TECHNICAL FEATURE
This article was published in ASHRAE Journal, February 2022. Copyright 2022 ASHRAE. Posted at www.ashrae.org. This article may not be copied and/or distributed
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Air-Cooled Chillers
Chilled-Water VAV
System Configuration
And Design
BY NABIL NASSIF, PH.D., P.E., MEMBER ASHRAE
The air-handling unit (AHU) of a typical variable air volume (VAV) system includes
two coils connected to two separate water loops. One coil, the cooling coil, is
connected to cooling sources through a chilled-water loop. The other coil, the heating coil, is connected to heating sources through a hot water loop. This separate water
loop configuration eliminates the opportunity to use both already-installed piping
systems and coils during full cooling or heating seasons and reduces both initial and
operating cost savings. This article proposes two possible improvements with several
configuration options for chilled-water VAV systems with air-cooled chillers.
Typical Chilled-Water VAV System
Figure 1 shows typical chilled and hot water loop configurations. Figure 1a shows the heating coil (HC) connected to the hot water loop and Figure 1b shows the
cooling coil (CC) connected to the chilled-water loop.
Pre-reheat coils may be also added to the hot water loop.
The hot water loop also serves VAV box reheat coils. For
clarity and discussion, an example will be presented on
all figures. It is assumed to have two 5 MBtu/h (1.5 MW)
boilers (B1 and B2) and two 500 ton (1.8 MW) chillers
(Ch1 and Ch2) required to serve multiple AHUs and
zone VAV boxes. One of the AHUs provides 10,000 cfm
(4719 L/s) at design cooling and heating loads. The
design heating and cooling performance data across the
coils, chillers and boilers are also illustrated in Figure 1.
This traditional configuration will be replaced by several
proposed options as discussed below using the same
example numbers for comparison.
Option 1: One Common Coil
In this case, one common coil (Cl) in each AHU will
be used instead of two coils, as shown in Figure 2. This
single two-pipe coil will be used either for cooling or
heating as needed. The water loop connected to the
common coil (Cl), called Loop 1, contains both heating
and cooling sources, as shown in Figure 2b and Figure 2d.
Nabil Nassif, Ph.D., P.E., is an associate professor of architectural engineering at the University of Cincinnati.
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TECHNICAL FEATURE
400 gpm
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70 gpm
28 gpm
70 gpm
800 gpm
800 gpm
800 gpm
400 gpm
400 gpm
800 gpm
28 gpm
400 gpm
In typical VAV systems, no need exists
FIGURE 1 Typical chilled and hot water configurations.
for mechanical cooling at a relatively
Design Heating Conditions
Design Cooling Conditions
A.
B.
low outside air temperature (OAT)
115°F
57°F
(e.g., 50°F [10°C]) when the system is
Chilled Water Loop
Hot Water Loop
Air
in full free-cooling economizer mode.
B1
B2
Ch1 Ch2
Air
DB = 38°F
CC
Air
At this temperature, there is also no
HC
5
5
500
500
DB
=
85°F
10,000 cfm
DB = 70°F
Air
heating provided by the heating coil
MBtu/h MBtu/h
Tons Tons
DP = 65°F
352 kBtu/h
DB
=
55°F
10,000
cfm
due to warm air recirculation. Thus,
DP
=
54°F
1,600
gpm
800 gpm
the same coil can be used for heating
43.2
Tons
42°F
140°F
at a lower OAT and for cooling at an
To Other AHU
To Other AHU Coils
elevated OAT.
Coils
And VAV Boxes
The second water loop, called
Loop 2, connects the VAV boxes to
FIGURE 2 Option 1 Configuration. One common coil in each AHU.
heating sources, and it is always
Design Cooling Conditions
A.
B.
available in heating conditions.
57°F
Figure 2a and Figure 2b shows the
Cooling
design cooling performance, and
Air
Cl
Figure 2c and Figure 2d shows the
DB = 85°F
B1
Ch1 Ch2 B2
Air
DP
=
65°F
design heating performance. In
Off
On On
Off
DB
=
55°F
10,000 cfm
heating mode, Cl raises the supply
DP = 54°F
Loop 2
Loop 1
air temperature (SAT) to a certain
43.2 Tons
Off
42°F
value, for instance 70°F (21°C), and
1,600 gpm
To Other AHU Coils
VAV Boxes
the VAV box reheat coil adds additional heat, for instance, to 90°F
Design Heating Conditions
C.
