## 4 Conclusions  
This article presents experimental studies of a heat pump integrated into a ventilation unit. The experiments involved varying the operating modes of the unit by changing the compressor speed, the position of the expansion valve, and the system volume. The comparison of the performance characteristics showed that the performance trends of the variable-volume heat pump are similar to those of the typical constant-volume heat pumps controlled by standard means (compressor and expansion valve). The experimental results show that the trends are similar, indicating a sufficient degree of accuracy. Additional control was achieved by changing the volume of the heat pump circuit, which, when combined with conventional controls, extended the heating power range. Increasing the volume of the system loop enables the heat pump to operate at a lower heating capacity than in the original case (V0), allowing the unit to operate with stability over a wider range of outdoor air temperatures. Increasing the volume of the heat pump loop was found to decrease its efficiency. This phenomenon is similar to the case of partial refrigerant leakage from the heat pump system. The analysis of the experimental heat pump operation cycles in a pressure-enthalpy diagram revealed that altering the compressor speed had a negligible impact on the temperature differences between the superheating and sub-cooling stages of the cycle. Increasing the compressor speed raises the condensation temperature and lowers the evaporation temperature. On the other hand, modifying the flow rate of the expansion valve has a more pronounced effect on the evaporation temperature than on the condensation temperature. The superheating and sub-cooling temperatures of the cycle were influenced by adjusting the heat pump with the expansion valve. An increase in the difference between these temperatures was achieved by reducing the valve's permeability. The operation cycle of the heat pump was significantly affected by increasing the volume of the heat pump circuit. Increasing the volume can lead to a more uniform \"lowering\" of the cycle in the diagram, resulting in lower temperatures for the condensation and evaporation isotherms. However, it is important to note that controllability is positively affected when the increase in volume is 17% or less. Beyond this point, performance is negatively impacted as the evaporation and condensation process cannot be achieved. Volume variation can benefit the cooling or refrigeration processes without altering the compressor speed or throttle capacity. By increasing volume, the evaporating temperature can be lowered with minimal impact on the operating efficiency. Figure 17: The operation cycles of the HP in a _p-h_ diagram (compressor speed: 1800 rpm; expansion valve capacity: 30%; volumes: V0, V1, V2, V3). Experimental investigations have revealed a significant influence of control components on the performance of a variable-volume heat pump. When controlled by varying the volume, the heating capacity parameters of the heat pump respond with the highest degree of sensitivity compared to control by the compressor or expansion valve. This sensitivity indicates the potential for developing novel control methods and components through the implementation of variable circuit volume technology in heat pump regulation. Since there are few new heat pump control components emerging in commercial devices (with only the existing components being improved instead), this technology could expand the operating ranges of heat pumps in the future, thereby helping to reduce energy consumption during their production and operation. This study represents initial steps in exploring the potential of variable-volume heat pump control. Future research is recommended to investigate the volume change control of the heat pump over a wider range of outdoor air temperatures, as well as in devices of different capacities.