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Evaluate and implement operating room airflow setback.

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Operating room HVAC systems use a significant amount of energy delivering and conditioning air when an operating room is occupied. Setting back the system when the room is not in use reduces energy consumption.

  • Project Talking Points

    • A typical operating room delivers 20-25 air changes per hour (ACH) in occupied mode with 4 ACH of outside air, and many older designs have air changes in excess of 30 ACH.
    • ASHRAE 170 (now part of the 2010 Facility Guidelines Institute’s Guidelines for Design and Construction of Health Care Facilities) requires operating rooms to have a minimum of 20 air changes per hour.  ASHRAE 170 and many state codes also allow the number of air changes to be reduced “when the space is unoccupied providing that the required pressure relationship to adjoining spaces is maintained while the space is unoccupied and that the minimum number of air changes indicated is reestablished anytime the space becomes occupied” (7.1-Subsection 1.c).
    • A typical operating room HVAC system cools room air to as low as 52° F and then reheats the air back to room temperature, an energy-intensive activity. Humidity control can be energy-intensive as well, so reducing the volume of air treated will reduce energy consumption.
    • Depending on the pattern of use of an operating room, which may be unoccupied 40 percent or more of the time, HVAC setbacks represent a significant opportunity to save on energy use.
  • Triple Bottom Line Benefits

    • Cost benefits: Reduced heating and cooling results in direct operating cost benefits. Savings will vary depending on local climate, number of operating rooms, and operating room usage patterns.
    • Environmental benefits: Setting back HVAC system settings reduces energy use and emissions associated with operating the facility’s HVAC system.
    • Social benefits:  Reductions in hospital operating costs provide for a more sustainable health care system and help reduce the costs to consumers.
  • Commissioning Connections

  • How-To

    1. Start by reading the ASHE monograph Operating Room HVAC Setback Strategies, which provides a good primer for getting started with this strategy.
    2. Your facility engineer, a trusted third-party engineer, or a trusted contractor with intimate knowledge of your existing HVAC system is your best resource for implementing this strategy as what needs to be done will vary according to the age of the system, the type of system, and its existing controls. It is recommended that you enlist the assistance of a trusted third-party engineer to evaluate the existing system and recommend an appropriate solution. Multiple vendors offer a range of solutions, and it can be difficult to evaluate solutions with respect to what best suits your system, budget, and user preferences.
    3. Coordinate with the facility’s infection control officer on the allowed minimum turndown rates as the minimum air flow rate to maintain a sterile environment may be higher than the air flow rate to maintain pressurization. Coordinate setbacks with testing procedures to determine if there is an increased build up of dust or debris on surfaces when air changes are reduced. Consider a staged approach by reducing air flow gradually over time to find the HVAC system’s sweet spot for energy savings and infection control.
    4. Establish a baseline energy consumption level for your facility’s operating rooms that accounts for fan, cooling, humidification/dehumidification, and reheat energy during normal operating hours.
    5. Identify additional key performance indicators such as user satisfaction, code compliance, and efficiency.
    6. Understand how your HVAC system currently operates. Specifically:
      • How are the operating rooms controlled and monitored?
      • How is the pressure differential maintained for the operating rooms?
      • What is the condition of the existing HVAC system serving the operating rooms – dampers, ductwork, controls, and so on? You may have the opportunity to discover and address other opportunities to make the existing system more efficient.
      • What is the air leakage in the existing operating rooms? This rate will affect how much supply air will be required relative to return air to maintain a positive pressure differential.
      • How many and which air-handling units serve which spaces? Does the air-handling unit that serves the operating room(s) also serve any non-operating room spaces?
      • The humidity in operating rooms must be maintained between 20 and 60 percent at all times. How does your system maintain this level?
    1. Once you understand your current control system, you’re ready to assess the options for a control strategy that can reduce air changes while maintaining appropriate pressure relationships.  Operating room setback is more complicated than setback for other types of spaces because the ORs must maintain positive pressure relative to all adjacent spaces whether they are occupied or unoccupied. As well, the humidity levels must be maintained. Ventilation and humidity in operating rooms can be controlled in a number of ways, and these options need to be well understood before a setback methodology is designed.
      • Generally speaking, pressure relationships can be maintained using two-position VAV boxes on the supply and return air ducts; pressure independent valves on the supply and returns; or a modulating control damper on the return duct.
      • Identify codes that may restrict the minimum number of air changes during setback (e.g., California). Verify the minimums required by local codes.
      • Simplify controls by modulating airflow only. Although some facilities may allow humidity and temperature to vary during HVAC setback, it is generally preferable to reduce only the airflow, while maintaining temperature and humidity. With this strategy, an operating room can achieve ready mode extremely quickly.
    1. Involve all affected users early in the process including: surgeons, clinicians, infection control staff, facility staff and operators, owners, and any trusted external contractors with intimate knowledge of the existing systems. It is critical to understand any perceptual barriers as well as preferences for how the upgraded system will respond to and interact with users.
    2. Following is a list of user interface options that may be employed:
      • Time schedule (simple, good for non-emergency operating rooms that operate on a predictable schedule, but requires an override in case of unforeseen complications where a surgery may go long)
      • Occupancy sensors (these can combine audio, infrared, and motion detection)
      • Manual switchover (these require staff training and open the possibilities for human error—this is not a preferred option)
      • Combined interface/control methods (e.g., time schedule with occupancy sensors and manual override for unscheduled events)
      • Integration of medical information system for patient and procedure tracking into building automation system (BAS).
    3. Set up a system to track key performance indicators.
  • Tools

