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ASHRAE draft standard specifies required PUE efficiency

Cooling body courts controversy with 90.4P standard, by specifying PUE levels

ASHRAE, the professional body for cooling engineers, is working on a standard for data center efficiency, which could create controversy in the industry. 

The new standard includes recommendations for the maximum power usage effectiveness (PUE) that should be achieved by data centers in a given location (see table below), but some in the data center industry believe the body is being too prescriptive, in an echo of the criticism which surrounded ASHRAE’s previous foray into this field, in 2010. 

Emerson Liebert EFC free cooling unit

Source: Emerson Network Power

Mandatory PUE?

The ASHRAE 90.4 Energy Standard for Data Centers (draft published here) standard has been in prospect for some years from the American Society of Heating Refrigeration and Airconditioning Engineers (ASHRAE).

It was first announced in Feburary 2015, and a further draft is now available for comment. 

ASHRAE  is the source of the 90.1 standard which applies to energy efficiency in buildings generally, andwhich is widely referred to in building regulations.

That standard contains an addendum covering data centers, which caused controversy over fears that it might mandate a particular type of cooling technology (the “economizer”), although ASHRAE maintained that innovation would be allowed by facilities meeting the standard. 

Since then, the group  has decided that a separate building standard document is needed - and once again raised fears that, it will be too prescriptive. In particular, mandating specific PUE scores may not be popular.  

“You shouldn’t use formal standards in a rapidly growing, fast-moving industry,” said Don Beaty of DLB Associates, a past chair of ASHRAE Technical Committee 9.9 (TC9.9) for data center technology, according to a report by Rich Miller on Data Center Frontier.

The standard includes sections covering the building itself; its heating, ventilation and air conditioning; its electrical systems; its water heating; its lighting; and its other systems. Most of these simply refer to the existing demands of the 90.1 standard, but the electrical and heating and ventilation systems have more detail. 

Within the secton for heating, ventilation and air conditioning, there are two “compliance paths”. One is based on the mechanical load component (MLC), defined in the standard and calculated from the specification and/or performance of the climate control equipment in the facility. There is an alternative, based on the data center achieving better than a specified PUE, according to its location.

Climate zone tables listing most countries are available here, the zones within the US are listed here, and on a map here (and elsewhere). As examples, Singapore would be in 1A, the US spans from 1 to 8, the UK is in 4 and 5. 

Comments on the draft are still open and  can be made here

Power Usage Effectiveness (Design PUE) Maximum
Climate Zone Design PUE
1A 1.61
2A 1.49
3A 1.41
4A 1.36
5A 1.36
6A 1.34
1B 1.53
2B 1.45
3B 1.42
4B 1.38
5B 1.33
6B 1.33
3C 1.39
4C 1.38
5C 1.36
7 1.32
8 1.30

Readers' comments (5)

  • 'mandate a particular type of cooling technology (the “eocnomizer”), which ASHRAE eventually pacified.' - economizer misspelled - and what does 'pacified' mean in this case?
    Next paragraph - 'mandating specific PUE scores maqy' may?
    Two paragraphs later has a full stop where it shouldn't, so should read 'The standard includes sections covering the building itself - its heating, ventilation and air conditioning; its electrical systems; its water heating; its lighting; and its other systems.' :)

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  • Thanks Paul. We seem to have crowdsourced the proofreading on this one.


  • The Climate Zones don't appear to include the regions within the United States.

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  • You are right. I'll add a link to US zones.


  • The PUE mandates must be PUEe (electrical) otherwise evaporative cooling systems will be excluded as PUE (revised) accounts for water evaporative energy and standby generator energy (fuel). Direct evaporative cooled facilities with SGE limitted to 100 hours operation should be able to achieve PUE in the range of 1.4 to 1.5. Indirect evaporative cooled facilities will be lucky to make a PUE of 2.0.

    ASHRAE appears to be deviating from the true definition of PUE. Suggest they get in-synch with The Green Grid on proper use of PUE especially as PUE is subject to on-going revisions to further accommodate new energy sources / forms within the data center eco-system.

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  • From my point of view this is a "nice" thing for targeting the renewal of old DC site. For new DC sites it will be definitely not enough to get away from DCs a an energy hog.
    We keep our old DC site (est. 1992) from 2,1 down to 1,35 (TR3++, 5 MW) but this is currently not the end (target 1,25).
    If you really want to get an DC site which is prepared for a future demand, you need a complete new design and technic. Double layer raised floor and CRAC systems exists since the early industrial time. From my point of view this is not the right thing to cool a actual DC even you put it into fancy pants (f.e. cold aisle).
    Getting under 1,2 with a TR3+ site might be a challenge from this point of view. I claim throw a lot of this old things over board. The building currently is only used as a physical shelter (same for modular boxes). If you want really lower the PUE you have to think in a different way.

    A DC is one big machine powered by electrical energy. Any thing in desing of one component has an impact on all others. Some more others less. The secret of success in this case is to find out which component has the main influence on your machine. Nobody will desingn a ship or a plane as a cube so why a DC?

    That should be something DC "designers" have to think about.


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  • The design PUE for zone "1", based on what type of cooling?

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  • I think the idea was to use whatever cooling would achieve that PUE.

    However, the idea of required PUE levels has now been abandoned in the published standard,

    Peter Judge

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