## 1 Introduction  
Hospitals operate 24/7 with high rates of occupancy and stringent air quality requirements. Accordingly, they generally present greater end-use energy consumption and greenhouse gas emissions per unit of floor area in comparison with residential and other commercial buildings. Thus, it has been recently reported that the average energy intensity of hospitals in the United States is 738.5 kW/m\({}^{2}\)/year, which is approximately 2.6 times greater than that of other buildings of the tertiary sector [1]. In turn, the energy consumption of hospitals in Greater London (UK) ranges from 194 to 1270 kWh/m\({}^{2}\)/year [2], whereas the average values for Germany and Spain are approximately 270 kWh/m\({}^{2}\)/year [3, 4]. In addition, a recent study on the CO\({}_{2}\) emissions derived from the energy consumption in Spanish hospitals estimates that the average annual ratio is 100 kg per m\({}^{2}\) of floor area [5]. The scientific literature offers a wide range of investigations evidencing that there is a large potential for energy savings and CO\({}_{2}\) emissions reduction in hospitals worldwide. Such studies mostly focus on enhancing the efficiency of heating, ventilation and air conditioning (HVAC) systems, as they are typically responsible for the greatest share of the total end-use energy consumption in hospitals, with ratios ranging from 50% to 75% [6, 7]. These investigations were conducted worldwide (e.g., in Europe [8] and Australia [9]), achieving similar results. For example, in China, a comprehensive study indicates that electricity consumption in hospitals accounts for 64% of the total energy, mostly from air conditioning systems [10]. Other publications include the evaluation of different retrofitting strategies, such as renovating the building envelope [11; 12], introducing renewable energy sources [13] or recovering energy from medical waste incineration [14]. For instance, it has been estimated that the yearly electrical energy consumption of a Greek hospital could be reduced by 45% through the implementation of energy policies, and the installation of new LED luminaries and photovoltaic panels [15]. Likewise, [PERSON] et al. have reported that it is possible to save up to 6.88 kWh and 4.82 kg of CO\({}_{2}\) per m\({}^{2}\) and year by implementing a series of relatively low-cost strategies that include improving the HVAC and lighting systems, introducing the use of renewable energies, enhancing the envelope thermal insulation or optimizing the maintenance management [16]. However, it is common that the implementation of energy efficiency measures in hospitals is hindered by a series of issues intrinsically related to the healthcare sector, including technical obstacles, political barriers and behavioral concerns [17; 18; 19; 20]. Operating rooms (ORs) have rigorous ventilation, hygothermal, lighting and functional requirements. In consequence, they present a large energy use intensity compared to other areas in a hospital, principally due to the energy consumption of HVAC systems. In fact, a recent study on the energy performance of the ORs of a Spanish hospital reveals that their average thermal energy demand is 1685 kWh/m\({}^{2}\)/year, or 1021 kWh/m\({}^{2}\)/year if the adjunct facilities, such as scrub rooms, sterile storage areas and preoperative rooms, are considered [21]. HVAC systems play a fundamental role in preventing infections during surgical operations while maintaining acceptable indoor thermal conditions for patients and medical staff [22], which makes them consume large amounts of energy. Thus, elevated constant ventilation rates, together with an efficient air filtration and air distribution, are required to remove airborne bioparticles, anesthetic gases, fumes and other contaminants, as well as to keep a constant positive differential pressure in the surgical areas [23; 24; 25; 26].