerpt from the Technical Institute Manual #3 available from the ONLINE STORE at RSES.ORG (Refrigeration Service Engineers Society) The HVACR Training Authority.