Hot Aisle/Cold Aisle Layout

  • Most modern IT equipment takes in cold air via the front of the unit and exhausts hot air out of the back of the unit. If servers are logically placed in rows with the front of the racks (and servers) all facing the same direction, then one has achieved a consistent airflow direction throughout the rows of racks. However, if several parallel rows of racks are placed with the same orientation, a significant efficiency problem arises. The hot exhaust air from the first row of racks gets sucked into the "cool" air intakes of the second row of racks. With each progressive row, the air temperature increases as hot air is passed from one row of servers to the next. (See Figure 2 below.)
    Poor Server Configuration

    Figure 2: Poor server row orientation (CRAC stands for computer room air conditioner)

  • To overcome this problem, the rows of server racks should be oriented so that the fronts of the servers face each other. In addition, the backs of the server racks should also face each other. This orientation creates alternating "hot aisle/cold aisle" rows. Such a layout, if properly organized, greatly reduces energy losses and also prolongs the life of the servers.15 (See Figure 3.)
  • Hot aisle/cold aisle row layout should not be used with server racks equipped with solid Plexiglas or glass doors. Perforated doors are necessary for hot aisle/cold aisle implementation.
    Hot Aisle Cold Aisle

    Figure 3: Hot aisle/cold aisle server row orientation. (Photo courtesy of

Savings and Costs

  • Hot aisle/cold aisle arrangements lower cooling costs by better managing airflow, thereby accommodating lower fan speeds and increasing the use of air-side or water-side economizers. When used in combination with containment, DOE estimates reduction in fan energy use of 20% to 25%.16
  • PG&E's experience with hot aisle/cold aisle retrofits indicated that the payback was greater than two years.17
  • Costs to consider include:
    • Server downtime while moving the racks,
    • Adjustments to the HVAC system,
    • New cabling for repositioned racks,
    • Electrical costs to reconfigure power distribution to the racks, and
    • Associated labor, overtime, and vendor costs, if applicable.


  • Hot aisle/cold aisle row 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 has cost implications that should be taken into consideration.
  • 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 indicate room temperature. (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.
  • 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.
16 Best Practices Guide for Energy-Efficient Data Center Design, NREL, February 2010.
17 Conversation with Mark Bramfitt, former Principal Program Manager, High Technology Energy Efficiency Team, PG&E, July 14, 2010.