Air-Side Economizer


  • An air-side economizer (see Figure 13 below) brings outside air into a building and distributes it to the servers. Instead of being re-circulated and cooled, the exhaust air from the servers is simply directed outside. If the outside air is particularly cold, the economizer may mix it with the exhaust air so its temperature and humidity fall within the desired range for the equipment.
  • The air-side economizer is integrated into a central air handling system with ducting for both intake and exhaust; its filters reduce the amount of particulate matter, or contaminants, that are brought into the data center.
  • Because data centers must be cooled 24/7, 365 days per year, air-side economizers may even make sense in hot climates, where they can take advantage of cooler evening or winter air temperatures. For various regions of the United States, Figure 14 shows the number of hours per year with ideal conditions for an air-side economizer.
Figure 13: Diagram of an air-side economizer

Figure 13: Diagram of an air-side economizer

Figure 14: Hours with ideal conditions for an air-side economizer. (Courtesy of DOE and The Green Grid)

Figure 14: Hours with ideal conditions for an air-side economizer. (Courtesy of DOE and The Green Grid)

Savings and Costs

  • Intel IT conducted a proof-of-concept test that used an air-side economizer to cool servers with 100% outside air at temperatures of up to 90°F. Intel estimates that a 500kW facility will save $144,000 annually and that a 10MW facility will save $2.87 million annually. Also, the company found no significant difference between failure rates using outside air and an HVAC system.38
  • A San Jose, California, data center estimates it can reduce its cooling costs by 60% through air-side economization. A Sacramento, California, data center projects a 30% savings over conventional data centers.
  • PG&E's experience with air-side economizer retrofits indicates that paybacks are greater than two years.39
  • Red Rocks Data Center in Morrison, Colorado retrofitted their facility with an air-side economizer and estimates that it will be able to utilize mountain air for 80% of the year.40
  • Retrofitting an existing data center may not be simple or inexpensive. Software giant Oracle did not find an economizer to be a practical retrofit.41

NetApp Data Center with Air-Side Economizers Earns Energy Star Label

  • A large part of the operational savings at NetApp's Global Dynamic Laboratory (GDL) –the first data center to earn the Energy Star label –is due to air-side economizers. Air-side economizers allow NetApp to make use of “free” cooler air from outdoors for much of its equipment cooling needs.
  • Using outside air allows the GDL to operate without a chilled water plant for more than 75 percent of the year. In addition, the company is able to use outside air for partial free cooling more than 98 percent of the time. Since the “return air” (air exhausted from server racks) is 90 degrees, any outdoor temperature below 90 degrees allows for at least partial free cooling. As a result, the data center's water plant operates nearly 90 percent less frequently than that of a typical data center.
  • This and other design approaches–such as elevated supply-side air temperatures, cold-aisle containment, pressure-sensors to minimize fan use, and the elimination of most ductwork–reduced building costs by over 66 percent and ongoing operating costs by roughly 60 percent.


  • Control systems are very important to the operation of the air-side economizer and must be properly maintained.
  • Excessive humidity control can cut into the savings achieved by the economizer. In certain geographic locations, for example, air can be very cool but very dry, and the system may spend excessive energy humidifying the air. Users will need to consider ASHRAE's recommendations, studies of their ambient climate, and their humidity preferences before considering implementation. If desired humidity ranges are too restrictive, net energy savings from an economizer can be limited. Proper management and controls are imperative to ensure that correct air volume, temperature and humidity are introduced.
  • PG&E, along with Lawrence Berkley National Labs, found that data centers with economizers have higher particle concentrations than those that introduce minimal, if any, outside air. However, an improved filter design may mitigate any contamination concerns.42 Using ASHRAE Class II, 85% filters, the authors of the report estimated that the particle counts in facilities with economizers would rival the small amount found in data centers without economizers. (In their study, data center CRACs and air handlers were equipped with ASHRAE Class I, 40% filters).