ASHRAE Journal - May 2013 - 24

ules for office occupancy call for a nearly constant occupancy
schedule of 95% and a nearly constant lights and receptacles
schedule of 90%. ASHRAE Research Project 15157 suggests
that realistic schedules are smaller than this and that a realistic
annual average part load ratio for office buildings is closer to
40% than 60%.7 At 40% part load ratio, the 0.5 cfm/ft2 (0.24
L/[s·m2]) ACB+DOAS design uses six times as much fan energy as the VAVR design and the 0.3 cfm/ft2 (0.14 L/[s·m2])
ACB+DOAS design uses four times as much.

Chilled Water

Chilled Beam

Chilled Beam

Chilled Beam

Cooling Energy

Heating Energy
Similar to cooling energy, the ACB+DOAS design has to
use more heating energy than the VAVR design because the
zone reheat load is higher, as described previously, and because the outside air heating load is higher. In the winter, the
ACB+DOAS design must heat 0.5 cfm/ft2 (0.24 L/[s·m2]) of
outside air while the VAVR design only has to heat 0.15 cfm/
ft2 (0.07 L/[s·m2]) of outside air, three times as much outside
air load.
Also, as with the cooling loads, the heating loads are met
by the campus central plant, so the heating efficiency is the
24

ASHRAE Journal

Heating
Hot Water

VAV Box

Figure 7: Hybrid zone.

Figure 8: 3-D image from EnergyPlus model.
30
25

Pumps
Fans
Heating
District Cooling

0.1
2.3

20
kBtu/ft2·Year

In non-economizer conditions (outside air temperature >
return air temperature), the ACB+DOAS design has a higher
cooling load than the VAVR design because it has a higher
outside air load (0.5 cfm/ft2 [0.24 L/(s·m2)]) compared to
0.15 cfm/ft2 [0.07 L/[s·m2]). In economizer conditions, the
ACB+DOAS design also has higher mechanical cooling loads
because it does not have an air economizer while the VAVR
design does have an air economizer and thus benefits from
economizer free cooling. The ACB+DOAS design also has
higher reheat/recool loads than the VAVR design because
the fixed primary airflow rate (0.5 cfm/ft2 [0.24 L/(s·m2)])
and fixed supply air temperature (63°F [17°C]) mean the
ACB+DOAS design is always providing about 6 Btu/h·ft2
(19 W/m2) of cooling even when the actual load is lower. The
VAVR design, on the other hand, provides no more than about
3 Btu/h·ft2 (9 W/m2) at minimum flow, even if the supply air
temperature is fixed at its minimum of 55°F (13°C). Therefore, the ACB+DOAS design has a higher cooling load under
all conditions compared to the VAVR design.
In this building, all mechanical cooling loads are met by
the campus central plant, so the cooling efficiency is the same
for all three designs. Therefore, the ACB+DOAS design must
have higher cooling energy than the VAVR design; the loads
are higher and the cooling plant efficiency is the same (or
worse when pump energy is included). To take advantage of
the warmer chilled water supply temperature required by the
chilled beams, a medium temperature chilled water plant is
required, preferably with an integrated water-side economizer.
One lesson learned from this analysis is that connecting to a
45°F (7°C) central plant loses all cooling efficiency benefits of
medium temperature systems such as chilled beams or radiant
cooling.

5.1
0.1
0.7
2.6

15
10

0
1
1.5

17.9
13.6

12.8

5
0
ACB Design

Hybrid ACB/VAVR

VAV Reheat

Figure 9: Simulated HVAC energy use intensities (site energy).
same for all three designs. Therefore, it is not surprising that
the ACB+DOAS model uses more than twice as much heating
energy as the VAVR model. To take advantage of the reduced
temperature hot water required by the chilled beams in heating
mode, a condensing boiler plant is required. As with cooling,
connecting to a conventional central plant loses all heating efficiency benefits of chilled beams. Note that the VAVR design
and the hybrid design can also take advantage of condensing
boilers, but not to the extent of the ACB+DOAS design, because they have smaller hot water coils.
ashrae.org

