Aisle containment is a proven technique for optimizing data center cooling. It builds upon the best practice of arranging racks and cabinets with the fronts of racks in one row facing the fronts of racks in an adjacent row. That way, hot exhaust air from the back is less likely to be drawn into nearby equipment. An aisle containment solution further minimizes hot and cold air mixing by capturing the air within a given aisle.

There are two choices — cold-aisle containment and hot-aisle containment — and there isn’t much guidance out in the marketplace as to which way to go. However, there are a few variables that can help you decide. It depends on how the data center is cooled and where the heat is rejected.

Cold-aisle containment is by far the more common technique. Generally, doors are installed at each end of the aisle so that the aisle fills with chilled air. Sometimes a roof is used as well. In addition to focusing chilled air on the intake side of the equipment, cold-aisle containment makes the environment more comfortable for IT staff.

Why Hot-Aisle Containment

Hot-aisle containment has value as well, although it’s somewhat less intuitive. The goal of hot-aisle containment is to trap exhaust air at its hottest point and provide it with a direct path into the AC return. Hot-aisle containment increases the cooling capacity of AC units, which is measured as the ratio of the difference between supply and return air temperatures. 

With that in mind, there are three situations when hot-aisle containment would be the preferred method.

Chilled air is not effectively getting into the aisle. If you’re going to box off the cold aisle, you need to ensure that chilled air is getting into the space. Otherwise, the chilled air will simply blow over the cold aisle into the hot aisle and not cool the equipment.

The data center has a slab floor. Cold-aisle containment is most often used in environments where cold air is generated outside the containment area and brought in through the floor. If you have a slab floor, you’d have to bring the containment system up to the ceiling of the cold aisle to capture the chilled air and keep the hot air out.

You’re using in-row cooling. In-row cooling units are most efficient when the return air is as hot as possible, so hot-aisle containment should be used. Also, if the in-row cooling unit fails in a cold-aisle containment configuration, chilled air won’t get to the IT equipment.

Aisle Containment Best Practices

Whichever type of aisle containment you use, it’s important to install blanking panels where racks are missing within a row, or equipment is missing within a space in the rack. If these spaces are not filled, hot and cold air mixing can occur.

Temperature and humidity monitoring are also important. Sensors can be placed throughout the environment and monitored remotely to ensure there are no hotspots or other problems that could harm equipment.

You might want to consider raising the roof to allow more headroom. Most containment systems have a flat roof that goes over the tops of the racks. Many data centers are using 48U and 52U racks, which means that you need a ladder to get to the gear at the top. We recommend raising the roof four to six inches so that you don’t have to bend your head when you’re on a ladder.

There are no hard and fast rules as to the choice between cold-aisle and hot-aisle containment. But if you understand how AC systems work and how your data center cooling is designed, you can identify situations where hot-aisle containment is preferred.