ASHRAE Journal - September 2013 - 12

Letters

this document is the points cap that the LEED District
Energy supplement imposes for the Option 1 modeling
method. Experience on a recent project has brought to
my attention that the energy modeler can apply ASHRAE
Standard 90.1-2007 Addendum ai in lieu of using the
District Energy Supplement. As many energy modelers
may be aware, LEED allows design teams to apply ASHRAE
standards addenda as long as the chosen addenda
are applied uniformly across the project. In this case
Addendum ai alters the Appendix G modeling methodology to include purchased district thermal utilities.
This addendum does not include a points cap for taking
this approach and our design team received EAc1 approval
from GBCI for the full 19 points. This addendum can make
a significant difference on LEED EAc1 performance for
design teams that want to take the purchased district utilities approach. Other Standard 90.1-2007 addenda can come
in handy in other applications as well (another one of my
favorites is Standard 90.1-2007 Addendum dn, which allows
one to model heating-only systems).

The Author Responds
Mr. West is correct in that the column primarily focused on the LEED supplemental document
“Treatment of District or Campus Thermal Energy in
LEED V2 and LEED 2009 – Design & Construction.” The
column was written on a basis to provide guidance and
direction for professionals on where to look for direction in the event they found themselves in a similar
situation. However, I agree that I should have included
the information concerning the addenda to Standard
90.1-2007.
One of the biggest drawbacks of the LEED supplemental document is indeed the points cap concerning
Option 1 when the modeling scenario has the potential to gain more than 10 points for EA Credit 1. Thank
you for pointing this out for all the column readers to
understand that there is more than one direction when
dealing with central energy plants serving multiple
buildings.
Jared A. Higgins, P.E., Member ASHRAE, Lubbock, Texas

Scott P. West, P.E., Member ASHRAE, Ft. Worth, Texas

VAV Reheat Versus
Active Chilled Beams & DOAS
By Jeff Stein, P.E., Member ASHRAE; and Steven T. Taylor, P.E., Fellow ASHRAE

everal recent articles claim that dedicated outdoor air systems

Genesis of Competition
The UC Davis Medical Center Graduate Studies Building (GSB) will be a
56,500 ft2 (5249 m2) ofﬁce building.

(DOAS) plus active chilled beam (ACB) systems are superior to vari- The space program is fairly evenly split
between private ofﬁces, open ofﬁces and

able air volume reheat (VAVR) systems on energy efficiency, first cost, classroom/conference rooms. Chilled
air quality, etc.1–4 The ASHRAE Golden Gate Chapter recently decided

to hold a head-to-head competition to put these claims to the test.
Three mechanical engineering ﬁrms
with ofﬁces in the Bay Area provided
a Design Development (DD) level design for a real ofﬁce building currently
in design, the UC Davis Medical Center Graduate Studies Building (GSB)
in Davis, Calif. One ﬁrm designed an
ACB+DOAS system, another ﬁrm designed a VAVR system, and the third
designed a hybrid combination of these
two systems. A fourth engineering ﬁrm
simulated each of the three designs using the EnergyPlus energy simulation
program. Finally, a major mechanical
contractor provided a detailed HVAC
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ASHRAE Journal

construction cost estimate for each design.
The VAV reheat system had the lowest ﬁrst costs and the lowest energy
costs of the three systems. The analysis showed that many of the supposed
advantages of ACB+DOAS relative to
VAVR, such as improved indoor air
quality and a lower ﬂoor/ﬂoor height,
also turned out to be largely overstated.
Note that the results of this analysis are
only strictly applicable to these three
designs and this building and climate,
but the conclusions also may apply
more broadly.

water and hot water will be provided by
the campus central plant.
When UC Davis ﬁrst decided to
build the GSB, it started with a traditional plans/speciﬁcations approach
with Firm A as the engineer-of-record.
Firm A, which has designed more than
1 million ft2 (92 900 m2) of chilled
beam buildings, chose an ACB+DOAS
design. In early 2012, when the design
was in the design development (DD)
stage, the owner decided to switch
About the Authors
Jeff Stein, P.E., and Steven T. Taylor, P.E., are
principals of Taylor Engineering in Alameda, Calif.
Stein is a consultant to SSPC 90.1 and a member
of SPCs 155P and 195. Taylor is a member and
former chair of SSPC 90.1 and is vice chair of TC
4.3, Ventilation Requirements and Infiltration.

ashrae.org

May 2013

to a design/build approach, and Firm
A’s design became the bridging documents.
One of the design/build teams bidding on the project proposed a VAVR
system (designed by Firm B, the authors’ ﬁrm) and also carried the design through about the DD level in its
bid. Another bidder proposed a hybrid
chilled beam + VAV reheat system, designed by Firm C. The three designs
included equipment schedules, detailed
equipment layouts and zoning plans.
The team with the hybrid design was
awarded the job, but the project has
been placed on hold since.
After UC Davis selected a design/build
team, the ASHRAE Golden Gate chapter
decided to use this building for a competition between chilled beams and VAV reheat
since it had already been designed with
both systems and with a hybrid combination of the two. All three ﬁrms were eager to
participate. The results of the competition
were presented at a seminar sponsored by
the ASHRAE Golden Gate Chapter at the
Paciﬁc Gas & Electric Energy Center in
San Francisco on Oct. 17, 2012.

