# Safety and Energy Implications of Setback Control in Operating Rooms during Unoccupied Periods
## 3 Results and Discussion
### Energy and Emissions Savings  
Table 6 gathers the energy and emissions savings calculated under the setback operation of the OR HVAC systems for the nonoccupation periods. It can be seen that, with the exception of test 3, OR2 was unoccupied almost the whole period (90-99% of the time), while OR1 remained unoccupied only during 50-70% of the time. However, in the wintertime (tests 1 and 2), the thermal energy savings are lower for OR2. This can be explained by having a closer look at the setback operating conditions: whilst in OR1, the positive pressure was maintained with an average supply airflow of only 860-970 m\({}^{3}\)/h; in OR2, the required supply airflow could not be reduced below 1540 m\({}^{3}\)/h without compromising the required relationships. Electric energy savings in the fans are nonetheless higher for the cases when the setback operates during longer periods. This is because the power required in the fans does not vary so importantly for different airflows; indeed, the frequency converter for a supply airflow of about 860 m\({}^{3}\)/h is close to 20 Hz, which results in approximately 850 W, while, for 1540 m\({}^{3}\)/h, it keeps close to 35 Hz, implying 914 W.  
Figure 4: Pressure relationships registered in OR1 during test 1. Figure 3: Nonconforming pressure relationships (% time) and average pressure differences registered. During the summer season (tests 4 and 5), electric energy savings are due not only to the reduced airflow in the fans but also to the lower ventilation cooling loads, hence the reduced energy consumption in the chiller. For the climate of the location, the midsseason test performed (test 3) did not imply a cooling demand; consequently, from test 1 to test 3, all electric savings were due only to the reduced airflow in the fans. A heating demand also occurs beyond the winter period due to the low nighttime temperatures; since the ORs have no external walls, ventilation is the only thermal load during nonoccupation, being necessary to heat the outdoor air up to the indoor set temperature. In these cases, the influence of the higher setback supply airflow required in OR2 was not a determinant of the heating energy savings because of the smaller differences between the outdoor and supply air temperatures. Therefore, the actual energy savings of the setback control will depend on the particularities of each OR, which will result in different needs of the setback supply airflow. Figure 5 shows the hourly energy savings achieved through the setback for the winter and summer tests--that is, the energy savings achieved related to the number of hours when the setback is applied during these tests. It is clearly seen that the expected energy savings, either for natural gas or electricity, will strongly depend on the OR. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Test**} & \multirow{2}{*}{**OR**} & **Setback** & **Natural Gas** & **Fans Electric** & **Total Electric** & **Total Carbon** \\  & & **Operation** & **Energy Savings** & **Energy Savings** & **Energy Savings** & **Emissions** \\  & & **(\%Time)** & **(kWh)** & **(kWh)** & **(kWh)** & **(kgCO\({}_{2}\))** \\ \hline \multirow{3}{*}{1} & OR1 & 66\% & 541.0 & 54.9 & 54.9 & 149.1 \\  & OR2 & 99\% & 517.4 & 76.7 & 76.7 & 150.6 \\  & OR1 & 71\% & 614.9 & 58.6 & 58.6 & 168.2 \\  & OR2 & 90\% & 489.3 & 70.1 & 70.1 & 141.6 \\  & OR1 & 0\% & na & na & na & na \\  & OR2 & 55\% & 155.5 & 29.4 & 29.4 & 47.3 \\  & OR1 & 69\% & 50.8 & 47.5 & 78.4 & 28.1 \\  & OR2 & 93\% & 97.9 & 60.5 & 86.5 & 43.7 \\  & OR1 & 51\% & 32.9 & 41.6 & 75.0 & 21.7 \\
5 & OR2 & 90\% & 50.2 & 71.6 & 130.8 & 35.8 \\ \hline \hline \end{tabular}  
* na: not applicable/no cases registered.  
\end{table} 
Table 6: Energy and emissions savings under the setback operation. Figure 5: Hourly energy savings achieved at each OR during the summer and winter periods.