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A cold aisle is the space in front of server racks, where conditioned air reaches equipment intakes. A hot aisle is the space behind the racks, where equipment releases heated exhaust. In the conventional layout, rack fronts face one another across cold aisles, while rack backs face one another across hot aisles.
How the layout works
This arrangement assumes equipment draws air in at the front and exhausts it at the rear. It groups supply air and exhaust air into separate paths: cool air is directed toward equipment intakes, and heated air is collected from equipment exhausts.
Separating the paths is intended to limit two airflow problems: hot exhaust returning to server intakes, and cool supply air bypassing equipment without removing heat. The U.S. Department of Energy’s 2024 Best Practices Guide for Energy-Efficient Data Center Design says strict hot-aisle/cold-aisle configurations “can significantly increase the air-side cooling capacity” of a data center cooling system. That is a qualitative statement, not a promised result for every room.
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How containment differs from aisle orientation
Aisle orientation arranges rack fronts and backs to organize airflow. Containment adds physical barriers to further separate supply and return air. Barriers may include doors, panels, or flexible materials, depending on the system.
#1 Best Overall
| Approach | What it encloses | Design consideration |
|---|---|---|
| Hot-aisle containment (HAC) | The exhaust aisle | Must suit the facility’s exhaust-air return path and what the room can accommodate. |
| Cold-aisle containment (CAC) | The supply-air aisle | Must suit the facility’s supply-air design and what the room can accommodate. |
| Rack-based containment | A rack-level area | Another option identified by ASHRAE; its fit depends on the facility design. |
ASHRAE describes these as design approaches, not a universal ranking. The choice depends on the room’s supply and return air design, possible doors or roof panels, equipment airflow, and HVAC operation.
What to check in an existing data center
- Equipment airflow: Verify that equipment intakes and exhausts match the assumed front-to-rear direction. Nonstandard airflow may require different rack arrangements, cabinets, or air deflectors.
- Open rack spaces: Fit compatible blanking or filler panels in unused rack spaces to reduce unintended bypass and recirculation paths.
- Cable openings: Seal openings where practical so they do not create unintended airflow paths.
- Room airflow and HVAC: Rack orientation affects airflow. ENERGY STAR advises involving facilities staff to evaluate HVAC needs when rack orientation changes.
What cooling savings can you expect?
There is no guaranteed savings figure for an individual facility. ENERGY STAR’s hot-aisle/cold-aisle guidance gives a general estimate of 10 to 35 percent cooling savings, but the page does not show a publication date, and the range should not be treated as a forecast for a particular data center.
Rank #2
ENERGY STAR also reports DOE estimates of 20% to 25% lower fan energy and 20% lower chiller energy in combination with variable-speed fan drives. The cited ENERGY STAR passage does not provide the underlying DOE study citation, so those figures require that attribution and combination context rather than serving as stand-alone expected results.
ENERGY STAR’s page also reports that a 2014 Uptime Institute survey found 30% of surveyed operators had at least three-quarters of their data center using some form of containment. This is a dated survey statistic, not a current adoption rate. Actual cooling performance depends on room airflow, rack loads, equipment orientation, and HVAC design.
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