Move to a Hot Aisle/Cold Aisle Layout

A Time-tested Technique

The hot aisle /cold aisle data center layout was originated by IBM in 1992 and it is one of the oldest ways to save energy in the data center.1  Hot aisle/cold aisle layout involves lining up server racks in alternating rows with cold air intakes – the fronts of servers – facing each other (the “cold aisle”) and hot air exhausts – the backs of servers – facing each other (the “hot aisle”).  (See Figures 1 and 2.)  Raised floors are commonly used in data centers to provide an efficient way to deliver cold air from the computer room air conditioner (CRAC) unit to server racks. CRAC units direct conditioned air into the sub-flooring.  This pressurized cool air rises through perforations in floor tiles into cold aisle, where it is subsequently drawn into the front of servers to cool them.  Hot exhaust air rises from server racks in the hot aisle and returns along the ceiling back to the to the CRAC unit intakes.

A recent NYSERDA survey of data center operators found that roughly two-thirds of larger data centers (e.g., enterprise and mid-tier) have implemented hot aisle/cold aisle layouts.2

Side View of Raised Floor

Figure 1: A side view of hot aisle/cold aisle layout.  

Hot Aisle Layout

Figure 2: Hot aisle/cold aisle layout. (Image credit:

What happens if you don’t use hot aisle/cold aisle layouts?

If servers are placed in rows with their fronts all facing the same direction, a significant problem arises. The hot exhaust air from the first row of racks gets drawn into the fronts of the second row of racks. With each progressive row, the server inlet temperature increases as hot air is passed from one row of servers to the next. (See Figure 3 below.)  The only way to keep equipment in the rows farthest from the CRAC unit from overheating is to “over-cool” the room, wasting energy.

Don't Do This

Figure 3: Don’t do this: poor server row orientation

No raised floors?  No problem!

Hot aisle/cold aisle layout can still be used in server rooms without raised floors: distinct hot and cold aisles can be created by rearranging server rack locations and then reconfiguring the ductwork above.  Return registers and ducts should be placed above the hot aisle.

Savings and Costs

  • The hot aisle/cold aisle layout helps prevent cold “supply” air from mixing with hot exhaust air, resulting in cooling savings from 10 to 35 percent.3
  • More efficient airflow allows for slower fan speeds and more frequent use of “free cooling” from air-side or water-side economizers.  When used in combination with variable speed fan drives, DOE estimates that containment can reduce fan energy use by 20% to 25% and chiller energy use by 20%.4
  • Potential costs to for consideration  include:
    • Server downtime while moving the racks
    • HVAC system adjustments/tuning
    • New and/or reconfigured cabling for repositioned racks, and
    • Reconfiguration of power distribution to the racks

Tips and Considerations

  • Hot aisle/cold aisle layout should not be used with server racks that have solid Plexiglas or glass doors.  Solid rack doors should be removed or replaced with perforated doors to permit airflow.
  • Hot aisle/cold aisle layout makes sense for the vast majority of new data centers or data center expansions. However, retrofitting an existing data center with a new layout may require downtime and can be costly.  For example:
    • All equipment in a server rack must be shut down prior to moving it, and cables must be labeled and unplugged
    • An electrician may need to realign the power pathway so all devices can be plugged into an outlet in their new location
    • The new server rack orientation will substantially change the airflow in a room, so it will be necessary to work with facilities to evaluate the data center's new HVAC needs.
  • Many computer room air conditioning (CRAC) units use return air temperature to govern their operation. (Return air refers to warm air entering the CRAC for cooling.) This will not work in a hot aisle/cold aisle configuration because the return air has been concentrated in hot aisles and its temperature is therefore substantially higher.  Instead, server inlet temperature (cold aisle temperature) should be used to govern CRAC unit operation.
  • Implementation risks include rendering the existing HVAC system ineffective, inefficient, or inadequate for the new layout, and creating new hot spots in the data center.

1 (PDF, 55.5 KB)

2 Table 12 of report found here:

3 (PDF, 69.2 KB)

4 Best Practices Guide for Energy-Efficient Data Center Design, NREL, February 2010. (PDF, 899 KB)