ASHRAE Journal - October 2024 - 46

TECHNICAL FEATURE
heating and cooling. Also, as other options,
both Coil 1 and Coil 2 operate in cooling mode
to meet cooling demands when OAT exceeds
a predefined threshold (e.g., 75°F [23.9°C]).
This configuration offers the same advantages
as Option 4 but with enhanced efficiency, as
some heat transfer from Coil 1 to Coil 2 occurs
internally instead of solely relying on heat
absorption from the environment.
Option 6: Centralized Water-Source Heat Pump
(WSHP) System. Option 6 entails installing two watersource
heat pumps, one dedicated to providing cold
air to the cold deck and the other delivering warm air
to the warm deck. This configuration offers numerous
advantages in terms of efficiency and flexibility. It shares
the same benefits as Option 4, but in this case, the heat
is not rejected or absorbed from the environment;
instead, it uses a water loop. Furthermore, the water
loop provides flexibility by accommodating various heat
sources and facilitating heat transfer between different
equipment units (Figure 10). The loop can be connected to
a solar heating system or any waste heat.3 Additionally,
a domestic water heat pump could be connected to the
loop, enhancing its versatility and efficiency.
Cooling
Tower
Waste Heat
Exchanger
Summary and Conclusion
The traditional dual-duct VAV system relies on a
central fan and a system-level heating coil, in contrast to
the single-duct VAV system with fan-powered VAV boxes,
which uses local fans and zone-level reheat. In the dualduct
VAV system, the additional cost associated with hot
deck ducting and extra zone VAV boxes can be mitigated
by repurposing them for cooling to support the cold
deck VAV boxes during summer. Consequently, two
significant improvements have been proposed for the
traditional dual-duct VAV system. 1) By incorporating a
fresh air mechanism for both cold and hot decks, the hot
deck can provide little to no fresh air while the cold deck
can deliver 100% fresh air. 2) Allowing either the hot or
cold AHUs to operate in cooling or heating mode enables
both to function in cooling mode during warm summers
to manage peak loads, reducing the size of AHUs and
associated ducts. Option 1 shows the basis for those
two improvements using four coils, two in each unit.
However, to further reduce cost, Option 2 uses one water
loop to provide either hot or chilled water to the coils,
and Option 3 uses one loop switching between cooling
46
ASHRAE JOURNAL ashrae.org O CTO B ER 2024
FIGURE 10 Option 6: Dual-duct VAV system with centralized WSHPs.
Water-Return Pipe
Valve
Solar Heating
System
Heat Added
Boiler
Water-Supply Pipe
and heating as needed. Options 2 and 3 make dual-duct
chilled water VAV systems more cost-effective compared
to traditional VAV system as they reduce the size of AHUs,
fans, ducting and piping system and VAV boxes.
For dual-duct DX VAV system, Option 4 using airsource
heat pumps provides one DX coil solution in
each AHU, providing cooling or heating as needed and
reducing the size of AHUs, fan, ducts and VAV boxes
compared to the traditional dual-duct VAV system.
Option 5 uses a centralized VRF system with a single
outdoor unit and dual-coil AHUs for efficient internal
heat transfer, reducing component sizes. Option 6
features a centralized WSHP system with heat pumps
for cold and warm decks, using a water loop for internal
heat exchange and integrating alternative heat sources.
Both Option 5 and Option 6 optimize heat transfer,
lower energy consumption and reduce component sizes,
offering cost-effective and sustainable HVAC solutions.
In conclusion, the analysis of various configurations
for dual-duct VAV systems highlights the potential for
significant improvements in efficiency, flexibility and
cost-effectiveness. By considering innovative strategies
such as incorporating fresh air mechanisms, using
water loops and deploying air-source heat pumps, it
becomes evident that there are viable alternatives to
traditional systems. These options reduce the size and
complexity of HVAC components and enhance overall
system performance. Embracing these forward-thinking
approaches will be crucial in meeting the evolving
demands of commercial buildings, ensuring optimal
comfort, energy efficiency and sustainability.
References
1. Nassif, N. 2020. " Using dual-duct, dual-VAV to reduce cooling &
heating loads. " ASHRAE Journal 62(11):14 - 21.
2. Nassif, N. 2022. " Chilled-water VAV system configuration and
design, water-cooled chiller. " ASHRAE Journal 64(4)12- 18.
3. Nassif, N. 2021. " Increasing efficiency by using four-pipe watersource
heat pumps. " ASHRAE Journal 63(4):24- 31.
WSHP
Serving
Cl2
WSHP
Serving
Cl1
WSHP
Serving
Cl1 For
WSHP
Serving
Cl2 For
Other AHUs Other AHUs
Domestic
Hot Water
Heat
Pump
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