ASHRAE Strikes PUE from Data Center Efficiency Standard Draft

ASHRAE drew criticism from some prominent industry voices last year after it released the first draft of standard 90.4, whose goal is to set energy efficiency standards specifically for data centers, recognizing that large stand-alone data centers need a special approach and cannot be lumped in with other building types as they currently are in standard 90.1.

Standard 90.4 is being created to work together with 90.1 and references a lot of definitions in the older standard, meant for all building types, except low-rise residential buildings.

ASHRAE doesn’t enforce compliance with its building standards, but they are a big deal because local building officials in the US use them extensively in building inspection and permitting.

PUE Gone from the Standard

The most controversial part of the first draft of 90.4, released for review and comment in February 2015, was its reliance on PUE, or Power Usage Effectiveness, as the data center energy efficiency metric.

Developed and promoted by The Green Grid, it is the industry’s most widely used (and most widely misused) metric, but the draft’s critics argued that using PUE in the ASHRAE standard would disadvantage colocation providers, whose facilities often run at partial load for the majority of their lifespan.

Put simply, the metric compares power consumed by IT to total power supplied to the facility. The bigger the portion of total power that doesn’t make it to IT equipment on the data center floor, the more inefficient the facility’s infrastructure is. Therefore, the more power IT equipment consumes, the better the overall PUE. If the success of your business relies to a great extent on having available capacity for more servers, that unused capacity will theoretically have negative impact on your PUE.

Second draft of 90.4, released last August, got rid of PUE, introducing instead two new efficiency metrics – Mechanical Load Component and Electrical Loss Component – but made PUE one of the alternative options for compliance.

The latest draft, released for review this January, gets rid of PUE altogether, replacing it in the alternative compliance section with a metric that combines MLC and PLC.

As an example, here’s how Design MLC (there’s Design MLC and Annualized MLC) is calculated:

ASHRAE Design MLC formula

More details on MLC and PLC in the draft itself, which is worth reviewing and commenting on as soon as possible if your job has anything to do with data center design and operation.

The point of the alternative compliance method is to give data center operators more flexibility. If the mechanical system doesn’t meet the required MLC level, but the electrical system is so efficient that it compensates for inefficiency of the mechanical system, they can use the combined metric to comply with the standard.

Cooling Upgrade Requirements Clarified

Another complaint was that the initial draft could be interpreted in a way that would force operators to upgrade existing cooling systems.

The new standard should apply only to new data centers or data center expansions. But, if a data center expansion is getting some or all cold air from a previously existing cooling system, should that cooling system be upgraded too? That wasn’t clear in the first draft, but the current draft specifies that older mechanical systems supplying air to a new portion of a data center don’t have to be upgraded.

View the current draft of ASHRAE Standard 90.4 here. The comment period for this draft ends in less than two weeks, on March 14, so, if you have any comments, it’s time to let ASHRAE know.

by Yevgeniy Sverdlik on 

Later this month, join me and a panel of experts in a keynote session that will focus on ASHRAE 90.4 at the Data Center World Global conference in Las Vegas. The panelists are:

Don Beaty: founder of the engineering firm DLB Associates. Beaty has spent many years on ASHRAE committees dedicated to developing data center standards.

Chris Crosby: co-founder and CEO of data center developer and provider Compass Datacenters. Crosby is a data center industry veteran who was part of the team that founded Digital Realty Trust, the world’s largest wholesale data center developer and provider.

Rich Miller: founder and former editor in chief of Data Center Knowledge. Miller is currently editor in chief at Data Center Frontier, which he also founded.

Join Don Beaty, Chris Crosby, Rich Miller, myself, and 1,300 of your peers at Data Center World Global 2016, March 14-18, in Las Vegas, NV, for a real-world, “get it done” approach to converging efficiency, resiliency and agility for data center leadership in the digital enterprise. More details on the Data Center World website.

ASHRAE Learning Institute's Online Course Sched, Spring

ASHRAE Green Standard - User's Manual now available

ATLANTA – A newly published User’s Manual clarifies the intent and implementation of the building industry’s foremost green building standard by explaining its requirements and providing example applications.

The User’s Manual to ANSI/ASHRAE/USGBC/IES Standard 189.1-2014, Standard for the Design of High-Performance, Green Buildings Except Low-Rise Residential Buildings, is intended to help engineers, architects, and contractors understand and comply with the standard and to help code officials and others enforce its requirements. This new version of the User’s Manual updates the version published in 2010, which covered the content of Standard 189.1-2009.

Standard 189.1 addresses the areas of site sustainability; water-use efficiency; energy ef­ficiency; indoor environmental quality; the building’s impact on the atmosphere, materials and resources; and construction and plans for operation.  The standard is published by ASHRAE, the Illuminating Engineering Society of (IES) and the U.S. Green Building Council (USGBC). For complete information on the standard, including a readable copy, visit

“The 2014 standard updates the 2011 standard through 67 individual addenda that modified all sections of the standard,” Andrew Persily, chair of the Standard 189.1 committee, said. “The User’s Manual is vital in helping the industry understand these updated provisions, which will help further reduce building energy and environmental impacts through high performance design, construction and operation while providing indoor environments that support the activities of building occupants.”

Among the many changes in the 2014 edition of Standard 189.1 that are addressed in the User’s Manual are:

  • Change of all site requirements to mandatory, with the previous prescriptive and performance options moved to the mandatory requirements.
  • Addition of minimum storm water management requirements for minimizing the impact of storm water discharge from the project site.
  • Inclusion of new requirements for bicycle parking; preferred park for low-emission, hybrid and electric vehicles; and a predesign assessment of native and invasive plants.
  • Adjustment of minimum on-site renewable energy requirements to provide two clear prescriptive paths for including on-site renewable energy in a project.
  • Establishment of two performance options for demonstrating compliance with the minimum energy efficiency requirements.
  • Addition of lighting quality to the scope of the indoor environmental quality section as well as requirements for lighting controls in specific space types.
  • Clarification of the requirements for air sealing of filtration and air-cleaning equipment; and new requirements for preoccupancy ventilation.
  • Addition of minimum moisture control requirements to reduce negative impacts associated with unintentional moisture migration through the building envelope.
  • Addition of new requirements for multiple-attribute product declaration or certification and maximum mercury content of certain types of electric lamps.
  • Update of requirements related to the environmental impacts associated with the idling of construction vehicles and new requirements to reduce the entry of airborne contaminants associated with construction areas.


The cost of the User’s Manual is $105 ($89, ASHRAE members).

To order, visit or contact ASHRAE Customer Contact Center at 1-800-527-4723 (United States and Canada) or 404-636-8400 (worldwide) or fax 678-539-2129.

ASHRAE, founded in 1894, is a global society advancing human well-being through sustainable technology for the built environment. The Society and its more than 54,000 members worldwide focus on building systems, energy efficiency, indoor air quality, refrigeration and sustainability. Through research, standards writing, publishing, certification and continuing education, ASHRAE shapes tomorrow’s built environment today. More information can be found at