D.
(32°C). Thus, the boiler can serve
85°F
115°F
hot water to the Cls at a lower temHeating
Heating
perature (e.g., 110°F [43.3°C]) and
Air
CI
thereby operates at a higher effiB1
Ch1 Ch2 B2
DB = 38°F
ciency compared to a traditional
On
Off
Off
On
Air
10,000 cfm
DB = 70°F
system. Coil Cl can be designed and
Loop 2
Loop 1
352 kBtu/h
selected for cooling (typically with
110°F
four to eight rows), as the heating
140°F
To Other AHU Coils
will be done mostly by the Loop 2
VAV Boxes
local reheat VAV boxes.
Let us now look at the control. When
the OAT drops below 45°F (7.2°C), for instance, Loop 1
in each AHU should be available in heating or coolswitches from cooling to heating, so the cooling sources
ing, accordingly, which is based on the OAT. The SAT
should be locked off. When the OAT increases above 50°F resetting algorithm can also be identical to the one in
(10°C), the loop will switch from heating to cooling so
a traditional VAV system.1 However, during the heatthe heating sources should be locked off. Five degrees
ing period, the supply air temperature setpoint may
is a dead band to avoid cycling between cooling and
be adjusted upward or downward to split equally the
heating.
heating loads between Loop 1 and Loop 2 in a way that
The supply air temperature control loop could be
the heating load will be shifted from Cl to the VAV
identical to when two coils are used in a traditional
box or vice versa. Loop 1 serving the VAV box provides
VAV system as shown in Figure 3. The only difference is
always hot water to VAV box coils for heating whenever
that the same coil is controlled in cooling or heating
needed as in a traditional VAV system.
mode, and associated Loop 1 serving this single coil
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TECHNICAL FEATURE
Option 2:
One Common Coil With Heat Pumps
FIGURE 3 Supply Air Temperature Control for Option 1 and Option 2. Single coil with and without heat pumps.
OAT = 45°F
OAT = 50°F
Water Loop 1 in Heating
Water Loop 1 in Cooling
100
Valve Position (%)
The main advantage of this configuration is the use of one coil instead
of two coils in each AHU, leading to
smaller AHU size, less piping and
control valves and lower fan energy
use due to reduced flow resistance.
This configuration also eliminates the
need for a preheat coil.
Free Cooling
Economizer
Damper Control
CI
0
0
100
Control Signal
CI
200
300
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28 gpm
400 gpm
800 gpm
70 gpm
Some or all air-cooled chillers and
FIGURE 4 Option 2 Configuration. One common coil with heat pumps.
boilers in Option 1 can be replaced
with air-source heat pumps (HPs).
Design Cooling Conditions
B.
A.
Figure 4 shows two chillers replaced
57°F
Cooling
by heat pumps of the same capacity.
Air
Figure 4a and Figure 4b show the design
CI
DB = 85°F
B1
HP1 HP2
cooling performance, and Figure 4c
Air
DP = 65°F
Off
On
On
DB = 55°F
and Figure 4d show the design heat10,000 cfm
DP = 54°F
Loop 2
Loop 1
ing performance. This configuration
Off
43.2 Tons
holds the same advantages of Option 1
42°F
but with fewer boilers to be installed. VAV Boxes
3,200 gpm
To Other AHU Coils
This option reduces burning fuel at
the site and provides a pathway to
Design Heating Conditions
C.
D.
the future trend in electrifying and
115°F
85°F
decarbonizing buildings. The coil
Heating
Heating
design and control are identical to
HP2
Air
CI
B1
HP1 (May Be
Option 1 discussed earlier and shown
DB
=
38°F
Dual-Fuel
Air
On
On
in Figure 3.
10,000 cfm
HP)
DB = 70°F
Loop 2
In cold climates when the air-source
Loop 1
352 kBtu/h
400 gpm
heat pump efficiency reduces sig140°F
110°F
nificantly, dual-fuel heat pumps can
VAV Boxes
400 gpm
To Other AHU Coils
be used. In addition, at lower OATs,
when the heating load is too high and
when the SAT is maintained by the heating coil rather
any of those coils in heating or cooling modes. To achieve
than the economizer dampers, the SAT setpoint can be
that, each water loop contains both heating and cooling
reset to a lower value (e.g., 60°F [16°C] or even lower) to
sources as shown in Figure 5 and Figure 6. Loop 1 serves
shift the heating to the local reheats served by the boiler the first AHU coil (called Cl1), one chiller and one boiler.
and consequently reduce the load on HPs when they
Loop 2 serves the second coil (Cl2) and contains one
are not that efficient. This will also reduce the load on
chiller and one boiler. The zone VAV boxes will be conthe coil (Cl) that is designed and selected for the cooling
nected to Loop 1.
season.
