ASHRAE Journal - July 2013 - 124

DATA CENTERS
disaster recovery plans, commissioning,
and maintenance practices.

Redundancy & Energy Efficiency
In addition to redundancy, there are a
number of strategies regarding whether
the redundant equipment is:
• Connected and running;
• Connected but not running; and
• Not connected but on the premises
(i.e., ready for a cold swap out).
Historically, the data center industry
tends to minimize risk and, therefore,
picked the most conservative of redundancy strategy (i.e., connected and
running). However, with an increased
focus on energy consumption, these
strategies are being revisited by some.
This is particularly true on the electrical
side since this is where the concurrent
operation of parallel paths results in the
largest energy inefﬁciencies.

Data Center Zoning
Another alternate approach to the
overall data center tiers from the Uptime Institute and the subsystem tiers
described in TIA-942 is to consider splitting the data center’s computer rooms
(also called raised ﬂoor area or white
space) into multiple zones and then to
adopt different redundancy strategies per
zone. Dividing the computer room into
many zones facilitates the planning and/
or implementation of various cooling
strategies (e.g., airﬂow pattern, cooling
source, etc.).
Figure 2 shows a simple zoning pattern, and Table 2 is an example of how
the zones from Figure 2 could be designed with variable criteria including
varying tier levels and provisions for
future tier level upgrades. This zoned
variable criteria approach also ties into
treating cooling systems more like a
service as described in the November
2012 column.

Conclusion
Data centers may require redundant
power and/or redundant cooling depending on the software applications.
Additional redundancy considerations
may apply to other building systems or
124

ASHRAE Journal

the applications themselves. In other
cases, very little to even no redundancy
is required.
The reliability of systems that are
made up of multiple components can
be increased through the addition of
redundant parallel components (eliminate any single points of failure), but
the right balance and tradeoffs need to
be determined with other considerations
(e.g., capital expenditure, energy efﬁciency, etc.).
Various industry redundancy topologies (tiers) have been deﬁned to help
quantify and compare data center resiliencies. Alternate strategies including
varying the redundancy levels through
zoning your data center computer room
space and applying different tier levels
per zone could yield better results (energy efﬁciency, total cost of ownership,
etc.) than a single topology approach.
A zoned strategy can also create greater
flexibility in future changes of the
infrastructure to better align with the
rapid change occurring on the software
applications. A more holistic approach
based on the real needs of the software
applications can ensure a better balance of costs, efﬁciency, schedule, and
availability.

Bibliography
Institute of Electrical and Electronics
Engineers. 1991. IEEE Recommended
Practice for the Design of Reliable

Z3
Figure 2: Simple data center computer
room zoning.
Industrial and Commercial Power Systems/Standard 493-1990 (IEEE Gold
Book).
Beaty, D.L. 2005. “Reliability engineering for datacom cooling systems.”
2005 ASHRAE Transactions (111):1.
Telecommunications Industry Association. 2005. ANSI/TIA-942 – Telecommunications Infrastructure Standard for
Data Centers. Arlington: Telecommunications Industry Association.
Uptime Institute. Uptime Tier Ratings.
http://uptimeinstitute.com.
Donald L. Beaty, P.E., is president of
DLB Associates Consulting Engineers,
in Eatontown, N.J. He is publications
chair of ASHRAE TC 9.9.

Topic

Zone 1

Zone 2

Zone 3

Localized Cooling

None

Rear-Door

Row-Based

Allowable Intake Class

Air

Air & Liquid

Liquid

IT Airﬂow Conﬁguration

F-R

F-t

Air Distribution

VUF

VOH

Cold Aisle Containment

Yes

Tier

III

Tier Upgradable

Yes

Cooling Source

Table 2: Variable criteria per zone.
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