hot air aisle containment rendering

Hot vs Cold Aisle Containment: What You Need To Know

When it comes to cooling systems for a data center, one of the most traditional approaches has been hot-air and cold-air aisle containment. Both dependability and efficiency can be increased by using the method of containment to eliminate hot spots and prevent IT machines from overheating. Separating the cold air from the warm air is the primary objective when it comes to containment, achieved through modifications to the air flow such as fans, vents or air conditioners. To keep the hot-air and cold-air aisles apart, containment strategies require that IT racks be arranged with the front of the racks facing each other, creating alternating aisles of cool and warm air.

Hot Aisle Containment

Hot-air aisle containment has been established as saving the most money when it comes to energy costs. With hot-air aisle containment, the savings on energy are a direct result of the increased economizer hours it can provide. Economizer hours occur when the rack system’s chiller can be turned off because the temperature of the racks has not reached a raised temperature setting (remained cool). Cooling systems can limit economizer hours because they often have to be operating at much lower temperatures in an effort to prevent hotspots. Hotspots are made as hot air is pushed upward from the cold air passing from the cooling unit to the front of the rack and concentrating there.


hot aisle containment airflow rendering

Cold Aisle Containment

In a cold-air containment system (CACS), the CACS encircles the cold aisle to limit the blending of warm and cool air, creating a hot-air return plenum (an enclosed space for airflow) in the data center. Better heat exchange and an increased cooling capacity are the result of using containment in this way, where hot air is returned to the cooling unit and the cold air supply can be raised in temperature. The cold-air aisles use a chiller to reach the desired temperature, cooled by the chiller expanding and compressing a refrigerant, typically set around 45°F/7°C. Economizer hours can be increased by raising the chiller water supply temperature so that it more closely matches the outside air temperature. Air supplied by the cooling unit can also be ushered to the front of the IT rack without ever mixing with the hot air, creating uniform IT inlet air temperatures. When the two air flows are kept apart, economizer hours can increased and so can the supply air temperature.

cool vs hot aisle cooling rendering

Efficiency and Benefits of Aisle Containment

  • Cooling systems can be set to a higher supply temperature, which will save on energy and still supply the system with safe operating temperatures
  • Elimination of hot spots and mixing of hot and cold air
  • Saves energy when outdoor temperature is lower than indoor temperature as the cooling compressors don’t need to work as hard
  • Humidification / Dehumidification costs are greatly reduced when hot and cold air no longer mix
  • Overall better physical infrastructure system utilization, which results in equipment running at higher efficiencies

Humidification and dehumidification is another area where costs can be reduced when the warm air flow is kept separated from the cool air flow. Energy and water can be saved when the air being supplied to the racks is kept above the dew point, as humidity will not be removed and the addition of humidity is not necessary.

Other Considerations

Traditional cooling can be more expensive because the equipment involved, including fans, may need to be oversized to accomplish the task, so physical infrastructure is a legitimate consideration. Appropriately finding the correct size of all of the system’s components is essential in increasing efficiency and reducing the waste of energy needed to cool the area. The separation in the air temperature can be enhanced using something as a physical barrier such as a plastic curtain, panel walls, blanking panels or containment strips at the ends of the cold-aisle racks to further guide the cool air from reaching the warm air. Raised floors are another physical means of maximizing cooling because they allow, through perforated tiles, the cool air to be directed to the desired area. It is important to keep in mind that the use of something like ventilated tiles can defeat the objective of containment, despite cooling effectively.

To the reverse, a hot-aisle containment system (HACS) encloses the hot aisle in the effort of containing the IT equipment’s hot exhaust air, creating a cold-air return plenum. Through a consistent row-oriented (hot-air/cold-air) pattern, the HACS will, similarly to the CACS, keep the warm air from mixing with the cool air. To advance the control over the warm air that is exhausted into the HACS, ducting may be employed to connect the area to a computer room air handler (CRAH) or a remotely located air conditioner.

This method is preferred in purpose-built data centers because of the efficiency benefits through air-side economizers which may already be operating and available for use on site. Drawbacks to this approach, however, may be an increased demand for fan power for large volume rooms and the simple fact that people still need to be able to work in the environment. OSHA actually has regulations, to that point, which enforce the setting of a reasonable room temperature for workers of such a work environment. Decreased efficiency can come as a result of a temperature set too high in a HACS hot-air aisle (the rack backs), as it could require ventilation too occur more frequently. Temperature is also a consideration when it comes to non-racked IT equipment that is housed in the same area of the data center because some may not operate effectively if the temperature is not moderate enough, requiring ductwork to draw in cool air. This can also include things often overlooked like light switches, fire suppression systems and electrical outlets which may require less or more heat to operate at peak performance.

This is more of a room-oriented, rather than the row-oriented layout of the CACS, approach but the two methods are not much different in overall theory but more so in physical concerns like fan operation. The room-oriented method is generally considered to be a better choice for data centers which are especially large or have recently been constructed rather than retrofits or smaller data centers.

In general, a preference for HACS does appear in the numbers when comparing it with a CACS, though both manners of containment carry their own advantages.