The Cl1 coil can be designed for heating with two
rows, as an example, and the Cl2 coil can be designed
Option 3: Two Coils
for cooling with four to eight rows. Cl1 will provide the
Now let us examine a different scenario when two coils second stage of cooling during warm weather when no
in each AHU and two separate water loops are installed
heating is needed. Cl2 will provide the second stage of
as in a traditional system but with the possibility of using
TECHNICAL FEATURE
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35 gpm
HC
5.2 gpm
400 gpm
400 gpm
400 gpm
400 gpm
22.8 gpm
35 gpm
CC
heating during cold weather when no
FIGURE 5 Typical Air-Handling Unit AHU vs. Option 3 or Option 4. Two coils.
mechanical cooling is needed as disExhaust Air (EA)
Return Air (RA)
Exhaust Air (EA)
Return Air (RA)
cussed later in Figure 7.
Many advantages result from this
CI1 CI2
Supply Air
Supply
(SA)
configuration, but all depend on how
Air (SA)
Outdoor
Outdoor Mixed
Mixed
the coils are selected. If the sizes of
Air (OA)
Air (OA) Air (MA)
Supply Fan
Supply Fan
Air (MA)
coils and water loops are kept the
VAV Boxes
VAV Boxes
Water Loop 1 Water Loop 2
Chilled Water Loop Hot Water Loop
same as in a traditional system, this
B. Typical AHU
A. Option 3 or Option 4
configuration can reduce pump
energy uses as chilled or hot water
will split between the two coils during FIGURE 6 Option 3 Configuration. Two coils.
full cooling or full heating periods. It
Design Cooling Conditions
B.
A.
can enhance heat transfer between
57°F
67°F
water and air as more surface areas
Cooling
Cooling
Air
(both coils instead of just one coil) are DB = 63°F
Air
Air
CI1
B1 Ch1
Ch2 B2
used. However, to reduce the size of
DP = 63°F CI2
DB = 85°F
DB = 55°F
Air
Off
On
On
Off
DP = 65°F
DP = 54°F
AHU and water loop pipes, combined
DB = 63°F
10,000
cfm
21.6 Tons
coils and associated water loops can
DP = 63°F
Loop 2
Loop 1
400 gpm
21.6 Tons
be designed to meet the peak cooling
52°F
42°F
loads and the peak heating loads, cut400 gpm
VAV Boxes
ting the size of those coils and piping
diameters compared to a traditional
Design Heating Conditions
C.
D.
system.
115°F
85°F
Heating
Heating
Another advantage is related to
Air
DB = 44°F
the opportunity to run the chiller
Air
CI1
B1 Ch1
Ch2 B2
CI2
10,000
cfm
DB
=
38°F
Air
in Loop 1 serving Cl1 at an elevated
On
Off
Off On
Air
10,000 cfm
DB = 70°F
chilled-water supply temperature
DB = 44°F
286 kBtu/h
Loop 1
(e.g., 52°F [11°C]). Looking at the
66 kBtu/h
Loop 2
800 gpm
400
gpm
example in Figure 6b, the first coil
110°F
140°F
(Cl1), which is typically designed for
VAV Boxes
heating with two rows, cools the air
to an elevated value—for instance
to 63°F (17°C)—and the chillers in Loop 1 can produce
be locked off, and Loop 1 will be available in heata higher supply water temperature of 52°F (11°C) and
ing so the VAV box reheat coil can provide heating if
operate more efficiently than the chillers in traditional
needed. Loop 2 continues providing cooling through
configurations.
