The automatic hot-gas by-pass technique is applied to reduce the capacity of refrigeration and air-conditioning systems when operating at part load. In this scheme, hot gas from the compressor is extracted and injected into the suction side of the compressor through a hot-gas by-pass valve. Thus, the refrigerant gas by-passes the evaporator and reduces the system capacity. In this paper, three hot-gas, by-pass schemes for HFC-134a are discussed and analyzed on the basis of the first and second laws of thermodynamics. A comparative study is performed of the schemes in terms of the system coefficient of performance (COP) and irreversible component losses. Second-law-based thermodynamic analysis indicates that the total irreversible losses of the by-pass valve increase substantially as the capacity decreases.