## 2 Materials and Methods
### The HVAC System  
The HVAC system installed in the building provides space heating and cooling and DHW production. In the present work, the energy consumption linked to the preparation of DHW is not calculated since this part of the system has not been considered in the numerical model. In Figure 3, the simplified scheme of the whole HVAC system is illustrated. As evident from Figure 3, the system is based on a ground-coupled heat pump (GCHP), coupled to a borehole heat exchanger (BHE) field consisting of two vertical double U-tube boreholes having a depth of 120 m. Each U-tube is 3.7 mm thick, with an external diameter of 40 mm. A mixture of water and 20% ethylenic glycol is used as the heat-carrier fluid in the BHE field. The primary loop (heat pump side) and the secondary loop (user side) are decoupled by a hydraulic separator. In order to increase the thermal inertia of the primary loop, a small thermal storage of 80 L is placed in the heat pump return branch. Two hydraulic pumps are installed in the primary loop. In detail, one circulator (rated electric power input of 75 W) is inserted within the borefield side and the other one (rated electric power input of 47 W) within the load side of the heat pump. Figure 3: Simplified scheme of the HVAC system dedicated to air-conditioning. Figure 2: Layout of the complete building–HVAC system model developed in TRNSYS. The heat emission system comprises radiant floor surfaces divided into two main loops, one per story. Each loop is composed of a manifold, a circulating pump with a rated absorbed electric power of 13 W and a three-way mixing valve. Each main loop consists of nine separate loops, controlled by means of on-off valves. The radiant floor heating pipes have a pitch of 10 cm. Additionally, a mechanical ventilation unit and a dehumidifying system are installed in the building to provide optimal hygothermal comfort conditions to the tenants. More in detail, a double-flow-controlled mechanical ventilation system with heat recovery guarantees the renewal of fresh air in all rooms. Finally, in each story of the building, a dehumidifier able to handle an air flow rate of 260 m\({}^{3}\) h\({}^{-1}\) is installed. The mechanical ventilation system must maintain the conditioned thermal zones to different set-point values, depending on the season. In particular, the set-points are equal to 20 \({}^{\circ}\)C and 26 \({}^{\circ}\)C during winter and summer, respectively. On the other hand, internal relative humidity is maintained below 50% throughout the whole year. As mentioned before, the HVAC system is based on a reversible geothermal heat pump. The generator, placed in the technical room on the ground floor, is a mono-block unit equipped with a BLDC inverter by means of which the compressor rotating speed can be varied continuously. The adopted refrigerant fluid is R-410A, but the heat pump is an \"A2L-ready\" model able to operate with new refrigerants having lower GWP values and a low inflammability level [30]. Therefore, different fluids can be used with no modifications to the refrigerant cycle components, control logic, lubricating oil, measuring sensors and electric components. Thanks to this peculiarity of the heat pump, the energy and environmental performance of both the traditional fluid R-410A and the alternative fluid R-454B have been investigated by means of dynamic simulations. In Table 1, the main thermophysical properties of the two refrigerants considered in this work are reported. As evidenced in Table 1, the fluid R-454B is a zeotropic mixture and belongs to the A2L safety class (inflammable with a burning rate less than 10 cm s\({}^{-1}\)). Indeed, R-454B is one the most promising candidates to replace R-410A in air-conditioning units for buildings since these two fluids have similar thermodynamic properties. As an example, the maximum R-454B condensing pressure is slightly lower than that of R-410A (see Table 1). Moreover, an impactful feature of R-454B is that it can be used as drop-in refrigerant in existing units with the same charge of fluid.