The development of ASHRAE 90.4: Energy Standard for Data Centers

ASHRAE 90.4 is a new energy standard for data centers that calls for meeting minimum efficiency requirements.


Learning objectives:

  • Explain ASHRAE 90.4: Energy Standard for Data Centers.
  • Explore the performance requirements of ASHRAE 90.4 versus the prescriptive requirements of ASHRAE 90.1-2007, addendum bu.
  • Calculate energy efficiency in data centers.

When addendum “bu” was added to ASHRAE 90.1-2007: Energy Standard for Buildings Except Low-Rise Residential Buildings in 2010, it caught the data center industry off-guard. While data center operators had a considerable amount of representation on the ASHRAE TC9.9: Mission Critical Facilities, Data Centers, Technology Spaces and Electronic Equipment technical committee that shapes the environmental standards for data centers, the ASHRAE 90.1 Standing Standards Project Committee (SSPC 90.1) was something else altogether. This addendum added significant prescriptive requirements to ASHRAE 90.1 for air- and water-side economizers in data center HVAC systems. Up until that point, data center HVAC systems were effectively exempt from energy code requirements. The major players in the data center industry reacted strongly to these changes and the resulting firestorm served to help shape future standards development, namely ASHRAE 90.4-2016 Energy Standard for Data Centers.

Figure 1: This is a photo of multiple condensing units on the roof of an older colocation facility. The power usage of these condensing units would count toward the heat-rejection peak fan-power portion of the mechanical load component. What was 90.1-2007 addendum bu?

Addendum bu added a new definition to ASHRAE 90.1: “computer room.” This definition was as follows:

A room whose primary function is to house equipment for the processing and storage of electronic data and that has a design electronic data equipment power density exceeding 20 W/sq ft of conditioned floor area.

This definition has been altered substantially in recent versions of the code, which will be discussed.

The major change attributed to addendum bu was the addition of economizer requirements for cooling systems with fans that serve computer rooms. This more or less aligned data centers with requirements for HVAC systems in other types of buildings and affected sections, 6.5.1, and Table 6.8.1H within 90.1-2007. There were several exceptions to the economizer requirement including when:

  • The total combined design load of all computer rooms in a building is less than 3,000,000 Btu/h (250 tons) and not chilled-water cooled.
  • If chilled-water cooled, the room design load is less than 600,000 Btu/h (50 tons).
  • Less than 600,000 Btu/h (50 tons) of computer room cooling is being added to an existing building.
  • The authority having jurisdiction (AHJ) does not allow cooling towers.
  • Where at least 75% of the design load is critical in nature (i.e., NEC Article 708: Critical Operations Power Systems, Tier IV data centers, data centers that process financial transactions, etc.).
  • The economizer provision affected every climate zone except for zones 1A and 1B (in the United States, typically only tropical regions, such as the southern tip of Florida, Puerto Rico, Guam, and Hawaii are in these climates zones).

Why did the data center industry object to bu?

While the foreword to the addendum clearly outlined ASHRAE’s justification for the new requirements and exceptions contained in it, the data center industry still had unusually strong objections. The primary issue was that the requirements were “prescriptive,” mandating specific design solutions. The data center industry’s argument was that in a product sector experiencing explosive growth/change, they should not be limited with prescriptive requirements that stifle technical innovation. Rather, the preference was that the requirements be performance-based and focus solely on quantifying energy efficiency, not the exact methods used to achieve that efficiency.

The data center industry’s most public response to addendum bu was published on Google’s public policy blog in April 2010. Google traditionally reserves use of this widely read forum for official commentary on privacy, net neutrality, anti-trust, and similar major regulatory topics. This open-letter response was signed by the who’s who of the data center industry. ASHRAE did issue a formal response to the open letter, maintaining that the new exceptions in combination with alternative compliance paths, such as the energy cost budget method, reasonably addressed those concerns. By this point, however, the lines had already been drawn between the two groups.

Why did efficiency requirements suddenly apply to data centers?

Figure 2: A step-down transformer within a server room power distribution unit is shown. The inefficiency associated with the transformer would count toward the “ITE distribution segment” loss portion of electrical loss component. Like all new codes and standards, nothing happens quickly. Changes are generally the result of a long, deliberate sequence of events. ASHRAE has long considered addressing data center energy efficiency in ASHRAE 90.1. In August 2007, the EPA issued a report on data center energy efficiency to the U.S. Congress. The key takeaway from this report was that the nation’s data centers were responsible for about 1.5% (61 billion kWh) of total US electrical consumption in 2006. The report further forecasted that this use would double by 2012. Now with the issue quantified, the wheels of change could be set in motion. Given this dramatic projected increase in energy usage, adoption of new energy efficiency requirements would be justifiable per the key provisions of federal energy efficiency legislation in effect at that time, the Energy Policy and Conservation Act of 1975 (EPCA) and the Energy Policy Act of 1992 (EPAct).

Those projections ended up being wrong. Based on a Lawrence Berkeley National Laboratory report issued 9 years later in 2016, usage did not increase nearly as dramatically as predicted. Instead of doubling, data center electrical usage was estimated at about 1.8% (70 billion kWh) of total 2014 U.S. electrical consumption. This represents only a 15% increase from 2006 levels. Ironically, the forecasting error is mostly attributed to the emergence of hyperscale/cloud data centers—which include many of the signatories to Google’s open letter who have since embraced forms of air and water economizers in a significant percentage of their data centers. Google, in fact, has managed to reduce their fleetwide trailing 12-month power-usage effectiveness (TTM PUE) in 2016 to 1.12, even when using fairly conservative metrics.

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