A comprehensive review of inﬂuencing factors and energy efﬁciency improvement strategies for variable refrigerant ﬂow systems Weihua Lyu , Zhaowei Xu , Yi Xu , Zhichao Wang , Chunyan Jiang , Xiaofeng Li , Jiandong Li , Xiaoxi Gou a China Academy of Building Research, Beijing 100013, China b China Quality Certiﬁcation Centre, Beijing 100070, China c State Key Laboratory of Building Safety and Built Environment, Beijing 100013, China d Tsinghua University, Beijing 100084, China A R T I C L E INFO Keywords: Variable refrigerant ﬂow system Advanced components Design optimization Operation efﬁciency Control strategy ABSTRACT Variable refrigerant ﬂow (VRF) systems are widely applied in both residential and commercial buildings for air conditioning. For energy efﬁciency, this paper systematically reviews the key factors affecting the energy per- formance of VRF systems across three stages: the VRF unit development stage, the VRF system design stage, and the VRF system operation stage. Corresponding targeted improvement strategies are summarized. Results show that efﬁcient components like advanced compressors and microchannel heat exchangers boost nominal efﬁciency during the VRF unit development stage. Optimizing pipe layout and conﬁguration ratios enhances the designed energy efﬁciency ratios. During operation stage, dynamic control strategies, such as variable evaporating/ condensing temperature control and AI-driven methods, along with the selection of high-efﬁciency refrigerants and improving the capacity utilization rate of indoor units can enhance actual operating performance. However, most research focuses on lab testing and modeling, which do not reﬂect real-world complexities. Future work should focus on long-term real-world energy improvement, developing accurate, universal and inexpensive long- term ﬁeld energy efﬁciency monitoring methods, advanced sensors, performance assessment methods, and conducting extensive ﬁeld studies on energy efﬁciency and overall performance. 1. Introduction Variable refrigerant ﬂow (VRF) systems have emerged as a promi- nent solution for heating, ventilation, and air conditioning (HVAC) in both residential and commercial buildings (Wang et al., 2024, Park et al., 2025). These systems offer numerous advantages, including high energy efﬁciency (Hsu et al., 2025), ﬂexible installation, and precise temperature control (Kim et al., 2024). As global energy consumption continues to rise, there is a growing emphasis on enhancing the energy efﬁciency of HVAC systems to reduce operational costs and mitigate environmental impacts (Ildiri et al., 2025, Es-sakali et al., 2024). VRF systems are becoming increasingly widely used due to their high energy efﬁciency (Zhou et al., 2024). VRF systems typically have one outdoor unit and at least two indoor units. The outdoor unit and indoor units are connected by pipes ﬁlled with refrigerant. Compared with traditional room air conditioners, VRF systems have more complex structures and more complex controls (Kim et al., 2024). Many studies have focused on developing efﬁcient VRF units. Re- searchers have put a lot of effort into compressor optimization (Liu et al., 2024, Yang and Shao, 2024, Wan et al., 2021), such as improving the design and optimizing of compressors, adopting advanced frequency conversion technology and efﬁcient motor drive systems. Moreover, some researchers focus on improving the efﬁciency of heat exchangers by adopting new heat exchange materials (Ghaddar et al., 2025), opti- mizing the geometry design of heat exchanger (Choi et al., 2022) and improving the structure and layout of heat exchangers (Zha et al., 2025). In terms of VRF system design, researchers have noticed the impact of pipe length and height difference on system performance (Hong et al., 2016). They work on optimizing pipeline design (Dong et al., 2025), selecting appropriate pipe diameters and pipe materials, and adopting advanced pipeline connection technologies. In terms of system opera- tion, researchers devoted into reducing the energy consumption of VRF systems from many aspects, such as control strategy, defrosting * Corresponding author. E-mail address: wangzc@emcso.com (Z. Wang). Contents lists available at ScienceDirect International Journal of Refrigeration https://doi.org/10.1016/j.ijrefrig.2025.07.025 Received 12 April 2025; Received in revised form 27 July 2025; Accepted 28 July 2025 International Journal of Refrigeration 179 (2025) 27–43 Available online 29 July 2025 0140-7007/© 2025 Elsevier Ltd and IIR. All rights are reserved, including those for text and data mining, AI training, and similar technologies. methods, and behavioral energy conservation methods (Hernandez and Fumo, 2020, Qian et al., 2024). Both the simulation and experimental methods are used comprehensively to improve the energy efﬁciency of the VRF systems (Wang et al., 2024, Wan et al., 2020). However, these above studies are focused on a single technical aspect to improve the energy efﬁciency of VRF system, lacking a comparison of the improve- ment of