Design Considerations for Operating Room HVAC Systems
Healthcare Design Blog Series
HVAC systems play an important role in providing a clean operating room environment, which helps minimize surgical site infections. Compliance with ASHRAE 170-2017 Ventilation for Healthcare Facilities allows for proper HVAC system design of an operating room. This blog is a general introduction to the ASHRAE 170-2017 requirements for operating rooms.
Operating rooms shall be under positive pressure with respect to adjoining spaces so to prevent the infiltration of pathogens, which could contaminate an open wound, into the operating room.
Operating rooms are to be maintained at a minimum of +.01 in. of water positive pressure with respect to all adjoining rooms.
The positive pressurization for the operating room is achieved by airflow balancing. The operating room total supply airflow will be greater than the operating room total return airflow.
A room pressure monitor will be installed to constantly monitor the operating room differential pressure. The room pressure monitor shall include a visual indicator where someone can monitor the operating room differential locally.
Air Distribution Systems
Supply air and outdoor airflow to meet the minimum air change requirements listed in ASHRAE 170-170 Table 9-1
Minimum 20 total supply air changes per hour.
Minimum 4 outdoor air changes per hour.
Supply Air System
The supply diffuser array must provide a laminar airflow pattern (unidirectional, downward) over the patient and surgical team.
The laminar diffuser array has an average velocity range of 25 feet per minute to 35 feet per minute.
The coverage area of the primary supply diffuser array shall extend a minimum of 12 in. beyond the footprint of the surgical table on each side of the table.
Within the diffuser array, no more than 30% of the area of the diffuser array can be used for non-diffuser applications such as lights, gas columns, utility poles, access panels, sprinklers, etc.
Return Air System
Room total return airflow capacity will be lower than the room total supply airflow so that the required room pressurization can be maintained.
To provide a top-down supply to return airflow path, the room shall be provided with at least two low sidewall grilles located at opposite corners or as far apart as possible in the room.
The bottom of the return grilles are to be installed 8 inches above the floor.
It is recommended that the core velocity thru return grilles be sized for a maximum of 400 ft/min, as grilles located close to the floor are noisier and collect debris from the floor.
Operating rooms needs to be filtered in accordance with ASHRAE 170-170 Table 6.4, which indicates the MERV (minimum efficiency reporting value) for the pre-filtration and final filtration.
MERV 7 filtration shall be installed of upstream of the heating and cooling coils so that all mixed airflow is filtered.
MERV 14 filtration will need to be installed downstream of the cooling coil and the supply fan.
68°F is the minimum allowable room temperature.
75°F is the maximum allowable room temperature.
ASHRAE 170-170 Table 9-1 normative note O states that surgeons or certain types of surgical procedures may require that the room temperature range to exceed the minimum indicated range. This design team and healthcare facility should discuss this during the design process.
Room Relative Humidity Percentage
20% is the minimum relative humidity % requirement.
60% is the maximum relative humidity % requirement.
ASHRAE 170-170 Table 9-1 normative note O state that surgeons or certain types of surgical procedure may require that the room relative humidity % range to exceed the minimum indicated range. This design team and healthcare facility should discuss this during the design process.
Mark Rowlenson, LEED AP