ASHRAE Journal - May 2013 - 26

ACB + DOAS

HVAC Cost Model

(0.5 cfm/ft2)

VAV
Reheat
50,000

Related to VAV Reheat Peak Power (%)

A major mechanical contractor used the DD level drawings
cfm
30,000*
of each design to estimate the HVAC installed costs for each
2
0.53
0.88
cfm/ft
design. As noted previously, the cost of the return ductwork on
External Static Pressure
1.8
2.3
the floors and in the return shaft in the ACB+DOAS design was
Internal Static Pressure
2.0
1.7
not included in the cost estimate because return ductwork is
Total Static Pressure
3.8
4.0
not required for the ACB+DOAS design and was not included
the other two designs. As shown in Table 2, The ACB+DOAS
Fan Efficiency
69%
69%
design costs more than twice as much as the VAVR design and
Brake Horsepower
26
44
the Hybrid Design is about half way between the other two.
*There was some confusion as to the primary airflow rate in the GSB
The ACB+DOAS design is so much more expensive that even
ACB+DOAS design. 30,000 cfm (0.5 cfm/ft2) was used in the energy
if it used no HVAC energy at all it would still take 80 years to
model but 36,000 (0.6 cfm/ft2) was used in the cost model. Had the
energy model used 0.6 cfm/ft2 then the ACB+DOAS energy perforpay back the incremental first cost compared to VAVR.
mance would have been slightly worse. Had the cost model used 0.5
The cost breakdown in Table 2 sheds some light on the cost difcfm/ft2 then the ACB+DOAS costs would have been slightly better.
ferential. The VAVR design uses 30% more sheet metal than the
In both cases the difference is most likely “in the noise.”
ACB+DOAS design, but the ACB+DOAS design uses 30 times
more chilled water piping and five times more hot water piping. Table 1: Design fan power.
One reason the VAVR hot water piping costs are so much
100
lower is because the VAV boxes are grouped close to the
90
VAV Reheat
duct mains to minimize hot water piping (Figure 5). In
80
ACB + DOAS (0.5 cfm/ft2)
the ACB+DOAS design, the hot water piping must run
ACB + DOAS (0.3 cfm/ft2)
70
out to every chilled beam. The ACB+DOAS equipment
60
cost is double the VAVR equipment cost. This is due pri50
marily to the cost of the active chilled beams and to the
40
added pumps and heat exchanger used for the secondary
30
chilled water circuit. The ACB+DOAS subcontractor cost
20
also is double the VAVR subcontractor cost. This is due to
10
the higher controls costs (e.g., more chilled water valves,
0
humidity sensors, etc.), higher insulation costs (for all that
0
10
20
30
40
50
60
70
80
90
100
extra piping) and higher test and balance costs.
Part Load Ratio (%)
Many other potential costs are not included in these
figures that could make the ACB+DOAS design even Figure 10: Supply fan power vs. part load ratio.
more expensive relative to VAVR. One
VAVR
Hybrid
ACB + DOAS
example is seismic bracing for the
chilled beams. General contractors have
Total HVAC Cost ($)
1,371,000
1,980,000
3,341,000
reported that they have been caught off
2)
25
37
62
Total HVAC Cost ($/ft
guard by the amount of seismic bracing
Parameter Comparison
required for chilled beams and the resulMaterial Cost ($)
215,179
279,703
576,496
tant impact on other utilities. Another
Labor Cost ($)
584,058
877,138
1,509,349
potential additional cost is lighting.
Equipment Cost ($)
319,695
380,297
608,118
The chilled beams take up much more
ceiling space than conventional diffusSubcontractors ($)
252,067
442,862
647,037
ers. This increases the cost of coordinaPounds of Ductwork
38,000 lb
33,224 lb
28,612 lb
tion between mechanical and electrical
Linear Feet of Chilled Water Pipe
310 ft
5,963 ft
10,244 ft
trades and potentially the cost of lightLinear Feet of Heating Hot Water
2,085 ft
2,330 ft
9,630 ft
ing fixtures.
Table 2: HVAC cost model results.

Other Factors

The VAVR system was clearly better on both energy efficiency and HVAC costs compared to the ACB+DOAS system, but
what about the other supposed advantages of ACB+DOAS?

Floor-to-Floor Height
The impact of the HVAC system on the floor-to-floor
height is primarily a function of the size of the supply mains
26

ASHRAE Journal

on the floors. The bigger the mains, the more floor-to-floor
height is required. Other HVAC components, such as VAV
boxes, rarely affect the floor-to-floor height because they are
usually smaller than the mains and can be tucked up between
beams while supply mains must run under the beams.
On a typical floor of the GSB, the ACB+DOAS design has
four supply mains each sized at 40 × 12 in. (1 × 0.30 m), for
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 - 41
ASHRAE Journal - May 2013 - 42
ASHRAE Journal - May 2013 - 43
ASHRAE Journal - May 2013 - 44
ASHRAE Journal - May 2013 - 45
ASHRAE Journal - May 2013 - PSU Design Build Project
ASHRAE Journal - May 2013 - 47
ASHRAE Journal - May 2013 - 48
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
ASHRAE Journal - May 2013 - 61
ASHRAE Journal - May 2013 - Building Sciences
ASHRAE Journal - May 2013 - 63
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ASHRAE Journal - May 2013 - 65
ASHRAE Journal - May 2013 - 66
ASHRAE Journal - May 2013 - 67
ASHRAE Journal - May 2013 - 68
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
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