    If you have an ROI tool, calculator, or similar resources to share, please contact us or participate in the discussion tool at the bottom of this page.

  • Case Studies

    Do you have a case study to share? Please contact us.

    The following examples are in-progress implementations of OR setback that have not yet been formalized, evaluated and written as a case study. However, they provided practical examples of how OR setback can be utilized.

    Example 1

    A new hospital has 14 orthopedic operating rooms (OR) that operate at 20 ACH (4 ACH outside air) 24 hours per day, seven days per week. The hospital is primarily for scheduled procedures such as total joint replacements; however an occasional accident victim may be admitted for emergency procedures.

    The hospital will initially operate all 14 OR’s until operational baselines (patient census, infection control, etc.) can be established. Approximately six months after commissioning, the staff will reduce air change rates during unoccupied hours and measure relevant infection control metrics over time as air change rates are reduced.

    A variable air volume HVAC system has been designed to operate with a minimum turndown of 5 ACH to maintain pressurization.

    Based on the patient census and operational experience a decision will be made the hospital staff on how many OR’s may be setback over night.

    The control of the OR setback is based on data relayed by the medical information system (MIS). When a surgery is scheduled, the information is relayed to the BAS, which ramps up the air handling unit to 20 air changes per hour. Once the surgery is complete, the room is set as unoccupied in the MIS, which then relays that information to the BAS to setback the air flow rate. In addition to the air flow, the lights are shut off during unoccupied times.

    Example 2

    A new hospital addition includes four c-section operating rooms (OR) as part of a new birthing center than includes labor and delivery patient rooms, post partum patients rooms and support spaces. The c-section OR’s operate at 20 air changes per hour (ACH) with 4 ACH of outside air in a facility that operates 24 hours per day.

    A complex setback strategy is being implemented that utilizes inputs from the medical information system (MIS), which relays information to the building automation system (BAS) which controls the air handling unit (AHU). The AHU is setup to reduce the air flow rate from 20 ACH to 5 ACH (required to maintain pressurization). The actual setback air change rate will be determined experimentally over time. The building operators will coordinate with the infection control staff to determine a viable minimum air flow rate. In addition to setting back the air flow rate, the lights in the OR are shut off based on a signal from the BAS. Temperature is not setback as temperature has the longest lag time to meet the desired set point.

    C-sections are procedures that are sometimes scheduled, and sometimes unscheduled, which requires rapid reaction on the part of the HVAC system. This is somewhat impractical in this setting as it can take several minutes for a room to reach operating requirements. To allow for unscheduled c-sections, a strategy of having 1 OR always on standby was selected.

    The first OR is always operating at 20 ACH even when unoccupied. Once that OR becomes occupied based on a signal from the BAS, the second OR is brought up to operating conditions. This continues with the third and fourth OR’s as the OR’s are occupied. 

  • Regulations, Codes and Standards, Policies

    “Operating rooms shall be maintained at a positive pressure with respect to all adjoining spaces at all times. A pressure differential shall be maintained at a value of at least +0.01 in. wc (2.5 Pa).”

