## 1 Introduction
### Experiments on the Improvement of the Vapor Compression Cycle
In cold climate zones, such traditional systems face many problems as the outdoor air temperature drops, including a high-pressure ratio, degradation of the mechanical lubrication properties in the compressor, a decrease in the refrigerant mass flow rate, and a decrease in the heating capacity [42; 43]. The mass flow rate of the working fluid in the heat pump is a crucial aspect of the ventilation heat recovery system, affecting its efficiency. As the ratio of condensation and evaporation pressures increases during heat pump operation, the mass flow rate of the refrigerant decreases [44]. This problem is addressed in several ways. One option involves refrigerant vapour injection cycles. Experimental studies [44] show that using such a cycle helps maintain a nearly constant flow rate of the refrigerant passing through the condenser as the outdoor air temperature decreases. Another option is to use multiple lower-capacity heat pump combinations. In this case, air passes through individual heat exchangers one after another, gradually changing its parameters. This allows for the reduction of the pressure ratio of each heat pump in-volved in the process. This method prevents excessive reduction of the mass flow rate of the working fluid. Additionally, this approach enables exercising control over the heating capacity. Moreover, the problem of a high-pressure ratio can be alleviated by using an ex-tractable air heat pump, as the air passing through the integrated heat pump evaporator in the ventilation system has a higher (indoor) temperature compared to the (outdoor) temperature of the air passing through the condenser. From the perspective of the vapour compression cycle, this means that the refrigerant's evaporation temperature can be higher, and the condensation temperature can be lower compared to a traditional heat pump, where the evaporator is located outdoors, and the condenser indoors. The isotherms of the vapour compression cycle approach each other, eliminating the high compression degree in the cycle, thereby reducing the work performed by the compressor. Several studies are made to increase the performance of the heat pump compressor with programming. [PERSON] et al. [45] showed a variable-speed control optimization model of the heat pump. Machine learning (ML) was used to predict the thermal load and outdoor temperature, and mixed-integer programming for compressor control optimization. This approach allowed proposing optimal schedules for a heat pump operation in a selected location, reducing its electricity consumption by 9% [45]. [PERSON] and [PERSON] [46] studied the effect of the complexity and flexibility of the predictive control model of the heat pump on the energy demand of the building. Genetic algorithms (GA) are used to optimise the COPHPSEZ by combining the maximum heat pump load, compressor displacement and system operating parameters [47]. However, even in these recent studies, the focus is not on problem-solving by evaluating the possibilities of thermodynamic cycle control, which is the subject of a different set of studies [48]; however, these are specific and not directly related to the characteristics of HVAC systems. As can be seen from this literature review, the majority of work on improving heat pump efficiency has focused on traditional, well-known and widely used heat pump control measures and their optimal operation strategies. A little attention is given to exploring the thermodynamic cycle of the heat pump and seeking new technological measures to modify and control it. This potential direction could improve the performance indicators of the heat pump integrated into ventilation systems. The main benefit and novelty of this study is the experimentally proven potential of variable loop volume control technology to improve the performance of a variable-volume heat pump integrated into a ventilation system.