Air-Side Economizers

Utilize “Free Air” to significantly reduce your operation costs by implementing Air-Side Economizers

One of the most exciting and talked about best practices for data center efficiency is often labeled “free cooling”-the use of Air-side Economizers to dramatically reduce your HVAC related energy consumption and costs. Air-Side economizers have become a talking point, it seems, in any current data center cooling discussion. Facilities professionals will point out that economizers have been around for a long time, and some data centers have indeed used them for years. In the wake of exorbitant energy bills, the concept of economizers-and “free cooling”-is gaining traction throughout the industry. End users have become interested in the economics (CapEX, ROI), installation, and operation of these devices. The Green Grid recently announced the introduction of an interactive free cooling map to help data center managers determine the amount of days they may be able to take advantage of Air-Side Economizers in their geographical location. With this Green Grid tool, you will be able to enter in your zip code and see the number of days your location falls within the ASHRAE recommended temperature and humidity ranges.

What are Air-Side Economizers?
Mechanical cooling, depending on the source, is estimated to consume anywhere from 33% to 40% of a facility’s incoming electricity. Designed to accompany or circumvent this process, Air-Side economizers can bring Mother Nature into the data center whenever the ambient conditions are favorable.

The outside air is brought into building and distributed via a series of dampers and fans. The servers ingest the cool air, transfer heat, and expel hot air to the room. Instead of being recirculated and cooled, the exhaust is simply directed outside. If the outside air is particularly cold, the economizer may mix the inlet and exhaust air, ensuring that the resulting air temperature falls within the desired range for the equipment.

The economizer design is typically integrated into a central air handling system with ducting for both intake and exhaust. The equipment includes filters to reduce the amount of particulate matter or contaminants that are brought into the data center space.

Air-Side Economizer Configuration
A quick internet search shows a lot of discussion on the concept but very little practical information on installing an Air-Side economizer within an existing data center. Instead, most articles emphasize proper design and alignment within the larger cooling architecture of a new building. Data center authority Pacific Gas and Electric recommends that the economizer and its controls are engineered “at the schematic design stage, where the required architectural accommodations can be made at little or no additional cost.”

Software giant Oracle addressed the “architectural accommodations” in its existing facility during the EPA’s 2006 Conference on Enterprise Servers and Data Centers. To retrofit Air-Side economizers in its Tier IV, 45,000 sq ft facility, Oracle would require a 3400 sq ft opening to move 1,710,000 cfm of air. Within their current layout, they found no convenient way to distribute the outside air from the CRAC to the raised floor. For them, the economizer was not a practical retrofit.

It was, however, for Red Rocks Data Center in Morrison, CO. The company in 2008 added an Air-Side economizer to their portfolio, significantly reducing their dependency on mechanical air conditioning. The facility, nestled in the foothills of the Rocky Mountains, estimates that they’ll be able to utilize the mountain air for 80% of the year (Reuters, 2008).

Though the approach will vary by the facility, the goal for all economizers is the same: “for all data center air handlers to have access to 100 percent outside air as well as return air” (Rumsey, 2007). This air, properly controlled and distributed, can significantly cut into the energy bill.

Air-Side Economizer Environmental Considerations

The data center has traditionally been a very controlled environment. Though this ecosystem has come at a price-economically and ecologically-users are not going to compromise uptime, performance, or equipment longevity. Introducing outside air and its by-products -higher temperatures, humidity fluctuations, dust/dirt-to this space with thousands of dollars of servers, storage, and networking gear may seem illogical to some. Before we review these concerns, point by point, here’s an overview of ASHRAE TC 9.9’s temperature and humidity ranges:

ASHRAE TC 9.9 Recommended Allowable
Temperature 18-27 deg C (64.4-80.6 Degrees F) 15-32 deg C (59-90 Degrees F)
Humidity 5.5-15 deg C Dew Point (41.9-59 Degrees F) 20%-80% RH

Conditions reflect air ENTERING the IT equipment

Rackmount hardware requires a continuous supply of cool air, as indicated by the ASHRAE range. The more temperate his environment, it would seem, the more a particular user could utilize ambient air for this supply. But, as engineer Vali Sorell notes, virtually all environments can take advantage of the technology: “Common sense would normally dictate that an outside air economizer in the hottest climate would not have a good payback. That logic may be applicable to a typical office building where there are approximately 2,500 hours of use in a year. However a data center must run continuously, 24 hours a day, for a total 8,760 hours per year. “Hotter climates can leverage cooler overnight temperatures or seasonal changes to incorporate economization.

To assuage concerns over varying temperatures, the economizer works in tandem with the central air handling system to maintain a palatable supply temperature. Through its controls and measurement devices, the economizer will compare the relevant temperature settings (outside air, server inlet, server exhaust) to determine the most economical cooling approach, whether full economization (100% outside air), partial economization, or mechanical cooling.

At extremes, humidity presents challenges for computing equipment. Too high, users worry about condensate forming on the equipment. Too low, users caution about electrostatic discharge. ASHRAE’s guidelines, for these reasons, seek a happy medium. If facilities choose not to measure humidity, the economizer can operate strictly based on temperature differentials. With humidity included, the economizer must perform additional measurements. If the outside air is cool but too dry, the system may expend extra energy to humidify the air. Thus, proper management and controls, as discussed within the temperature section, are imperative to ensure the right volume of air at the right temperature and humidity are introduced into the space.