May 2013



ASHRAE Journal - May 2013

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

ASHRAE Journal - May 2013
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
VAV Reheat Versus Active Chilled Beams & DOAS
A Stable Whole Building Performance Method for Standard 90.1
Technology Award Case Studies:
PSU Design Build Project
Passive Cooling for School
Standing Columns
Building Sciences
InfoCenter
Refrigeration Applications
IAQ Applications
Engineer's Notebook
Products
Data Centers
Emerging Technologies
Classified Advertising
Advertisers Index
ASHRAE Journal - May 2013 - ASHRAE Journal - May 2013
ASHRAE Journal - May 2013 - Cover2
ASHRAE Journal - May 2013 - 1
ASHRAE Journal - May 2013 - 2
ASHRAE Journal - May 2013 - Contents
ASHRAE Journal - May 2013 - Commentary
ASHRAE Journal - May 2013 - 5
ASHRAE Journal - May 2013 - Industry News
ASHRAE Journal - May 2013 - 7
ASHRAE Journal - May 2013 - 8
ASHRAE Journal - May 2013 - 9
ASHRAE Journal - May 2013 - 10
ASHRAE Journal - May 2013 - 11
ASHRAE Journal - May 2013 - 12
ASHRAE Journal - May 2013 - 13
ASHRAE Journal - May 2013 - Letters
ASHRAE Journal - May 2013 - 15
ASHRAE Journal - May 2013 - Meetings and Shows
ASHRAE Journal - May 2013 - 17
ASHRAE Journal - May 2013 - VAV Reheat Versus Active Chilled Beams & DOAS
ASHRAE Journal - May 2013 - 19
ASHRAE Journal - May 2013 - 20
ASHRAE Journal - May 2013 - 21
ASHRAE Journal - May 2013 - 22
ASHRAE Journal - May 2013 - 23
ASHRAE Journal - May 2013 - 24
ASHRAE Journal - May 2013 - 25
ASHRAE Journal - May 2013 - 26
ASHRAE Journal - May 2013 - 27
ASHRAE Journal - May 2013 - 28
ASHRAE Journal - May 2013 - 29
ASHRAE Journal - May 2013 - 30
ASHRAE Journal - May 2013 - 31
ASHRAE Journal - May 2013 - 32
ASHRAE Journal - May 2013 - A Stable Whole Building Performance Method for Standard 90.1
ASHRAE Journal - May 2013 - 34
ASHRAE Journal - May 2013 - 35
ASHRAE Journal - May 2013 - 36
ASHRAE Journal - May 2013 - 37
ASHRAE Journal - May 2013 - 38
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ASHRAE Journal - May 2013 - 44
ASHRAE Journal - May 2013 - 45
ASHRAE Journal - May 2013 - PSU Design Build Project
ASHRAE Journal - May 2013 - 47
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ASHRAE Journal - May 2013 - 49
ASHRAE Journal - May 2013 - 50
ASHRAE Journal - May 2013 - 51
ASHRAE Journal - May 2013 - 52
ASHRAE Journal - May 2013 - 53
ASHRAE Journal - May 2013 - Passive Cooling for School
ASHRAE Journal - May 2013 - 55
ASHRAE Journal - May 2013 - 56
ASHRAE Journal - May 2013 - 57
ASHRAE Journal - May 2013 - 58
ASHRAE Journal - May 2013 - 59
ASHRAE Journal - May 2013 - 60
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ASHRAE Journal - May 2013 - Building Sciences
ASHRAE Journal - May 2013 - 63
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ASHRAE Journal - May 2013 - 69
ASHRAE Journal - May 2013 - InfoCenter
ASHRAE Journal - May 2013 - 71
ASHRAE Journal - May 2013 - 72
ASHRAE Journal - May 2013 - 73
ASHRAE Journal - May 2013 - 74
ASHRAE Journal - May 2013 - Refrigeration Applications
ASHRAE Journal - May 2013 - 76
ASHRAE Journal - May 2013 - 77
ASHRAE Journal - May 2013 - IAQ Applications
ASHRAE Journal - May 2013 - 79
ASHRAE Journal - May 2013 - 80
ASHRAE Journal - May 2013 - 81
ASHRAE Journal - May 2013 - 82
ASHRAE Journal - May 2013 - 83
ASHRAE Journal - May 2013 - Engineer's Notebook
ASHRAE Journal - May 2013 - 85
ASHRAE Journal - May 2013 - Products
ASHRAE Journal - May 2013 - 87
ASHRAE Journal - May 2013 - Data Centers
ASHRAE Journal - May 2013 - 89
ASHRAE Journal - May 2013 - 90
ASHRAE Journal - May 2013 - 91
ASHRAE Journal - May 2013 - Emerging Technologies
ASHRAE Journal - May 2013 - 93
ASHRAE Journal - May 2013 - 94
ASHRAE Journal - May 2013 - Classified Advertising
ASHRAE Journal - May 2013 - Advertisers Index
ASHRAE Journal - May 2013 - Cover3
ASHRAE Journal - May 2013 - Cover4
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