Energy Saving Performance of Variable Refrigerant Flow Systems with Different Heat-Recovery Fresh-Air Units in Residential Buildings

To address the issues of poor air quality and the lack of fresh air in traditional variable refrigerant flow (VRF) systems, fresh air units are commonly supplemented in practical engineering applications. However, these systems are typically controlled independently. To achieve coordinated operation between VRF systems and heat-recovery fresh-air units, a new integrated VRF fresh-air system was developed, with the aim of reducing system energy consumption and improving indoor comfort and air quality. Based on the full-condition performance model of the VRF system and the characteristics of the heat-recovery unit, simulations were conducted to evaluate the energy-saving effects of conventional VRF systems and of three types of integrated VRF fresh-air systems in residential buildings in Nanjing and Beijing. The results indicate that, compared with conventional VRF fresh-air systems, the system equipped with a constant air volume heat-recovery unit significantly reduced the operational energy consumption, achieving annual energy savings of 20.7% and 30.6% in Nanjing and Beijing, respectively. The energy-saving performance was positively correlated with the severity of cold-climate conditions. During the heating season, the new integrated system prioritizes the use of the heat-recovery mode and minimizes the fresh air volume. During the cooling season, the energy-saving performance can be further improved by introducing a bypass branch and combining it with a variable air-volume control strategy. Compared to the primary baseline system, the combined VRF fresh-air system achieved cooling-season energy-saving rates of 2.77% and 15.31% for Nanjing and Beijing, respectively.