Figure 1: Rendering of graduate studies building.

HW
Coil

VFD
CHW
Supply
Coil with
Fan
Bypass
Array

VFD
Exhaust

CH
Beam

Active Chilled Beam Design
The ACB+DOAS design consists of
a 100% outside air, constant volume air
handler serving two supply risers. The
air handler has a chilled water coil with a Figure 2: Active chilled beam design.
bypass damper and a hot water coil. The
air handler has a variable speed drive primarily for reducing cludes one set of chilled water pipes supplying 45°F (7°C)
fan speed when the bypass damper is open. The air handler chilled water to the air handler and a separate chilled water
provides the primary air to the active chilled beams. The aver- heat exchanger and chilled water pump in the building to
age DOAS primary airﬂow rate is about 0.6 cfm/ft2 (0.28 L/ maintain the chilled water supply temperature to the chilled
[s·m2]), which is considerably higher than the minimum ven- beams at 57°F (14°C). The design engineer felt that a heat
tilation in low occupant density spaces, like ofﬁces, but close exchanger was needed, rather than just a blending valve, as an
to the minimum ventilation in higher density spaces such as added layer of protection against condensation.
Each ACB has both heating and cooling coils. The primary
conference rooms and classrooms. A DOAS ﬂow rate higher
than the minimum ventilation in low density spaces is often air is maintained at 63°F (17°C), and the ACB heating and
needed to meet the space loads as the capacity of the chilled cooling coils are controlled by the thermostat to maintain
beams is a function of the primary airﬂow rates. In densely space conditions.
The ACB design also includes a partially ducted exhaust
occupied spaces, it also ensures space dew point does not rise
above the surface temperature of the chilled beams, possibly system—exhaust ducts extend from the air handler down the
causing condensation. It also improves the indoor air quality shafts and into the ceiling return plenum about two thirds of the
compared to code minimum ventilation.
way from the exhaust shafts to the building skin. This was in
Hot water and chilled water are provided by an existing response to perceived owner preference for ducted return. Howcampus central plant supplying 45°F (7°C) chilled water to ever, neither of the other two designs in the design/build compethe buildings on campus. However, 45°F (7°C) typically can- tition included ducted return and, therefore, the return ductwork
not be supplied to the chilled beams because of the likelihood was not included for any design in the cost model or energy
of condensation and dripping. Therefore, the ACB design in- model for the Golden Gate ASHRAE Chapter competition.
May 2013

ASHRAE Journal

4/22/2013 11:27:01 AM

018-032_Stein.indd 19
4/22/2013 11:26:59 AM

018-032_Stein.indd 18

VAV Reheat vs. Chilled Beams
The May article “VAV Reheat Versus Active Chilled Beams
& DOAS” was certainly interesting. However, some misconceptions in the article should be cleared up.
The article purports to compare the energy consumption of a VAV system to an active chilled beam system.
What it really compared was the energy consumption of
a modern VAV system to an old fashioned 1960s constant
volume reheat system. The only difference was that the
reheat coil is not duct mounted, but in the chilled beam.
This is hardly a contest. I would certainly hope a VAV
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ASHRAE JouRnAl

ashrae.org

Septem ber 2013

system is more efficient than a constant volume reheat
system. Constant volume reheat systems fell out of favor
in the 1970s when Tempmaster and Trane introduced
the more efficient VAV system.
To add insult to injury the chilled beam system was
modeled with all the DOAS air as 100% fresh air to be
heated and cooled without any heat recovery. I am not
sure what the reasoning to this is?
The VAV system was sized at 0.9 cfm/ft2 of total airflow
with fresh air in the range of 0.15 cfm/ft2 at design. The
chilled beam + DOAS unit was sized at 0.6 cfm/ft2 of
100% fresh air. A third system was modeled as a hybrid
VAV chilled beam. This was sized at 0.7 cfm/ft2. The article does not say how much of that was fresh air, but I am
assuming it is in the range of 0.15 cfm/ft2 at design, the
same as the VAV system.
The article states that the reason the DOAS unit for
the chilled beam system was sized at 0.6 cfm/ft2 was
that this “improves the code minimum ventilation,”
yet the modeling of the VAV system assumed that the
interior VAV boxes’ minimum was zero? The exterior zones for the VAV system were modeled with CO2
demand ventilation control with the boxes’ minimum
at 0.15 cfm/ft2. Since the DOAS unit was modeled as

ASHRAE Journal - September 2013

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