The use of air-to-air heat pumps in modern air handling units is becoming more popular for building heating and cooling supply. However, the constantly changing state of outdoor air (temperature, humidity) means that energy conversion modes of heat pumps need to be flexible and able to efficiently respond to ambient air parameters. The ability to adapt to changes in ambient air conditions in an efficient and rational manner is a crucial factor to consider when selecting the operating mode and control of energy transformers. Modern commercial heat pumps typically have two control degrees of freedom, which include a variable-speed compressor and an electronic expansion valve. This combination of control components has previously helped to improve the seasonal efficiency of heat pumps. However, for a long time, focus was placed mainly on refining electronic control tools, without considering how changes in the thermodynamic cycle of heat pumps are correlated with energy demands for air preparation that correspond to outdoor temperatures. The authors of the article propose introducing an additional component into traditional heat pump control measures to provide a third degree of freedom for the cycle control. The experiments were conducted to assess the impact of the heat pump system volume on its thermodynamic operating cycle and it was found that the system volume parameter has the potential for regulation capabilities in controlling the operation of a heat pump. This technological solution has the potential to increase efficiency indicators of the air preparation process and warrants further development and study.