## 2 Materials and Methods
### The HVAC System
Finally, it is important to stress that the R-454B GWP value is 78% lower than that of R-410A, assuring its employment compliant with limits imposed for the near future by the F-gas Regulation. Figure 4 shows the GCHP characteristic curves in the heating and cooling mode, evaluated at the minimum (30 Hz) and maximum (110 Hz) inverter frequency, as functions of the fluid temperature at the BHE outlet. The heat pump experimental performance data, validated by the manufacturer, are reported for both the refrigerants considered in this analysis. The curves have been obtained considering fixed values of the water temperature at the heat pump inlet/outlet, i.e., 35/40 \({}^{\circ}\)C and 20/15 \({}^{\circ}\)C in heating and cooling working modes, respectively. Considering these mentioned criteria and a geothermal fluid temperature at the heat pump inlet/outlet of 4/7 \({}^{\circ}\)C and 33/28 \({}^{\circ}\)C in winter and summer, respectively, the heating capacity and the COP of the installed GCHP are equal to 7.4 kW \begin{table}
\begin{tabular}{c c c} \hline \hline
**Refrigerant** & **R-410A** & **R-454B** \\ \hline Composition (\(w/w\)) & R-32: 50\% & R-32: 68.9\% \\  & R-125: 50\% & HFO-1234 yf: 31.1\% \\ GWP & 2088 & 457 \\ ODP & 0 & 0 \\ Safety class & A1 & A2L \\ Molar weight (kg/kmol) & 72.6 & 62.6 \\ Critical temperature (\({}^{\circ}\)C) & 71.3 & 78.1 \\ Critical pressure (kPa) & 4901 & 5334 \\ Glide (K) & 0.1 & 1.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Thermophysical properties of the considered refrigerants. and 4.85, respectively, while the corresponding values in the summer season are 7.9 kW and 5.7, respectively. In addition, the geothermal fluid temperature difference across the external heat exchanger remains the same whatever refrigerant is adopted. The heat pump control logic is based on a PID controller, which modulates the inverter working frequency according to the required thermal load, coupled to an on-off controller operating through a hysteresis cycle, characterized by a 5 K dead band. The inverter operating frequency is calculated by comparing the monitored variable, namely the water temperature at the heat pump outlet, with the corresponding set-point value, set to 38 \({}^{\circ}\)C during the heating period and to 15 \({}^{\circ}\)C in the cooling one. When the minimum inverter frequency is reached and no further modulation is possible under partial load conditions, on-off cycles are performed to modulate the heat pump thermal capacity. In order to generalize the findings of this numerical study, an additional heat generator has been considered in this work. In detail, an air-to-water heat pump from the same manufacturer, having the same size as the ground-source unit described before, has been modeled in TRNSYS. Additionally, in this case, the air-source heat pump (ASHP) is an \"A2L-ready\" unit and can operate with both R-410A and R-454B. Furthermore, it is worth mentioning that the air-source unit is equipped with the same inverter-driven compressor of the GCHP installed in the building and, moreover, is based on the same control logic (i.e., a PID controller to regulate the inverter frequency and on-off controller for the further modulation of the thermal capacity). Therefore, the energy performance of the air-source heat pump, working with the two different refrigerants, has been assessed by means of dynamic simulations and compared to that achieved by the ground-coupled unit in order to investigate the potential energy and environmental savings of R-454B with respect to R-410A for different heat pump typologies. In Figure 5, the characteristic curves of the considered air-source heat pump, evaluated in heating and cooling mode for minimum and maximum inverter frequencies, are reported as functions of the external air temperature for both refrigerants. Reported data have been validated by the manufacturer with experimental tests.