ASHRAE Journal - April 2022 - 14

TECHNICAL FEATURE
towers to maintain the water temperature
within minimum and maximum
limits. Loop 3 may be equipped
with a waterside economizer (ECO)
that receives cold water from CT1
and/or a free cooling heat exchanger
(HX) that receives cold water from
Loop 5.
Figure 3 shows the case when three
chillers are replaced by heat pumps
with the same capacity. Dual-fuel
HPs (DF-HP) may be used in Loop
2 and a boiler in Loop 1 to provide
heating during low OAT in cold climates.
During mild weather (e.g., at
60°F [16°C] OAT), HP3 in Loop 2 may
operate in cooling and HP1 in Loop 1
may operate in heating to provide
hot water for local reheat coils. In
addition, if process cooling exists,
HP4 in Loop 3 will operate in cooling
and reject heat to Loop 5 where
HP3 recovers this heat and operates
efficiently.
During cold weather, all heat pumps
FIGURE 3 Option 2 configuration: one common coil with heat pumps.
VAV Box Reheat
(Hot) Water Loop
(Loop 1)
B1
HP1
Tank
Common Water Loop
(Loop 5)
HX
B2
B3
CT4 CT3 CT2
FIGURE 4 Option 3 configuration: two coils.
VA Box Reheat
AHU Coil Cl1
Water Loop
(Loop 1)
operate in heating, except HP4 in Loop
3, so the Loop 5 water temperature will
drop near the minimum limit so full
or partial free cooling can be obtained
through HX; no need exists to run HP4
and/or the cooling tower for the ECO
heat exchanger. HP4 can simply be a
chiller, but its condenser loop needs
to be connected as well to Loop 5.
The SAT control could be identical to Option 1 or a traditional
VAV system. In heating mode and when all zones
are in heating mode, the AHU coil raises the systemlevel
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 heat pumps serving
the AHU coils can operate in heating mode at decreased
supply hot water temperatures and thereby higher
efficiency. During heating mode at low OAT, the SAT setpoint
can be reset to a lower value (e.g., 60°F [16°C] or
even lower) to reduce the load on HPs when they are not
very efficient and shift the required heating to the local
reheats served by the boiler(s). This configuration holds
B1
B2
B3
Ch1
Ch1
CT4 CT3 CT2 CT1
Condenser Water
Loop
(Loop 4)
the same advantages of Option 1 but with fewer chillers/
boilers to be installed.
Option 3: Two Coils in AHUs
In this option, the two coils in each AHU and two separate
water loops in a traditional VAV system are kept.
However, the traditional cooling coil, now called Cl1, can
provide heating during the winter when no mechanical
cooling is needed, and the heating coil, now called
Cl2, can provide cooling during the summer when no
heating is needed. To achieve that, as shown in Figure 4,
Loop 1 and Loop 2 are both equipped with heating and
cooling sources. As an example, Loop 1 that serves the
first AHU coil (Cl1) and the zone VAV box reheat coil
14
ASHRAE JOURNAL ashrae.o rg
A P R I L 2022
DF-HP2
HP3
ECO
HX
AHU Coil
Water Loop
(Loop 2)
CT1
Ch1
Condenser Water Loop
(Loop 4)
Cooling Loads
Process Cooling
Chilled Water Loop
(Loop 3)
HP4
AHU Coil Cl2
Water Loop
(Loop 2)
B3
Ch2
Ch3
ECO
Cooling
Process Loads
Process Cooling
Chilled Water Loop
(Loop 3)
DF-HP2
HP1
HP3
HP4
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ASHRAE Journal - April 2022

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