ASHRAE Journal - September 2013 - 14

Letters

constant volume its minimum is 0.6 cfm/ft2 with no
CO2 demand ventilation control?
Interestingly a chilled beam system for this building
would use a DOAS system in the range of 0.23 cfm/ft2 to
satisfy all the needs for fresh air in the building. This DOAS
flow rate meets the three requirements for outside air:
• Enough dry treated outdoor air to pressurize the
building to prevent the infiltration of warm moist outdoor air (latent cooling of outdoor air),
• Enough outdoor air for ventilation for people per
codes, and
• Enough subcooled air for latent cooling of internal
loads, mostly from people.
We at Taco have been involved in a number of chilled
beam projects around the country from Florida to
Alaska. A sampling of recent projects had DOAS airflow
ranging from 0.19 to 0.25 cfm/ft2 including our new
Innovation and Development Center at Taco, Inc., which
has a DOAS airflow of 0.22 cfm/ft2. This system has a
DOAS unit with a heat recovery wheel with VAV boxes
supplying CO2 demand controlled air to active chilled
beams.
The article is hardly an apples to apples comparison.
It is obvious that the authors are feeling threatened
by chilled beams. They make a flat statement that the
authors’ firm “has conducted detailed life-cycle cost
analysis…and has yet to come across a single case
where active chilled beam was more efficient or lower
cost.”
The authors are tilting at the wrong windmill. The system that is the biggest threat to VAV is not chilled beams,
but VRF. In a VRF system, there is no VAV system. An
optimally designed chilled beam system would include
a VAV system. The authors describe this option in their
article, a hybrid chilled beam + DOAS + VAV, but it is
mostly VAV. The combination of chilled beams and VAV is
the most efficient HVAC system on the market today.

The question on heat recovery is addressed in the article on Page 31.
DCV could have been added to the ACB+DOAS design
to reduce the ventilation penalty but this would have
required adding VAV boxes to all zones, which would have
significantly increased the first
cost. The ACB+DOAS design
ASHRAE
is already so expensive that it
JOURNAL
would take 80 years to pay for
itself even if it used no energy
at all.
Mr. Cunniff boldly states
that chilled beam + VAV is the
Comparing Performance
most efficient system on the
Active Chilled Beams + DOAS or VAV Reheat
market today. If the system
does not include a fully sized
airside economizer, then we do not believe that it would
be more efficient than a VAVR system, certainly not in
mild California climates. If it does have a full airside
economizer then you have purchased two entire HVAC
systems: a chilled beam system and a VAVR system. While
this double system should be more efficient than a VAVR
system alone, there is no way it could possibly be more
cost effective. The payback for the ACB+DOAS system
alone was 80 years if it used no energy. The ACB+VAVR
system would then require something like 100 years to
pay back if it used no energy and 200 years to pay back
given the likely energy savings.
MAY 2013

THE MAGAZINE OF HVAC&R TECHNOLOGY AND APPLICATIONS

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Whole Building Performance Method for Standard 90.1 | Rocks Don’t Burn |
Natural Ventilation in Health Care | Air-Distribution Design

MayCover.indd 1

4/22/2013 9:18:50 AM

Jeff Stein, P.E., Member ASHRAE, Alameda, Calif.

 The May 2013 article comparing a VAV system to
a chilled beam/DOAS system was thought-provoking.
However, the approach burdened the chilled beam/
DOAS system with unnecessary requirements.
It is true that a chilled beam system may need 0.4 to
0.5 cfm/ft2 for proper operation, but that does not necessarily mean the DOAS has to be sized for this. As noted
in the article, consideration could be given to “adding
Greg Cunniff, Life Member ASHRAE, Great Falls, Mont.
parallel or series fan-powered boxes to the ACB to reduce
primary air” (as footnoted to Livchak). The DOAS could
The Author Responds
We agree that the ACB+DOAS design left some room for probably be sized closer to 0.1 cfm/ft2. This would greatly
reduce its energy consumption. The total district heating
improvement but it is not accurate to call it a constant
and cooling shown on page 24 for the ACB design might
air volume reheat (CAVR) system. A CAVR design would
2 of 55°F supply air. The ACB+DOAS
go from 23 to 4.6 kBtu/ft2·year.
require 0.9 cfm/ft
As noted in the article, the cost of the chilled beam/
design included 0.5 cfm/ft2 (in the energy model) of 63°F
supply air or about one-third of the supply air capacity of DOAS system was more of a consideration than its energy
consumption. If the primary air from the DOAS were cut
CAVR.
14

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Septem ber 2013

ASHRAE Journal - September 2013

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