## 2 Materials and Methods
### Experimental Part  
During the study, an experimental set-up was built where, in addition to standard means (compressor speed control, expansion valve position change), the power, condenser and evaporator parameters are also controlled with an introduction of volume change. The schematic diagram of the experimental set-up is shown in Figure 3.  
Figure 3 shows that the main objective of the operation of the AHU is to provide the required indoor air quality. The purpose of an integrated HP is to recover heat and to preheat the outside air. Table 1 shows the technical characteristics of the main components of the experimental test bench. Figure 3: The schematic diagram of the experimental test bench with its main components. Figure 2: The dependence of condensation, evaporation temperatures and the COP on the heating power of HP [49]. The test bench is equipped with two four-way valves to allow the HP to be used for cooling. A hot steam line is installed to defrost the evaporator when it freezes. This paper presents a study addressing only the heating season. The speed of the compressor is changed using a Lenze (Hamelin, Germany) i550 frequency converter. The expansion valve position is changed using an Arduino (Ivrea, Italy) Mega 2560 microcontroller board. This experimental bench has the advantage of being equipped with a multi-stage volumetric enlarger, which allows the system volume to be increased. The multi-stage volumetric enlarger is located between the evaporator (EV) and the electronic expansion valve (EEV). A schematic diagram of the multi-stage volumetric enlarger is shown in Figure 4.  
The multi-stage volumetric enlarger consists of a number of shut-off ball valves and piping. The combination of these components allows the internal volume of the heat pump to be gradually increased by 17.0% (+0.81 dm\({}^{3}\)) (V1), 26.4% (+1.26 dm\({}^{3}\)) (V2), 34.4% (+1.64 dm\({}^{3}\)) (V3) in relation to the initial volume (V0 = 4.76 dm\({}^{3}\)). A test cycle is performed with the volume expansion. After the cycle, the volume is increased and the test is repeated. Figure 5 presents the images of the experimental test bench. \begin{table}
\begin{tabular}{c c c} \hline
**The Component of the Experimental Bench** & **Marking in the Diagram** & **Technical Characteristics** \\ \hline Supply and exhaust fan RS-315L EC & F\({}_{\text{s}}\) and F\({}_{\text{e}}\) & Air flow rate: 430 m\({}^{3}\)/h \\ \hline EC-10 Fan motors potentiometer & - & Control signal: 0–10 V \\ \hline Heat pump compressor GMCC (Foshan, & & Refrigerant: R410A; voltage: 380–415 V; \\ China) YA281X3 CS-4 MT & CM & heating capacity: 6.92 kW \\ \hline Frequency inverter Lenze i550 & - & Voltage: 380–415 V \\ \hline Expansion valve Carel (Breganze, Italy) & EVV & Maximum differential pressure: 35 bars; full \\ E2V14 BSF01 & & closing steps: 480. \\ \hline Condenser & CN & DX coil heat exchange surface: 4.5 m\({}^{2}\); air \\  & & flow rate: 400 m\({}^{3}\)/h, pressure drop: 25 Pa \\ \hline Evaporator & EV & DX coil heat exchange surface: 5.9 m\({}^{2}\); air \\  & & flow rate: 400 m\({}^{3}\)/h, pressure drop: 18 Pa \\ \hline Microcontroller board Arduino Mega 2560 & - & Operating voltage: 5 V; digital I/O: 54; \\  & & analog input: 16; flash: 256 KB; SRAM: 8 KB. \\ \hline \end{tabular}
\end{table} 
Table 1: The technical characteristics of the main components of the experimental test bench. Figure 4: A multi-stage volumetric enlarger for increasing the volume of a heat pump. Figure 5 shows digital photos of the experimental test bench where part (a) shows the experimental bench at the production stage, when there is no thermal insulation or volume expansion installed yet, and part (b) shows the bench, which is insulated and equipped with volume expansion and additional components, such as flexible ducts that supply and extract air. In addition, the main components are presented and marked in the pictures: (1) the compressor; (2) the evaporator; (3) the expansion valve; (4) the condenser; and (5) the volume expansion unit.