-Monday's story updated with Tuesday response from ASHRAE-

Six data center heavyweights have signed an open letter, objecting to a proposal by an air conditioning industry organization to define the newest set of data center energy efficiency standards in terms of the types of equipment or design methods used to cool IT equipment as opposed to looking at actual energy efficiency achieved.

"We believe that for data centers, where the energy used to perform a function (e.g., cooling) is easily measured, efficiency standards should be performance-based, not prescriptive," read the statement, posted on one of the official blogs of Google, one of the letter's signees. "In other words, the standard should set the required efficiency without prescribing the specific technologies to accomplish that goal."

Authors of the letter to the American Society of Heating, Refrigerating and Air-Conditioning Engineers, including top data center executives from Digital Realty Trust, Google, Amazon and Microsoft, among others, took special issue with a proposed requirement that an efficient cooling system must use airside economizers.

While recognizing the energy efficiency benefits of using free cooling, the six executives said deploying one did not guarantee an efficient system.

"Future cooling methods may achieve the same or better results without the use of economizers altogether.

An efficiency standard should not prohibit such innovation."

In its response, ASHRAE said the proposed changes would not preclude data center designs without the use of economizers.

"The addendum includes eight exceptions to requirements for the use of economizers in data centers," ASHRAE representatives wrote in an email.

"The addendum does not change the portion of the standard that already allows, through the Energy Cost Budget method (an alternate method of compliance), for data centers to be designed without economizers if other energy saving methodologies, including power usage effectiveness (PUE), are employed."

The industry letter's authors pointed out an existing metric for data center energy efficiency that has recently been given official status by industry groups and government agencies in the US, Europe and Japan: Power Usage Effectiveness.

The US Environmental Protection Agency already uses the metric, developed by The Green Grid, as the basis for its Energy Star rating program for data centers.

The set of standards in question is ASHRAE Standard 90.1 - a standard that defines energy efficiency for different types of buildings.

Data centers were recently included in this standard.

The letter was signed by Chris Crosby, SVP at DRT, and Hossein Fateh, president and CEO at DuPont Fabros.

Both companies are two of the world's largest data center developers and wholesale colocation providers.

The other signees were James Hamilton, VP and engineer at Amazon, Urs Hoelzle, SVP of operations at Google, Mike Manos, VP of service operations at Nokia and Kevin Timmons, GM of data center services at Microsoft.