ASHRAE Journal - February 2022 - 41

TECHNICAL FEATURE
In typical VAV systems, no need exists
for mechanical cooling at a relatively
low outside air temperature (OAT)
(e.g., 50°F [10°C]) when the system is
in full free-cooling economizer mode.
At this temperature, there is also no
heating provided by the heating coil
due to warm air recirculation. Thus,
the same coil can be used for heating
at a lower OAT and for cooling at an
elevated OAT.
The second water loop, called
Loop 2, connects the VAV boxes to
heating sources, and it is always
available in heating conditions .
Figure 2a and Figure 2b shows the
design cooling performance, and
Figure 2c and Figure 2d shows the
design heating performance. In
heating mode, Cl raises the supply
air temperature (SAT) to a certain
value, for instance 70°F (21°C), and
the VAV box reheat coil adds additional
heat, for instance, to 90°F
(32°C). Thus, the boiler can serve
hot water to the Cls at a lower temperature
(e.g., 110°F [43.3°C]) and
thereby operates at a higher effi -
ciency compared to a traditional
system. Coil Cl can be designed and
selected for cooling (typically with
four to eight rows), as the heating
will be done mostly by the Loop 2
local reheat VAV boxes.
Let us now look at the control. When
FIGURE 2 Option 1 Configuration. One common coil in each AHU.
Design Cooling Conditions
A.
57°F
Cooling
Air
B1
Off
Loop 2
VAV Boxes
C.
115°F
Heating
B1
On
Loop 2
140°F
VAV Boxes
the OAT drops below 45°F (7.2°C), for instance, Loop 1
switches from cooling to heating, so the cooling sources
should be locked off . When the OAT increases above 50°F
(10°C), the loop will switch from heating to cooling so
the heating sources should be locked off. Five degrees
is a dead band to avoid cycling between cooling and
heating.
The supply air temperature control loop could be
identical to when two coils are used in a traditional
VAV system as shown in Figure 3. The only difference is
that the same coil is controlled in cooling or heating
mode, and associated Loop 1 serving this single coil
Ch1
Off
Ch2
Off
B2
On
Design Heating Conditions
85°F
Heating
Air
DB = 38°F
10,000 cfm
Loop 1
110°F
CI
Air
DB = 70°F
352 kBtu/h
To Other AHU Coils
Off
Ch1
On
Ch2
On
B2
Off
DB = 85°F
DP = 65°F
10,000 cfm
Loop 1
1,600 gpm
42°F
Cl
Air
DB = 55°F
DP = 54°F
43.2 Tons
To Other AHU Coils
D.
B.
FIGURE 1 Typical chilled and hot water configurations.
Design Heating Conditions
A.
115°F
Air
DB = 38°F
10,000 cfm
HC
Hot Water Loop
Air
DB = 70°F
352 kBtu/h
800 gpm
140°F
To Other AHU Coils
And VAV Boxes
B1
5
B2
5
MBtu/h MBtu/h
Ch1
500
Tons
B.
Design Cooling Conditions
57°F
Ch2
500
Tons
Chilled Water Loop
Air
DB = 85°F
DP = 65°F
10,000 cfm
1,600 gpm
42°F
CC
Air
DB = 55°F
DP = 54°F
43.2 Tons
To Other AHU
Coils
in each AHU should be available in heating or cooling,
accordingly, which is based on the OAT. The SAT
resetting algorithm can also be identical to the one in
a traditional VAV system.1 However, during the heating
period, the supply air temperature setpoint may
be adjusted upward or downward to split equally the
heating loads between Loop 1 and Loop 2 in a way that
the heating load will be shifted from Cl to the VAV
box or vice versa. Loop 1 serving the VAV box provides
always hot water to VAV box coils for heating whenever
needed as in a traditional VAV system.
F E B R U A RY 2 0 2 2 ashrae.o rg ASHRAE JOURNAL
41
28 gpm
400 gpm
400 gpm
400 gpm
800 gpm
800 gpm
400 gpm
800 gpm
800 gpm
28 gpm
70 gpm
70 gpm
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ASHRAE Journal - February 2022

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