different technologies. It is valuable to review and summarize these research ﬁndings, which contributed to providing technical sup- port for practical engineering applications. Regarding the review of VRF systems, researchers have made some detailed and in-depth summaries of the research progress on VRF sys- tems. Wang et al. (Wang et al., 2024) focus on the modeling of VRF systems. Modeling methods based on empirical performance curves, component working principle, and machine learning have been analyzed and summarized comprehensively. In addition, the tools used for modeling, including EnergyPlus and Modelica and the validation of the modeling method are also introduced. Wan et al. (Wan et al., 2020), examined the latest advancements in VRF systems, focusing on system architecture development, modeling, experimentation, control strate- gies, fault detection and diagnosis, and defrosting. The authors review ﬁndings show that studies mainly focus on temperature and humidity control strategies. Machine learning and data mining techniques are usually applied in the studies of VRF modeling, control, and fault detection and diagnosis (FDD). Aynur (Aynur, 2010) and Lin et al., reviewed the development of VRF before year 2015 while Zhang et al. (Zhang et al., 2019), focused on the recent studies since 2015. Research related to refrigerants, innovative components, control strategies, simulation and fault detection since 2015 are reviewed completely. VRF systems used in the residential buildings and commercial buildings are reviewed by Hernandez and Fumo (Hernandez and Fumo, 2020) and Yau et al. (Yau et al., 2024), respectively. The review reveals that the energy performance of VRF and thermal comfort are inﬂuenced by conﬁgurations of compressors, placements of electronic expansion valve, and airﬂow and the number of indoor units. Saryazdi et al. (Saryazdi et al., 2024), conducted a comprehensive literature review of uncertainty analysis studies of buildings equipped with VRF systems in terms of uncertainty factors, sensitivity analysis, and sampling. In gen- eral, these existing review articles have touched upon some related studies on energy efﬁciency improvement of VRF systems. However, there are two main issues with existing review articles. First, the review of inﬂuencing factors and improvement methods for energy efﬁciency is insufﬁcient and lacks comparison between different methods. This results in the potential for energy efﬁciency improvement at different stages remaining unclear. Second, the existing review arti- cles were published before 2020 and thus lack the latest research progress. In fact, VRF systems have made signiﬁcant progress in the past ﬁve years. Therefore, this paper aims to systematically review the key factors that affect the energy efﬁciency of VRF systems at each stage and to summarize the latest research on energy efﬁciency improvement methods. The structure of this paper is shown in Fig. 1. The review of inﬂuencing factors and methods to improve VRF system efﬁciency is summarized in three aspects: improving the energy efﬁciency of VRF units themselves during the equipment development stage; improving VRF systems efﬁciency during the system design stage; and improving VRF systems operation efﬁciency during the operation stage. According to Fig. 1, this review paper consolidates fragmented ad- vances of VRF systems into a uniﬁed framework. Key points related to what has already been solved, what are the current research hotspots and what should the ﬁeld focus on next are highlighted. This paper provides engineers with practical strategies to move from nominal performance to actual, long-term system efﬁciency, and provides re- searchers with a roadmap for next-generation, carbon-neutral VRF technologies. 2. Inﬂuencing Factors for energy efﬁciency of VRF systems in each stage Generally, after the VRF unit is produced, it is ﬁrst tested in the laboratory to obtain its performance under the nominal working con- ditions created by the laboratory. Then, before being installed on site, the VRF system design is carried out. According to the actual engi- neering environment, different pipe lengths and height differences are designed, and the system performance under the design working con- ditions can be obtained. Finally, the VRF unit is installed in actual projects and operates to obtain the actual operating performance. Many researchers have found that the energy efﬁciency of VRF systems in actual operation is much lower than that shown on their nameplates. To understand the inﬂuencing factors for the energy efﬁciency of VRF systems, it is necessary to clarify the relationship between the energy performance in laboratory conditions, design conditions, and actual operating scenarios. Drawing on the relationship between the energy performance in laboratory conditions, design conditions, and actual operating scenarios for air source heat pump heating systems as Fig. 1. Description of the paper framework. W. Lyu et al. International Journal of Refrigeration 179 (2025) 27–43 28