    This requirement applies during both occupied and unoccupied modes. However, per Section 7.1 of Standard 170:

    “For spaces that require a positive or negative pressure relationship, the number of air changes can be reduced when the space is unoccupied, provided that the required pressure relationship to adjoining spaces is maintained while the space is unoccupied and that the minimum number of air changes indicated is reestablished anytime the space becomes occupied. Air change rates in excess of the minimum values are expected in some cases in order to maintain room temperature and humidity conditions based upon the space cooling or heating load.”

    • ANSI/ASHRAE/ASHE Standard 170 Addendum d reduces the minimum design humidity for short-term patient care spaces from 30 percent to 20 percent. The humidity range must be maintained between 20 and 60 percent at all times (occupied and unoccupied mode). 
  • Cross References: LEED

    • LEED for Existing Buildings: Operations + Maintenance
      • Energy & Atmosphere Prerequisite 1: Energy Efficiency Best Management Practices—Planning, Documentation, & Opportunity Assessment, document the current sequence of operations, develop a building operating plan, develop a systems narrative, create a preventive maintenance plan, and conduct an energy audit that meets the requirements of the ASHRAE Level I - Walk-through assessment.
      • Energy & Atmosphere Prerequisite 2: Minimum Energy Performance, establish the minimum level of operating energy efficiency performance by achieving an energy performance rating of at least 69 using the EPA’s ENERGY STAR® Portfolio Manager tool; or, demonstrate energy efficiency at least 19% better than average following the LEED Reference Guide for Green Building Operations & Maintenance; or use the alternative method described in the LEED Reference Guide for Green Building Operations & Maintenance AND have energy meters that measure energy use.
      • Energy & Atmosphere Credit 1: Optimize Energy Efficiency Performance, achieve increasing levels of operating energy performance relative to typical buildings of similar type utilizing any of the methods described in Energy & Atmosphere Prerequisite 2.
      • Energy & Atmosphere Credit 3.1: Performance Measurement—Building Automation System, utilize a computer-based building automation system (BAS) that monitors and controls major building systems.
    • LEED for Healthcare: New Construction and Major Renovations
      • Energy & Atmosphere Prerequisite 1: Fundamental Commissioning of Building Energy Systems, designate a commissioning authority to commission the heating, ventilating, air condition systems and associated controls.
      • Energy & Atmosphere Prerequisite 2: Minimum Energy Efficiency Performance, establish the minimum level of energy efficiency for the proposed building and systems by a whole building energy simulation; or a prescriptive compliance path utilizing the ASHRAE Advanced Energy Design Guide (AEDG) for Small Hospitals and Healthcare Facilities; or a prescriptive compliance path complying with ASHRAE Standard 90.1.
        • Setting back air changes in OR’s during unoccupied periods is not a standard energy conservation measure within ASHRAE 90.1, and therefore may not be accepted by the LEED reviewer.
        • Sensor and non-schedule based setbacks are more likely to be accepted by the LEED reviewer as they are more similar to CO2 demand control ventilation and lighting savings via occupancy sensors.
        • Scheduled setbacks, which are similar to whole building lighting schedules are unlikely to be accepted as an energy savings over the baseline. However, reducing overall building energy consumption in both the baseline and proposed models will improve the relative energy savings of the building.
        • Determining how often to setback air flow is based on a patient census, and is difficult to estimate for new facilities. Medical planners will have a good sense of how many procedures will be conducted each day for new facilities as that is what the building program is largely based off of. For existing facilities, metrics are typically available. 
      • Energy & Atmosphere Credit 1: Optimize Energy Efficiency Performance, to achieve increasing levels of energy performance beyond the prerequisite standard to reduce energy usage.
      • Energy & Atmosphere Credit 5: Measurement and Verification, develop a Measurement and Verification plan by incorporating the Calibrated Simulation method; or the Energy Conservation Measure Isolation as specified in the International Performance Measurement & Verification Protocol Volume III.
  • PIM Synergies

  • More Resources

  • PIM Descriptors


    Level: Advanced

    Category List:

    • ENERGY
    • HVAC
    • Operations

    PIM Attributes:

    • Optimize Operations
    • Repair or Optimize Existing Systems (fix what you have)
    • System Upgrades

    Improvement Type:


    • Engineering/Facilities Management
  • This resource was underwritten by  Johnson Controls


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