Does the H1 class mean air cooling is no longer acceptable?

No. Air-cooled A-classes remain valid for equipment within their power density range. The H1 class describes the envelope for liquid-cooled equipment operating above what air cooling can reasonably support.

Does simulation alone satisfy the resilience bulletin's requirements?

The bulletin's guidance concerns operational and facility design practices; simulation demonstrates thermal margin under normal and fault conditions but does not substitute for the facility's own operational procedures.

Can existing air-cooled rack models be extended to liquid-cooled analysis?

Often yes — the enclosure and airflow model can be extended with a cold plate and loop network model rather than rebuilt from scratch.

01ASHRAE TC 9.9 (5th edition + H1 class)

ASHRAE TC 9.9 — liquid-cooled class and resilience bulletin

ASHRAE TC 9.9's 5th edition formalized an H1 liquid-cooled equipment class alongside the existing air-cooled A-classes, and a September 2024 bulletin added resilience guidance specific to liquid-cooling loops (leak response, redundancy, facility water quality). The practical driver is rack power density: AI accelerator racks commonly exceed 30 kW, a range where the air-cooled A-classes no longer describe an achievable or efficient operating envelope.

When it takes effect

The 5th edition and H1 class are current guidance; the resilience bulletin was issued in September 2024. Facilities specifying new AI infrastructure are increasingly asked to demonstrate compliance with the liquid-cooled class rather than the legacy air-cooled classes.

Who it affects

Data center operators, colocation providers, and OEMs designing racks, cold plates, or manifolds for AI accelerator or high-density compute deployments above roughly 30 kW per rack.

What evidence satisfies it

Rack- and room-level airflow and thermal simulation (for any remaining air-cooled fraction of the load) plus cold plate and liquid-loop thermal simulation demonstrating component temperatures within the targeted class's envelope, including at credible loop-fault conditions referenced in the resilience bulletin.

02How that evidence is produced

This evidence spans board-level cold plate design through full-rack and room airflow modeling.

Contracted alternative