Hot and Cold Aisle Containment Differences

Charmie Lyn FloresArticles, Blog

hot and cold aisle differences

Hot or cold aisle containment is a type of layout for server racks and other IT equipment inside a data center. The aim of hot or cold aisle containment is more efficient cooling resulting in improved energy consumption and lower cooling costs through airflow management.

A hot or cold aisle containment design is the lining up of server racks in alternating rows, with cold air intakes facing one side and hot air exhausts facing the other. Cold aisles refer to the rows composed of rack fronts. Cold aisles face the direction of air conditioner output ducts. Meanwhile, hot aisles face the direction of air conditioner return ducts.

Containment systems are used to isolate hot and cold aisles from each other. This is done to prevent hot and cold air from mixing. Initially, containment systems were physical barriers that separate hot and cold aisles using vinyl plastic sheets or Plexiglas covers. However, recently, vendors now offer plenums and other options that integrate containment with variable fan drives (VFDs), which further helps prevent the mixture of cold and hot air.

Best Practices for Hot or Cold Aisle Containment

There needs to be a way for the cold air to travel to the containment system. In some cases in row coolers blow cold air directly into the cold containment aisles. Another design that is popular is raised access flooring where cold air is ducted through the floor and vented through perforated tiles to the cold aisle containment center. These perforated tiles should be capable of a high cubic feet per minute (CFM) of airflow. Up to 600 CFM is ideal.

Cold Aisle Containment

cold Aisle containment

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Benefits of Cold Aisle Containment 

Cold aisle containment systems are often easier to implement since they do not need to install additional architecture to contain the exhaust air and revert it to the cooling units such as drop ceiling or air plenum. It only requires the installation of doors at the ends of the aisles and a roof at the top. In general, cold aisle containment systems are less expensive and easier to retrofit to existing data centers. This is particularly true for data centers that have overhead obstructions such as existing ductwork, lighting and power, and network distribution. It is not required for a cold aisle to be placed on a raised floor. In case of a power and generator failure, a cold aisle system contains a reservoir of cool air that can be used for short periods of time.

Challenges of Cold Aisle Containment

When utilizing a cold aisle system, the rest of the data center becomes hot. This can make it uncomfortable for workers who need access to the computers.

You should also match the equipment thermal requirements with the containment aisle. If there is a lot fo equipment in an aisle that can operate at 25°C but there is one piece of equipment that needs 20°C you will be wasting energy over cooling most of the equipment. This reduction in supply air temperature also reduces the economic benefits of the containment system.

The conditioned air that leaks from the openings under equipment like PDUs and raised floor tend to enter air paths that return to cooling units. Due to this, the overall efficiency of the system is lessened. Full cold aisle containment also creates a “separate space” that needs to equipped with fire detection and suppression system (FDAS).

Hot Aisle Containment

Hot aisle containement

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Benefits of Hot Aisle Containment

When having a hot aisle set up, the data center is flooded with cold air. It is also considered to be generally more effective. Any leakages that come from raised floor openings in the larger part of the room go into the cold space. In fact raised floors are not even required, you can simple populate the data center area with precision air cooling units. Having a hot aisle containment setup is more suitable for network racks and stand-alone equipment like storage cabinets that are situated outside the containment system since they are able to stay in the lower temperature part of the data center.

Challenges of Hot Aisle Containment

Hot aisle containment systems are, in general, more expensive compared to cold aisle containment systems. They require a contained path in order for air to flow from the hot aisle to the cooling units. As a return air plenum, a drop ceiling is often used.

For technicians working on IT gear, the higher temperatures in the hot aisle may create intolerable conditions. However, recent developments in server manufacturing have made servers front-serviceable. Due to this, the hot aisle would not need to be entered as often.

Electric outlets, fire suppression, lighting, and other systems inside the room need to be evaluated for their ability to operate at higher temperatures.

Work Environment and Containment Aisle

work environment in data center

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People still need to conduct operations inside the hot aisle, so regardless of the containment system type, it is important that the temperature inside the work environment be kept as per OSHA regulations or ISO 7243 guidelines for exceeding wet-bulb globe temperature (WBGT). In cold-aisle containments, the working area in general, like the walkways and workstations, becomes the hot aisle. Meanwhile, for hot-aisle containment, the general working area becomes cold.

Allowing the hot-aisle temperature to get too high with cold-aisle containment systems can be a cause for concern for IT personnel who are permanently assigned inside. Meanwhile, for hot-aisle containment systems, the high temperatures in the hot aisle behind IT racks can be mitigated. This is done by momentarily opening the aisle to let cool air in. However, work environment regulations can still be met even if the hot aisle remains closed because the staff is not permanently assigned in the hot aisle. Furthermore, most operations usually take place in front of the IT rack. This leads to a maximum WBGT of 90°F or 32.2°C due to the work or rest regimen of 25% work/ 75% rest, allowing the temperature of the hot-aisle containment system to get as high as 117°F or 47°C. The key difference between the hot aisle containment system and the cold aisle containment system is the higher temperature allowed with the hot aisle containment system since this allows the CRAH units to have more efficient operations.

Monitoring with AKCP Thermal Maps

No system is perfect, and monitoring sensors should be in use to provide ample warning in the unlikely event that a hot spot begins to form. Of course, the AKCP monitoring solution provides data to data center operators to have critical information necessary to increase their efficiency. AKCP Monitoring can predict needed preventive maintenance and actions, such as filter replacement, which help ensure the cooling system is operating efficiently.

Thermal Map Sensors

AKCP Themal Map Sensor

Obstructions Within the Cabinet

Cabling or other obstructions can impede the flow of air causing high-temperature differentials between the inlet and outlet temperatures. The cabinet analysis sensor with pressure differential can also help analyze airflow issues.

Server and Cooling Fan Failures

As fans age, or fail, the airflow over the IT equipment will lessen. This leads to higher temperature differentials between the front and rear.

Insufficient Pressure Differential to Pull Air Through the Cabinet

When there is an insufficient pressure differential between the front and rear of the cabinet, airflow will be less. The less cold air flowing through the cabinet, the higher the temperature differential front to rear will become.

Power Usage Effectiveness (PUE)

When the data is combined with the power consumption from the in-line power meter you can safely make adjustments in the data center cooling systems, without compromising your equipment, while instantly seeing the changes in your PUE numbers.


Due to high costs and added energy consumption, many data center professionals are considering hot and cold aisle containment strategies. Preventing the mixture of hot and cold air is seen to be one of the most promising energy-efficient measures available for data centers today. Additionally, containment also allows for consistent IT inlet temperatures, and it eliminates hotspots that are usually present in uncontained data centers.

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Charmie Lyn FloresHot and Cold Aisle Containment Differences