# Safety and Energy Implications of Setback Control in Operating Rooms during Unoccupied Periods
## 1 Introduction  
Health care facilities are energy-intensive buildings where the energy supply is critical as they must operate 24 h a day yearly [1, 2]. Energy consumption in hospitals can be optimized through different energy efficiency measures and appropriate management but must not compromise the environmental requirements [2]. Energy use in hospitals can be dedicated to medical equipment, lighting, services, and, also, to Heating, Ventilation and Air Conditioning (HVACs), among others [1]. HVAC systems are highly energy demanding due to the large outdoor airflows required to remove indoor contaminants [2]. However, the Indoor Air Quality requirements vary by health function, where operating rooms are amongst the most demanding areas [3]. The implementation of energy conservation measures related to HVAC systems must comply with the safety standards at all times. The typical measures considered in health care facilities are the use of heat recovery equipment [2; 4; 5]; maintenance and replacement of old, low-performing equipment [1; 6]; enhancing ventilative cooling when possible [1; 7]; zonification and optimal control [1; 6], etc. In addition, the current Spanish and ASHRAE standards [8; 9] admit to the reduction of the minimum number of air changes and alterations of the set temperature and relative humidity in certain spaces during unoccupied periods, such as in operating rooms (ORs). This is also called a turndown or setback. Reducing the supply airflow to ORs during unoccupied periods can save a lot of energy from fans, as well as air heating and cooling. However, there are critical issues, such as [10]:  
* Maintaining the required positive pressure. It is necessary to control both the supply and return airflow (many old systems do not include the latter). * Maintaining an appropriate ventilation rate to remove contaminants from cleaning activities. * The control of unoccupied periods through occupancy sensors, timers, light switches, etc. * Rapid transition from unoccupied periods to occupied operations while maintaining the indoor set temperature and humidity. The large air change rates usually applied in ORs incur significant expenses for health care buildings [11]. The first evidence in the literature of ventilation control to reduce energy consumption in ORs dates back to 1986 [12]. In 2005, [PERSON] et al. [13] proposed controlling the airflow during nonworking hours of two operating rooms and achieved a reduction in the fan power consumption of up to 72% without any adverse effects observed. Recent research has proposed different percentages of supply airflow reduction to introduce energy savings, adjusting the return airflow to maintain the minimum positive pressure of the adjacent areas. Most authors have proposed a 50% reduction in the airflow rate supplied to an OR [14; 15; 16; 17]. [PERSON] et al. [15] ensured a positive pressure relationship of 1 Pa. [PERSON] et al. [17] introduced the 50% reduction in the air change rate after 30 min of no occupation and controlled the indoor air quality by particle counting. They observed that, while unoccupied, there was no source of contamination in the cleanroom. [PERSON] et al. [16] estimated 70% yearly energy savings, complying with the requirements for the patients' and staff's safety and health. Alternatively, [PERSON] [18] proposed a reduction of the supply outdoor air to 30% during unoccupied periods and achieved up to 6.5% savings in the annual primary energy demand. Other authors have evaluated the energy savings possible through air recirculation, achieving 24.1% and 44.31% reductions in the energy consumption for 25% or 50% recirculated air, respectively [19]. In those cases where the OR air handling unit is equipped with a heat recovery device, the reduction of the supply airflow prevents the system operations. However, it has been demonstrated that the supply airflow control can still entail larger energy savings than the operation of heat recovery units, especially when air humidification is required [5].