ASHRAE/IES Standard 90.1's Addendum bh, approved April 30, 2025, requires operating rooms, surgical cystoscopy rooms, procedure rooms, and Class 2 and Class 3 imaging rooms in new construction and additions to have HVAC systems capable of automatically reducing airflow during unoccupied periods, and it applies to any renovation project that alters more than half the zones on an affected system. Hospitals adopting the 2025 edition of Standard 90.1, or a state energy code that references it, now need commissioning and controls verification that specifically tests this turndown sequence in addition to the room's occupied-mode air change rate.
During unoccupied periods, the affected zones must reduce room airflow to no more than the larger of two thresholds: the airflow needed to maintain the room's required pressure relationship under ANSI/ASHRAE/ASHE Standard 170, or 20% of the airflow required to hit the room's minimum occupied air change rate. In plain terms, a room cannot turn down further than whichever number keeps its pressure relationship (positive or negative, depending on room type) correct, even if 20% of design airflow would otherwise be enough.
Zones return to full occupied airflow either on a programmed schedule or through a real-time trigger: an occupancy sensor detecting someone entering the room, or a manual override a clinician can activate. That override has to hold the room in occupied mode for a programmable duration rather than an instant, so airflow stays up for the full length of a procedure.
| Room Type | Turndown Requirement Applies | Governed By |
|---|---|---|
| Operating rooms | Yes | 90.1 Addendum bh, referencing ASHRAE/ASHE 170 |
| Surgical cystoscopy rooms | Yes | 90.1 Addendum bh |
| Procedure rooms | Yes | 90.1 Addendum bh |
| Class 2 and Class 3 imaging rooms | Yes | 90.1 Addendum bh |
| General patient rooms, corridors, offices | No | Not covered by this addendum |
Facilities may exempt up to 25% of zones per space type, or two zones, whichever is greater, from turndown to keep them continuously ready for emergency care. New construction and additions cannot use that exemption to skip turndown capability across the board. It is meant for a limited number of always-ready rooms, not a way around the requirement.
Addendum bh does not set its own air change numbers; it defers to ASHRAE/ASHE Standard 170 for what pressure relationship and air change rate each room type has to maintain, in both occupied and turned-down modes. The 2025 edition of Standard 170 adds its own clarification of unoccupied turndown requirements in outpatient procedure spaces, so a hospital working from an older edition of 170 needs to confirm which edition its authority having jurisdiction (AHJ) has actually adopted before assuming its rooms comply. Jurisdictions still enforcing an older edition of 170 may require the turndown hardware and controls to be installed and functional, without requiring the reduced-airflow mode to be actively configured yet.
A room's HVAC system now has two performance states to verify instead of one: full occupied airflow and pressure, and the reduced unoccupied state, plus the transition between them. That changes what a commissioning provider needs to functionally test:
Skipping the turndown-state test and only checking occupied-mode performance leaves a hospital with an OR that passes commissioning on paper while its unoccupied sequence has never actually been verified to hold the required pressure relationship.
Hospitals renovating existing operating rooms, cystoscopy suites, or imaging rooms, or building new ones, need functional testing that covers both airflow states this addendum requires, verified against whichever edition of ASHRAE/ASHE Standard 170 their AHJ has adopted. Aero Performance Group's monitoring-based commissioning services verify controls sequences like this one continuously rather than at a single point in time, which matters for a requirement built around an automated transition between two operating states.
For new construction or major additions where this turndown capability has to be designed in and commissioned before occupancy, request a compliance assessment to confirm scope before drawings are finalized.
Not directly. It applies to new construction, additions, and to existing systems where more than 50% of the zones served by that system are altered. A hospital with no current renovation plans is not required to retrofit turndown capability into existing operating rooms under this addendum alone, though a state energy code amendment could impose a different timeline.
Rooms can turn down to whichever is larger: the airflow needed to maintain the room's required pressure relationship under Standard 170, or 20% of the airflow needed for the room's minimum occupied air change rate. The pressure relationship floor typically ends up being the controlling number in most operating room configurations.
Addendum bh requires a programmable duration for the override hold and a functioning occupancy sensor or manual trigger, but it leaves the exact response time to the facility's commissioning agent and controls sequence design rather than setting one universal number. That response time needs to be documented and tested as part of functional performance testing.
Whichever edition your state or local authority having jurisdiction has formally adopted, which is not always the newest one. Contacting the AHJ directly to confirm the adopted edition is the only reliable way to know, since adoption timelines vary significantly by state and even by municipality.
No. The exemption covers up to 25% of zones per space type, or two zones, whichever is greater, and it exists to keep a limited number of rooms continuously ready for emergency care. New construction and additions cannot use it to exempt the whole suite from having turndown capability installed.
Yes. Once a state adopts the 2025 edition of Standard 90.1 or writes an equivalent requirement into its own energy code, a hospital's energy code compliance path for affected spaces needs to document turndown capability and the controls sequence behind it, alongside the occupied-mode design airflow that has traditionally been the focus of that documentation.