Cl2 if needed. When the OAT drops below 40°F (4.4°C)
A similar scenario for heating can be obtained as
(adjustable) so no mechanical cooling is needed, Loop
shown in Figure 6d: Cl1 heats the air to 44°F (6.7°C) for
2 and Cl2 become available for heating. The chillexample, the boilers in Loop 1 can operate at lower supers in Loop 2 should be also locked off. When the
ply water temperature (e.g., 110°F [43.3°C]), but this
OAT increases above 45°F(7.2°C) (adjustable), Loop 2
is possible only if the VAV box is designed for this low
and Cl2 become available for cooling. When the OAT
temperature.
increases above 70°F(21°C) (adjustable), Loop 1 and
Looking at the control, both loops and associated
Cl1 become available for cooling. Five degrees is a dead
coils (Cl1 and Cl2) will provide cooling when the OAT
band to avoid cycling between cooling and heating.
is greater than 65°F (18°C) (adjustable). The boilers in
The two coils could simply be controlled in sequence
both loops should be locked off. When the OAT drops
as shown in Figure 7. In cooling, the SAT is controlled by
below 65°F (18°C), the chillers in this loop should
Cl2, and when the valve position becomes fully open,
TECHNICAL FEATURE
22.8 gpm
In this option, some or all cooling and heating sources in Option
3 can be replaced by air-source
110°F
heat pumps. Here the two boilers
and two chillers were replaced by
two heat pumps as shown in Figure
8. A dual-fuel heat pump in Loop 1 serving hot water
to VAV boxes may be selected. In a cold climate when
HPs operate inefficiently, dual-fuel heat pumps in
both loops may be selected. Another other option is to
add an emergency gas-fired or electric boiler in Loop
1 and/or in Loop 2. This configuration holds the same
advantages of Option 3 but with additional benefits of
installing fewer boilers and chillers. The SAT control in
this option is identical to the one in Option 3 presented
in Figure 7.
Loop 2
35 gpm
800 gpm
HP2
On
HP1
On
DB = 38°F
10,000 cfm
Loop 1
110°F
400 gpm
CI1
5.2 gpm
Air
DB = 70°F
286 kBtu/h
400 gpm
Option 4: Two Coils With Heat Pumps
10,000 cfm CI2
400 gpm
800 gpm
Valve Position (%)
the control will modulate the Cl1 valve FIGURE 7 Supply Air Temperature Control for Option 3 and Option 4 Configurations. Two coils with and without
heat pumps.
position. In economizer mode, the
SAT is controlled in a similar way as
OAT = 40°F
OAT = 65°F
OAT
=
45°F
OAT = 70°F
in traditional control. In partial free
Loop 2 in Heating
Loop 1 in Heating
Loop 1 and Loop 2 in Cooling
cooling economizer mode, the SAT is
maintained by Cl2 when the outside
100
Max Valve Position
economizer damper is fully opened.
Full Free Cooling
CI2
Economizer
Damper
In full free cooling economizer mode,
CI2
Control
CI1
CI1
the SAT is controlled by the economizer dampers. In heating, the SAT
is maintained by Cl1, and when the
0
0
100
200
300
400
500
valve position reaches a maximum
Supply Air Temperature Control Loop Signal
value, the control will modulate to the
Cl2 valve position.
FIGURE 8 Option 4. Two coils with heat pumps.
As Loop 1 should serve hot water
Design Cooling Conditions
B.
A.
for both Cl1 and VAV boxes, the heat57°F
67°F
ing capacity of this loop should be
Air
carefully examined. Depending on
Air
DB = 63°F
Air
the design VAV box reheat loads, the
DB
=
85°F
CI
DP = 63°F CI2
DB = 55°F
HP2
HP1
DP
=
65°F
maximum value of the Cl1 valve posiDP = 54°F
On
On
Air
10,000 cfm
21.6 Tons
tion could be set to lower than 100%
DB = 63°F
DP = 63°F
Loop 2
Loop 1
(e.g., 70%) so the heating load will be
21.6 Tons
shifted from Loop 1 to Loop 2, and
52°F
800 gpm
800 gpm
VAV Boxes
42°F
consequently Loop 1 will have more
capacity to serve the VAV boxes. This
Design Heating Conditions
C.
value could be dynamically reset
D.
85°F
85°F
based on VAV box hot water valve
Air
position readings.
