Hot aisle containment
A data room layout that encloses the aisle where servers discharge hot air, with end doors and a roof or ceiling panels, so exhaust returns to the cooling units without mixing with the cold air reaching server inlets. Cold aisle containment encloses the supply side instead.
Also called HAC, cold aisle containment, aisle containment
What is hot aisle containment?
Hot aisle containment encloses the aisle behind server racks so their hot exhaust returns straight to the cooling units instead of mixing back into server inlets. Aisle containment cuts bypass air and recirculation, so cooling units can supply closer to the airflow the servers actually draw.
Hot aisle containment: how it works
Servers draw air in at the front and discharge it hotter at the back. In an open room some of that hot air curls over and around racks into server inlets, and some cold supply air returns to the cooling units without passing through any server. Enclosing the hot aisle separates the two streams, so cooling units receive hotter return air and deliver cold air only where servers draw it.
Where you meet the term
- In a data hall with doors at the end of each rack row and a ceiling or chimney over the hot aisle.
- In a high-density room where close-coupled coolers draw directly from a contained aisle.
- In a cooling calculation, where the bypass allowance can be set lower for a contained room.
How to check it
- Look for open gaps in the containment. Missing blanking panels, empty rack positions, and propped-open aisle doors let hot and cold air mix and should be closed. Owner check
- Measure inlet and return temperatures. A technician measures server inlet temperatures across the rack face and return air temperature at the cooling units to confirm the streams are separated. Technician
Hot aisle containment: what it is not
- Containment does not add cooling capacity. It lets existing capacity reach the servers by stopping the mixing.
- Containment does not replace airflow balance. If cooling units supply less air than the servers draw, air is pulled through gaps from the hot side.
Hot aisle containment questions
Is hot aisle or cold aisle containment better?
Each has trade-offs. Hot aisle containment leaves the rest of the room at supply temperature, while cold aisle containment leaves it at return temperature, which affects people working there and any equipment outside the contained rows.
Does aisle containment affect fire protection?
Enclosures and roofs can obstruct sprinkler discharge and smoke detection, so the containment design is coordinated with the fire protection engineer under NFPA 75 and the local code.
How much bypass air does containment remove?
It depends on how completely the aisle is sealed, including blanking panels and floor openings. The HVAC Bench cooling example allows 15 percent bypass for a room and notes that a well-contained room runs lower.
Where this definition comes from
Written from standards body publication material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- Thermal Guidelines for Data Processing Environments, Fifth EditionASHRAE Datacom Series book 1 defining the recommended IT equipment inlet envelope of 18 to 27 degrees Celsius with a dew point range of minus 9 to 15 degrees Celsius and an upper relative humidity limit, the wider allowable envelopes for equipment classes A1 to A4 with A1 at 15 to 32 degrees Celsius, and the change in the fifth edition to a dew point basis for the lower humidity limit.
- NFPA 75, Standard for the Fire Protection of Information Technology Equipment2024 edition setting minimum requirements for protecting information technology equipment and areas from fire and its effects of smoke, corrosion, heat, and water, covering construction, automatic detection and suppression including gaseous and water mist systems, HVAC systems, coolant systems, electrical service, and emergency response.