ASHRAE Journal - November 2020 - 18

TECHNICAL FEATURE

FIGURE 3 Dual-duct, dual-VAV systems including the Case 3 performance data when the SAT is constant at 55°F (12.7°C).

Flow = 1,193 cfm
RAT = 75°F
Return Air (RA)
Heating = 0 Btu/h
OAT = 58°F
OA Flow = 0 cfm

MAT = 75°F

AHU2
Supply Air

Mixing Air

Outdoor Air

Sensible Cooling = 23,801 Btu/h
Total Cooling = 23,801 Btu/h

OAT = 58°F
OA Flow = 7,212 cfm
Required Ventilation = 2,237 cfm

Flow = 7,212 cfm
SAT = 55°F

AHU1

Flow = 4,605 cfm
Reheat = 0 Btu/h

Flow = 1,000 cfm
Reheat = 0 Btu/h

Flow = 8,000 cfm
Reheat = 0 Btu/h

South (Zone 1)
Sensible Load =
14,285 Btu/h
Latent Load =
2,200 Btu/h

East (Zone 2)
Sensible Load =
14,438 Btu/h
Latent Load =
2,200 Btu/h

Core (Zone 3)
Sensible Load =
101,311 Btu/h
Latent Load =
16,400 Btu/h

West (Zone 4)
Sensible Load =
14,200 Btu/h
Latent Load =
2,200 Btu/h

North (Zone 5)
Sensible Load =
14,437 Btu/h
Latent Load =
2,200 Btu/h

Mode: Heating

Dual-Duct, Dual-VAV Systems
Figure 3 shows using two separate supply air ducts and
systems: one is used for cooling (AHU1), and the other
is used for heating (AHU2). One common return duct
is used. Both systems have been equipped with economizer and outdoor air dampers. This conﬁguration
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656 cfm

Flow = 1,000 cfm
Reheat = 0 Btu/h

Airﬂow rates in all zones except the core zone are at
minimum values. The zone reheat requirements (Table 2a,
Case 3) are 7,715.2 Btu/h (South Zone 1) , 7,561.6 Btu/h (East
Zone 2), 0 Btu/h, (Core Zone 3), 7,799.3 Btu/h (West Zone
4) and 3,163 Btu/h (North Zone 5), or 2261 W, 2216 W, 0 W,
2286 W, 927 W, respectively. To reduce heating use in those
zones, SAT reset strategy can be applied while ensuring
adequate cold air to meet the cooling load in Core (Zone
3). Tables 2b and 3b show the zone and system results when
the supply air temperature reset algorithm is applied.
For the same case (Case 3), the SAT is now 59.7°F (15.4°C)
(Table 3b). Mechanical cooling will not be required, but
some reheat is still required in the four exterior zones.
The airﬂow and, thereby, fan power are increased. There
are some energy savings from resetting the SAT or from
any advanced control strategies, but is this the best we can
do? Let us investigate the scenario when two systems and
two separate ducts are used instead.

ASHRAE JOURNAL

646 cfm

Flow = 1,000 cfm
Reheat = 0 Btu/h

Mode: Heating

18

4,605 cfm

144 cfm

656 cfm

0 cfm

351 cfm
649 cfm

354 cfm

Supply Air
Mixing Air
MAT = 58°F

344 cfm

Outdoor Air

Flow = 1,193 cfm
SAT = 75°F

Supply Fan

N OVEM BER 2020

Mode: Cooling

Mode: Heating

Mode: Heating

was successfully applied for schools.3 Two VAV box ﬂow
control dampers are used with the dual-duct mixing
control strategy.1 The objective of this study is not to lay
out the best control strategy for dual-duct or single-duct
VAV systems, but to provide a meaningful comparison
between these two systems and evaluate the possible
load reductions by the dual-VAV systems.
For that purpose, the following assumptions are
used: AHU1 uses the same SAT control strategies of the
single-duct VAV system discussed above. The AHU2
SAT is at the maximum value possible to maximize the
use of recirculation air and equals the AHU1 SAT when
system heating is required to be activated. The zone
airﬂow rates for both systems are maintained at the
minimum values (20% of maximum design ﬂow) during heating mode.3
Looking back at Case 3 in the previous example,
reheat for the single-duct VAV system is still needed in
most zones with or without SAT resetting (Figure 2 or
Tables 2b and 3b). However, as shown in Figure 3, using
dual-VAV systems, heating can be done completely by
AHU2, recirculating the warm return air; cooling can
be partially done by AHU1 for a constant 55°F (12.7°C)
SAT, introducing 100% cold outdoor air at 58°F (14.4°C).
Or cooling can be done completely by AHU1 if the SAT
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ASHRAE Journal - November 2020

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