Air
DB = 44°F
Air
DB = 44°F
66 kBtu/h
VAV Boxes
Cost Saving Estimation
The equipment cost savings due to the configuration
options presented are shown in Table 1. The initial costs
were obtained by consulting with local HVAC companies, and a conservative approach is used. The cost
range should be treated very carefully as this may vary
from project to project and location. The objective is not
to get exact cost estimation but rather to get approximate meaningful comparisons between those options
as opposed to the traditional design (base) shown in
Figure 1. The operating cost is obtained by running models for 40,000 ft2 (3716 m2) office building using the
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TECHNICAL FEATURE
energy simulation software
TABLE 1 Cost estimation for several option configurations.
EnergyPlus2 for seven cliCOST
EQUIPMENT
OPTION 1
OPTION 2
OPTION 3A
OPTION 3B
OPTION 4B
mate zone locations. Option
3a is when the same coils
Base –
Base –
–
Base –
Coil + Valve
Base (15% toBase
(40% to 50%)
(40% to 50%)
25%) (15% to 25%)
and water loops of the base
(traditional system) is used.
Base –
Base –
–
Base –
Pipe
Base (20% toBase
Initial
(10% to 20%)
(10% to 20%)
30%) (20% to 30%)
Cost of
Option 3b is when the comEquipment
Base –
–
bined coils are designed to
Listed Cooling or Heating
Base
Base
Base (25% toBase
Equipment
(15% to 25%)
35%)
meet the peak loads. Option
Base –
Base –
–
Base –
4b is identical to Option
Pumps
Base (15% toBase
(2% to 7%)
(2% to 7%)
25%) (15% to 25%)
3b but using heat pumps.
Base –
Base –
Base –
Base –
Dual-fuel heat pumps or
AHU Fan
Base
(20% to 30%)
(20% to 30%)
(5% to 15%)
(5% to 15%)
possible needed emergency
Operation
Cost of
Base –
Base –
Base –
Base –
Base –
heaters are not included in
Pump
Equipment
(5% to 10%)
(5% to 10%) (40% to 50%) (10% to 15%) (10% to 15%)
the cost as the needs of those
Listed
Base –
Base –
Base –
Base –
Base –
depend heavily on location.
Chillers/Boiler/HPs (2% to 7% Boiler)
(2% to 7% Boiler) (10% to 20%) (15% to 25%) (15% to 25%)
In all options, the initial and
operation equipment costs
are equal and lower than the base.
Using air-source heat pumps instead of traditional
boilers and chillers as in Option 4 lowers the number of
Conclusion
equipment to be installed, further dropping the system’s
The common design in traditional VAV systems, using
initial cost.
two separate chilled and hot water loops, reduces the
All recommended options, with a simple changeover
opportunity to achieve initial and operating cost savings. temperature control and SAT temperature control
Including heating and cooling sources in both loops can loop, provide huge energy and cost benefits for any
make any of those loops provide heating or cooling when buildings designed to be conditioned by traditional
needed, reducing the number of coils or cooling and
chilled-water VAV systems with air-cooled chillers.
heating equipment to be installed.
Attention should be taken to increase the time when
In the first configuration option, when a common coil
both coils and loops operate during mechanical coolis used instead of two coils, the required cooling and
ing-only seasons or heating-only seasons. The control
heating by AHUs is provided with fewer coils, smaller
can limit the coil valve opening to a maximum valve
AHU, fewer pipes and valves and reduced fan energy
position, so the loads can somewhat split between
use. Compared to the traditional design, this option
the coils. This maximum value can dynamically
reduces both initial and operating cost as indicated in
be adjusted based on the number of chillers, boilTable 1. If air-source heat pumps are used in the water
ers or heat pumps that operate in each loop. Also, it
loop, as in Option 2, fewer cooling and heating sources
requires careful examining of coil selections such as
are required to be installed, further reducing the projnumber of rows of tubes, fins, coil face area, etc., and
ect’s initial cost.
optimal design of those parameters so the same coil
Other possible improvements as in Option 3 are to
can provide effective cooling or heating in different
use the traditional two coils, each connected to a water
seasons.
loop equipped with both cooling and heating sources so
it can provide heating or cooling as needed. Thus, the
References
1. ASHRAE Guideline 36-2018. High-Performance Sequences of
combined coils and water loops are designed to meet
Operation for HVAC Systems.
the peak loads, reducing the size of coils, AHU and pipe
2. DOE. 2018. “EnergyPlus.” Version 9.0.1. U.S. Department of
diameters. It also reduces chiller, fan and pump energy
Energy. https://energyplus.net/
use. This option reduces significantly both initial and
operating cost as indicated in Table 1.
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