ASHRAE Journal - October 2024 - 41

TECHNICAL FEATURE
Alternatively, local reheat and fans can be
replaced by a central fan and system-level
heating coil, leading to a dual-duct VAV
system. However, adoption of a dual-duct
system requires additional VAV dampers to
regulate zone airfl ow, making it comparable to
VAV systems with variable speed fan-powered
VAV boxes. Figure 2 illustrates the typical
confi guration of a dual-duct, dual-fan VAV
system, incorporating two fans and two coils: a
cooling coil for the cold deck and a heating coil
for the hot or warm deck. In this arrangement,
the warm air-handling unit (AHU), known
as the secondary AHU, solely recirculates
return air from the zones without introducing
fresh air, while fresh air requirements are
met through the cold AHU, referred to as the
primary AHU. Air is supplied to the spaces via
dual inlet VAV boxes.
Comparing Figure 1 and Figure 2, it's evident
that the traditional dual-duct VAV system
uses a central fan and system-level heating
coil, whereas the single-duct VAV system with
fan-powered VAV boxes uses local fans and
zone-level reheat. Thus, the additional cost of
ducting for the hot deck and extra VAV boxes
should be compared to the cost of reheat coils
and local variable speed fans.
The primary advantage of this dual-duct
confi guration lies in its ability to mitigate
system-level simultaneous heating and cooling,
commonly observed in typical single-duct VAV systems.
By optimizing the use of warm return air for heating
purposes, it effectively reduces the workload on heating
equipment and extends the operational hours of free
cooling economizers, alleviating strain on cooling
equipment. However, the dual air distribution system
does entail an elevated initial cost and space required
for hot deck. Furthermore, the depicted confi guration
cannot entirely eliminate the potential for simultaneous
heating and cooling in zones. Similar to single-duct VAV
systems, the absence of fresh air in warm air requires
cold VAV boxes to remain open at minimum settings to
ensure the minimum ventilation requirements. This
practice, in turn, increases heating demands to prevent
zone overcooling.
Another confi guration of the dual-duct VAV system
FIGURE 1 Multizone VAV system with fan-powered VAV boxes.
Return Air (RA)
Exhaust Air (EA)
From Zones
Primary AHU
Outside Air (OA)
Mixing Air
(MA)
Cooling Coil
Supply Fan
To Other Zones
FIGURE 2 Typical dual-duct VAV system configurations-fresh air only in cold deck.
RA
EA
Secondary AHU
Heating Coil
Primary AHU
OA
OA
MA
Cooling Coil
Supply Fan
To Other Zones
involves using a common fresh air intake, as depicted
in Figure 3. This setup supplies fresh air to both the hot
and cold decks, consequently increasing the heating
load. In scenarios where heating is necessary during
economizer free cooling mode, typically observed, the
cold outside air would require elevated heating by the
heating coil in the secondary AHU. This contrasts with
the confi guration depicted in Figure 2, where fresh air
is solely introduced into the cold deck, necessitating
heating only for warm return air.
The dual-duct VAV system can use direct expansion
(DX) coil or chilled and hot water coils. When chilled
and hot water serve as cooling and heating sources for
the AHUs, the waterside confi gurations, exemplifi ed by
two chillers (Ch1 and Ch2) and two boilers (B1 and B2)
in a primary/secondary piping system, are depicted in
Figure 4.
O CTO B E R 2 0 2 4 ashrae.org ASHRAE JOURNAL
41
Supply Fan
From Zones
SA
SA
Cold Air
Warm Air
Supply Air (SA), Cold Air
Zone 3
Zone 2
Zone 1
Zone 3
Zone 2
Zone 1
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ASHRAE Journal - October 2024

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