ASHRAE Journal - July 2013 - 13

VAVR system is a better choice than ACB system, multiple
other detailed life-cycle cost analysis have indicated completely
different results with the ACB “winning” hands-down.
Bob Fagg, P.E., Member ASHRAE, Gaithersburg, Md.

The Author Responds
The fact that ACB systems have been deployed in Europe and
Washington D.C. does not prove that it is more efﬁcient or lower
cost than VAVR. Neither does anecdotally stating that they are a
better choice for ofﬁce buildings and most academic applications.
The analysis we presented was based on detailed designs for the
same building proposed by experienced designers with energy
costs and ﬁrst costs estimated by unbiased independent contractors. We encourage you to share the detailed, unbiased life-cycle
cost analyses that have resulted in ACB “winning” hands down.
We agree that a waterside economizer would improve the
performance of the ACB, but we are also convinced that air
economizers are more efﬁcient than waterside economizers. The
beams in the competition are designed for 57°F entering water
and 63°F leaving water. With typical tower and heat exchanger
selections, the outdoor air wet bulb would have to be about 45°F
or lower to turn off the chillers. There are only 1,000 hrs/yr
below 45oF WB in the Bay Area, and most of this is at night, so
“6,000 annual hours without chiller operation for ACBs” is not
accurate. Partial (integrated) waterside economizer precooling
requires outdoor air wet-bulb temperatures below about 52°F
which occurs about 3,500 hrs/yr in the Bay Area (again, mostly
at night). On the other hand, there are about 5,000 hrs/yr below
57°F DB in the Bay Area (full airside economizer) and over
8,000 hours below 75°F dry bulb (partial airside economizer).
Moreover, a waterside economizer requires cooling tower fans,
condenser water pumps, and chilled water pumps to run at or
near full speed in full or partial water economizer mode. Tower
fans and pumps are off in full air economizer mode and can run
at low speed in partial air economizer mode.
We also agree that a key to efﬁcient ACB design is to generate
chilled water at the warmer (57°F) temperature required by the
beams. Blending colder (44°F) supply water with return water
or via a heat exchanger saves no cooling energy at all (actually
cooling energy increases slightly due to higher pump energy).
Yet this is most commonly how we see the system designed.
The reason is that colder chilled water is needed for DOAS
dehumidiﬁcation, and providing two chiller systems with two
supply water temperatures (or DX systems for DOAS cooling)
further increases the ﬁrst costs of ACB/DOAS systems.
Speaking of ﬁrst costs, the ACB design in the competition
(without a water economizer) was so much more expensive
than the VAVR design that even if it used no energy at all it
would still have an 80-year simple payback. An ACB design
will always have a higher HVAC cost due to the cost of the
beams and the added piping. The only way an ACB design can
possibly compete on cost is if it can signiﬁcantly reduce the
ﬂoor-to-ﬂoor height. In the competition, the ACB design did
not allow a lower ﬂoor-to-ﬂoor height at all and we are skeptical
July 2013

that any ACB design will permit a signiﬁcantly lower ﬂoorto-ﬂoor height than VAVR. There are a number of techniques
such as multiple shafts, multiple takeoffs, and shallower beams
in selected locations that can be deployed to avoid raising the
ﬂoor-to-ﬂoor height in a VAVR design.
Jeff Stein, P.E., Member ASHRAE, Alameda, Calif.

Whole-Building Performance Method
Addendum bm, by backdating the baseline to a static 90.12004, accomplishes none of the authors’ stated goals in May’s
“A Stable Whole Building Performance Method for Standard
90.1.” LEED, federal tax credits, etc., use current versions. And
it will never demonstrate code compliance to Standard 90.1-2013.
The Addendum bm targeted reduction (45%) relative to
90.1-2004 is based upon a weighted average of many building
types. Backdating to an increasingly outdated standard will
not demonstrate code-minimum compliance. Mr. Rosenberg
co-authored the report on the 30% weighted average improvement of 90.1-2010 over 90.1-2004 (PNNL-20405), which shows
the wide variation in energy savings for many building types.
It may be appropriate for simplifying benchmarks (2030
Challenge), but comparison of the designed energy use of a
restaurant, hotel or ofﬁce relative to an average improvement
for all building types won’t show code compliance.
Addendum bm addresses a symptom, not the cure. It would
open a loophole for gaming tax credits and reopen the USGBC
to claims of awarding LEED credits to buildings that perform
worse than code-minimum buildings erected next door.
Mark Heizer, P.E., Member ASHRAE, Portland, Ore.

The Author Responds
Mr. Heizer is incorrect regarding the claim that “LEED,
federal tax credits, etc., use the current version.” Current federal commercial tax incentives are based on a 50% reduction
below 90.1-2001 (http://tinyurl.com/kbahjg3). Current LEED
prerequisites for new construction require a 10% reduction
below 90.1-2007.
If adopted, Addendum bm will in fact provide a path to
minimum code compliance. It seems Mr. Heizer’s issue is that
this path may not be equivalent to the prescriptive path from
an energy use or energy cost perspective. Equivalency with a
prescriptive path is not possible for a performance approach,
as the prescriptive path does not deﬁne a single level of energy
performance for each building type, but rather a wide range
of acceptable performance depending on which prescriptive
options are selected by the designer. Surely the more deﬁned
energy performance of a building design complying with Addendum bm will fall within the range of energy performance
allowed by the prescriptive path.
Mr. Heizer is correct that the goals of the Addendum bm
approach will not be realized simply by the creation of this
new approach. That will only happen if it is adopted by code
jurisdictions and beyond code programs.
Michael Rosenberg, Member ASHRAE, Eugene, Ore.
ASHRAE Journal

ASHRAE Journal - July 2013

Table of Contents for the Digital Edition of ASHRAE Journal - July 2013