Hospitals demand high energy use, and operating rooms in particular sit at the top of the list. The ANSI/ASHRAE/ASHE standards governing operating room (OR) HVAC systems require strict control over temperature, humidity, ventilation, and pressurization, so “turning things down” isn’t an option. But there are proven strategies to mitigate energy consumption while staying fully compliant.

Standard Hospital Air Change Rates (ACH) in ORs

Regulations vary by state, but many hospital codes require 15–20 ACH in operating rooms. In practice, many facilities run 20–25 ACH, and some even up to 30 ACH. Of that volume, at least 4 ACH must be outside air to maintain safe ventilation. 

Notably, ANSI/ASHRAE/ASHE standards permit reducing ACH during unoccupied periods, required pressure relationships are maintained in all modes of operation and full ACH is restored when occupancy resumes^[1].

Why Reducing ACH Saves Energy (and Doesn’t Violate Safety)

Operating room HVAC systems routinely cool supply air down to as low as 52 °F and then reheat it to room temperature, especially when high outside air volumes or humidity loads are involved. Lowering the treated airflow volume cuts both cooling and reheating energy (when pressure and air-quality requirements are maintained). 

In short: less air = less energy to condition it.

Strategy: Occupied vs. Unoccupied ACH Control

This method assumes a modern OR HVAC and BAS setup (supply & exhaust VAV control, pressure monitoring, and logic flexibility). Key steps:

1. Balance the airflow during occupied hours
   • Use an air balancer to set the flow so it just meets or slightly exceeds code requirements (e.g. 20–22 ACH).
2. Lower to a safe unoccupied setpoint
   • Maintain positive pressure (e.g. 0.01 in WG) even when reducing flow. In practice, many ORs land at ~8 ACH during unoccupied hours without losing stability.
3. Switch mode via logic or scheduling
   • For example: designate 20 of 23 ORs to go unoccupied between 6 p.m. and 6 a.m. The remaining 3 might remain in occupied mode for emergency readiness.
   • Use occupancy sensors in unoccupied rooms to trigger a return to occupied mode after ~5 min of continuous motion.

This operational logic ensures that safety and pressure requirements remain intact while capturing energy savings when rooms aren’t in use.

Savings in Action: A Real Hospital Use Case

At a large hospital with 23 operating rooms, Mantis helped reset the system from 28–30 ACH constant to a dual-mode control:

• Occupied mode: 22 ACH
• Unoccupied mode: 8 ACH

The result: > 900,000 kWh electric savings + ~11,000 therms of gas annually (before incentives). 

Many ORs can benefit from a similar reset, particularly those currently operating at high constant ACH.

Considerations & Nuances (Don’t Miss These)

• Humidity load & outside air temperature: In humid or hot climates, outside-air conditioning penalties may limit how low you can drop flow.
• Equipment heat load: Some ORs generate BYPASS heat (lights, sterilizers). Make sure fallback strategies prevent thermal comfort violations during mode switching.
• Transition timing: Avoid switching modes during surgeries or critical periods. Use buffer windows or manual lockouts.
• Failure mode logic: In the event of sensor faults or control failures, default to safe (higher ACH) mode.
• Validation & monitoring: Always commission the dual-mode logic (e.g. hot testing, pressure checks) and monitor over the first 4–12 weeks.
• • Pressure maintenance across modes: Confirm that your control logic sustains required positive pressure during all HVAC operating modes, including reduced-flow or seasonal operation^[1].

Next-Level Upgrades & Enhancements

Once you have dual-mode ACH control in place, you can layer on more advanced tactics:

• Demand-Controlled Ventilation (DCV): Use CO₂, differential pressure, or particulate feedback to modulate fresh air delivery.
• Heat recovery or energy recovery ventilators (ERVs): Reclaim enthalpy from exhaust to pre-condition supply air.
• Segmentation / containment strategies: Partition support areas or use local air scrubbers to reduce the burden on central HVAC.
• Predictive analytics for OR schedules: Correlate OR use forecasts with HVAC mode schedules to pre-adjust rather than “react,”  reducing wasted lead/lag energy.

Improving HVAC efficiency in your operating rooms doesn’t mean compromising safety; it means optimizing intelligently. Mantis can evaluate these considerations and engineer HVAC strategies tailored to each hospital’s operational realities. Our collaborative, data-driven process ensures every solution is right-sized for efficiency, safety, and compliance, helping you capture measurable savings without compromising care.

By implementing dual-mode ACH control and layering in advanced strategies like DCV, ERVs, and predictive scheduling, your hospital can maintain compliance, strengthen reliability, and capture substantial cost savings. Every facility’s systems and constraints are different, but the potential for impact is universal.

Contact Mantis today to learn how we can help you uncover the safest, smartest path to energy efficiency in your critical environments.