Why Indirect Air Economization Is Your Smarter Choice Compared to Direct Air Economization

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In my last post, I listed some common questions raised by data center design managers and users when considering direct and indirect air economization. In fact, the core issues of those questions are about which cooling topology is best, direct or indirect? What are the differences between the two and when / where are each used?

Both direct and indirect air economizer modes have pros and cons. Their individual benefits should be weighed against their associated costs, risks and other factors like geography, IT environmental setpoints, and so on. An ideal economizer mode takes advantage of a wide range of outside conditions to maximize the number of economizer mode hours, has the lowest operating and capital cost, while presenting the fewest risks to data center availability.

We’ve just released a new White Paper titled “Choosing between Direct and Indirect Air Economization for Data Centers”. In this paper, we discuss the benefits, associated costs, and risks of direct and indirect air economizer modes. Here are the main takeaways:


  • An indirect air system typically has a lower capital cost compared to a direct air system. Direct air systems typically require 100% DX backup in locations where the maximum allowable ASHRAE recommended dew point of 15°C (59°F) is violated. Finding adequate data center site locations with worst case dew point temperatures below this limit becomes challenging. Therefore, the majority of data center locations are likely to require 100% DX backup for direct air which is likely to push the capital cost above that of indirect air systems and also adds to the cost of upstream power distribution equipment and generators due to the extra DX load.
  • An indirect air system has a lower operating cost for the majority of locations in the world. Indirect air system operating costs are less dependent on geographic location compared to direct air. Direct fresh air systems perform best where the climate is favorable throughout the year. However, locations where the allowable dew point is violated, requires that the DX system (for direct air) operate in full mechanical mode which increases energy cost. We analyzed 54 cities around world and found that in 43 cities (80%), the indirect air system had a lower cooling load factor than the direct air system.


  • An indirect air system poses less or no environmental risks as a result of separation of inside and outside air streams. The availability risks posed by using direct air economizer systems are well known and can be mitigated with additional cost and operational maturity. However, we don’t believe this “insurance premium” makes financial sense given the two factors above. If data center operators cannot justify the risks of running direct air economizer systems without these safety nets, the logical decision is to use indirect air economizer systems.


If you want to know more about the detailed qualitative and quantitative comparisons between direct and indirect air economization, you can download and read this latest white paper. We also have a TradeOff tool – Cooling Economizer Mode PUE Calculator – that compares common cooling architectures to show you how they perform (PUE, economizer mode hours) in different geographies and under different operating conditions.


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