Particle Contamination
Pacific Gas and Electric, along with Lawrence Berkley National Labs, compared particle concentrations in data centers with economizers against those who introduce minimal, if any, outside air. The results were expected. Facilities with economizers have higher particle concentrations inside the data center. However, an improved filter design can mitigate any contamination concerns (Tschudi, 2007). Using ASHRAE Class II, 85% filters, the authors of the report envisioned that the particle counts in economizer facilities would rival the small amount found in non-economizer data centers (In their study, data center CRACs and air handlers were equipped with ASHRAE Class I, 40% filters).

Maximizing Economizer Efficiency

The efficiency gain for Air-Side economization is obvious. Running a data center on outside air or as some have said “opening the windows in the data center,” significantly reduces or eliminates the chiller’s energy consumption. Parts common in mechanical cooling, like compressors and pumps, are inactive, as the fans (both inlet and exhaust) become the primary mechanism in removing heat. As a result, most of the incoming electricity can be devoted to the mission critical hardware and applications.

Yet this process isn’t as simple as opening the windows in your home. There are several considerations for maximizing the efficiency of the economizer.

Containment Strategy
If cool outside air mixes with server exhaust air, the servers will experience an elevated inlet temperature and hot spots may occur. A containment strategy, either cold aisle or hot aisle, ensures that the server supply air remains cool.

Maximizing the ASHRAE ranges
An economizer’s operating hours ultimately depend on the user’s temperature and humidity settings; higher settings mean more economizer hours. With this information, engineers can utilize temperature BIN data and psychrometric charts to approximate the use and justify the expense of the economizer.

This point is especially important for humidity, as excessive humidity control can cut into the savings achieved by the economizer. In certain geographies, for example, air can be very cool, but very dry. Users will need to couple ASHRAE’s recommendations, studies of their ambient climate, and their humidity preference. If the settings are too restrictive, the economizer use, and potential savings, can be limited.

Understanding the Controls
The earlier emphasis on design is very important. A number of different parts must work together to optimize the economizer and to maintain the environmental integrity of the data center. The switchover process from mechanical to economizer and vice versa must be seamless and well-defined. As data center provider Equinix notes, Air-Side economizers are directly cooling the computer room; there’s very little, if any, room for error.

Air-Side Economizers Energy Efficiency and ROI

Due to its temperate climate and tech focus, California is often the centerpiece of Air-Side economizer studies. Some of the figures are staggering:

  • A data center in San Francisco can achieve full economization 97.7% of the year (Sorell, 2007)
  • A data center in San Jose can reduce cooling costs by 60% through Air-Side economization (Pacific Gas and Electric, 2006)
  • A Sacramento facility envisions, through Air-Side economization, a 30% savings over conventional data centers (Bowman, 2009)
  • For 3500 hours per year, metro Los Angeles has dry bulb temperatures below 59 degrees F. (EYP Mission Critical Facilities, 2006)

Not every climate is as economizer friendly as California. And it doesn’t have to be. The Equinix study, mentioned earlier, envisioned a facility in the northeastern United States saving $250,000 per year in energy costs through an airside economizer. Ultimately, any potential savings are determined by the cost of electricity, climate, and facility temperature and humidity preferences.

On this note, consider Intel, who conducted a proof of concept test using economizers at their New Mexico location. Outside their traditional data center, they built two makeshift computing compartments-one cooled by conventional air conditioners, the other strictly by an airside economizer. In the economizer section, they made little effort to control the environment. Outside air was brought in between 65-90 degrees F; if above that setting, it was only conditioned back to 90. Humidity was ignored and only minimal filtering-“a standard household air filter” as they describe it-was employed.

The approach seems illogical, especially when you consider 450 high density servers were subject to these conditions. Yet, at the end of the ten-month test, the servers held up well. The server failure rate was only slightly higher than their main facility, despite relative humidity ranges from 4% to 90% and server inlet temperatures as high as 92 deg F.

Applying economizers to a larger data center installation, Intel extrapolates $144,000 in annual energy savings for a 500kW facility and $2.87 million for a 10MW facility.

These results will inevitably vary per user and location. Yet the study encourages thoughtful discussion on the viability of Air-Side economizers and the resiliency of modern hardware. Perhaps Intel’s hands-off approach to environmental control is not your cup of tea. Yet, you may conclude your own environmental requirements needn’t be as stringent. As that range expands, so does the possibility for Air-Side economization.

Read about Water-side Economizer


Bowman, C. (2009, February 3). ‘Free cooling’with fresh air makes McClellan Park data center greener. Retrieved February 3, 2009, from The Sacramento Bee:…

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EYP Mission Critical Facilities. (2006, July 26). Energy Intensive Buildings Trends and Solutions: Data Centers. Retrieved February 2, 2009, from Critical Facilities Roundtable:

Intel. (2008, August). Reducing Data Center Cost with an Air Economizer. Retrieved January 28, 2009, from Intel:

Oracle. (2006, January 31). Case Study: Expanding an Existing Data Center. Retrieved January 28, 2009, from Energy Star:

Pacific Gas and Electric. (2006). High Performance Data Centers: A Design Guideline Sourcebook.

Reuters. (2008, November 17). Red Rocks Data Center Leverages Mountain Air for a Green Advantage. Retrieved February 3, 2009, from Reuters:

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Sorell, V. (2007, December). OA Economizers for Data Centers. ASHRAE Journal , pp. 32-37.

Tschudi, W. (2007). Data Center Economizer Contamination and Humidity Study. Pacific Gas and Electric/Lawrence Berkeley National Laboratory.