ASHRAE Journal - May 2013 - 32

Conclusions
In this competition, VAVR was the clear winner versus
ACB+DOAS. VAVR had much lower ﬁrst costs, much lower
energy costs, and similar ﬂoor-to-ﬂoor heights. These conclusions strictly apply only to the analyzed systems and this building, which is in a relatively mild climate. However, in the last
ﬁve years the authors’ ﬁrm has conducted detailed life-cycle
cost analyses comparing several ACB variations to VAVR for
several buildings across the country and has yet to come across

a single case where ACB was more efﬁcient or lower cost.
The authors’ ﬁrm has also reviewed life-cycle cost analyses by
other ﬁrms that appear to have gone out of their way to make
VAVR look bad by using unnecessarily high design ﬂow rates and
static pressures, single maximum zone controls with high minimums, ﬁxed supply air temperature, etc. A common technique to
inﬂate the VAVR energy use is to make it minimally compliant
with Standard 90.1-2004 or 90.1-2007, conveniently ignoring the
facts that 90.1 has raised the bar for VAVR since 2007 and that
a good VAVR design can far exceed 90.1.
A well-designed VAVR system (including
dual maximum zone controls, supply air
temperature reset, duct static pressure reset and CO2 controls in high density spaces) is hard to beat. Many others engineers
have come to similar conclusions.9
It is possible an ACB design with low
primary airﬂow and with medium temperature chilled water and a waterside economizer might be more efﬁcient than VAVR
in some applications, such as a building
with high sensible loads in a more extreme
climate where outdoor air economizers
are not as effective. But, it is doubtful
ACB can ever compete with VAVR on a
ﬁrst cost or life-cycle cost basis. The added costs of the piping and beams for ACBs
are simply too high and well-designed
VAVRs are simply too efﬁcient.

References

www.info.hotims.com/44632-33

32

ASHRAE Journal

1. Simmonds, P. 2013. “To beam or not to
beam?” Engineered Systems (1).
2. Hastbacka, M. 2012. “Emerging Technologies: DOAS, radiant cooling revisited.”
ASHRAE Journal (12).
3. Rumsey, P. 2007. “Chilled beams in
labs: eliminating reheat & saving energy on
a budget.” ASHRAE Journal (1).
4. Weidner, S. 2009. “Cooling with less air
using underﬂoor air distribution and chilled
beams.” ASHRAE Journal (12).
5. Taylor, S., et al. 2012. “Dual maximum
VAV box control logic.” ASHRAE Journal (12).
6. Taylor, S. 2007. “Increasing efﬁciency
with VAV system static pressure setpoint
reset.” ASHRAE Journal (6).
7. ASHRAE RP 1515, Thermal and air
quality acceptability in buildings that reduce energy by reducing minimum airﬂow
from overhead diffusers. Center for the Built
Environment, Taylor Engineering LLC,
Price Industries. In progress.
8. Livchak, A. 2012. “Don’t turn active beams
into expensive diffusers.” ASHRAE Journal (4).
9. Murphy, J. 2011. “High-performance VAV
systems.” ASHRAE Journal (10).
May 2013
ASHRAE Journal - May 2013

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