ASHRAE Journal - October 2024 - 45

TECHNICAL FEATURE
operate in cooling or heating mode. One loop
serves Cl1(s), while the other serves Cl2(s)
(Figure 8).
During warmer weather conditions, both
coils and their associated water loops operate
in cooling mode to meet the cooling load. As a
result, the size of these two water loops might
be roughly comparable to those of a single
traditional chilled water loop (Figure 4a). If the
primary AHU supplies the required fresh air
(acting as dedicated outdoor air), while the
secondary AHU cools the return air, Cl1 can
effectively remove all latent loads, allowing
Cl2 to operate dry with an elevated chilled
water temperature, enhancing heat pump
or chiller effi ciency. Despite the necessity for
two water loops in this confi guration, it offers
high fl exibility in terms of improved heating
control in zones.
In colder weather, a dual fuel heat pump or
additional natural gas-fi red heating source
(boiler) can be installed to ensure adequate
heating capabilities. This balances effi ciency
and fl exibility, offering optimized control
over heating and cooling operations while
mitigating the risk of overcooling in zones.
DX Coils
Option 4: Air-Source Heat Pump DX Coils.
Each unit can be equipped with an air-source heat
pump DX coil to facilitate both cooling and heating
operations. This setup resembles the confi guration
depicted in Figure 6, with the distinction that Coil 1
and Coil 2 are DX coils. These coils have the fl exibility
to switch between providing heating or cooling as
needed. During warmer periods, both will operate in
cooling mode. In moderate weather conditions, the
primary AHU DX coil (Cl1) will deliver cooling, while
the secondary AHU DX coil (Cl2) will assume heating
responsibilities. As temperatures decrease further,
Cl2 will supply heating, with Cl1 potentially providing
supplementary heating if required.
As partial and full free cooling can be attained
through the economizer in the primary AHU, Cl2
must meet the entire heating load. In colder climates,
a system-level furnace may be necessary. Moreover,
careful consideration should be given to adjusting the
FIGURE 8 Option 3: Dual-duct VAV system with water coils with heat pumps-two water loops.
Cl2
B1
Water Loop 2
HP2
HP1
Water Loop 1
Cl1
To Other AHU Coils
To Other AHU Coils
(A)
FIGURE 9 Option 5: Dual-duct VAV system with centralized VRF.
VRF Indoor
Coil
EA
Outdoor VRF
Unit
To Other VRF Indoor Coils-Other Dual VAV Systems
RA From Zones
Secondary AHU
VRF
Indoor
Coil
Coil 2
OA
MA
VRF
Indoor
Coil
Coil 1
Supply Fan
To Other Zones
secondary AHU supply temperature setpoint during
heating operations. This adjustment may involve raising
the setpoint based on warm VAV box positions to ensure
none are open more than 95%, particularly after the
duct static pressure reaches its maximum value. This
resetting strategy could also be applicable to Option 2
and Option 3. This option shares the advantages of
Option 2 and Option 3 in terms of reducing the size of
AHUs, duct systems and VAV boxes compared to the
traditional dual-duct VAV system.
Option 5: Centralized Variable Refrigerant Flow
(VRF) System. The VRF system can be integrated by
installing one coil in the cold deck and another in
the warm deck. Additionally, an extra coil may be
incorporated in the exhaust air to harness heat from
the warm exhausted air. Figure 9 illustrates that a single
VRF outdoor unit has the capacity to serve multiple
AHUs, equipped with two coils (Coil 1 and Coil 2), with
potential for an additional coil in exhaust air. This
approach presents signifi cant potential for cost savings,
particularly in situations requiring simultaneous
O CTO B E R 2 0 2 4 ashrae.org ASHRAE JOURNAL
45
Supply Fan
Primary AHU
(B)
Cold or Warm Air
SA
SA
Warm Or Cold Air
Zone 3
Zone 2
Zone 1
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ASHRAE Journal - October 2024

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