ASHRAE Journal - September 2022 - 34

TECHNICAL FEATURE
TABLE 1 Common multizone configurations and features.
CONVENTIONAL (HOT DECK/COLD DECK) MULTIZONE
* Two decks: one hot, one cold.
* Heating and cooling coils in respective decks.
* Both decks typically operated continuously, even when heating/cooling not
needed.
V
H
Supply Fan
SF
Hot Deck
Cold Deck
C
V
C
T
Temperature
Sensor
Digital Control
BYPASS (TEXAS STYLE) MULTIZONE
* Two decks: cold and bypass.
* No coil in bypass deck.
* Heating coil typically in each zone duct (allows heating in only zones that
need it).
* Uses unconditioned air (e.g., recirculated and outside air) in bypass deck for
partial " free heating " when not economizing, especially useful in hot/humid
climates that cool to 48°F - 52°F for dehumidifi cation needs.
* More effi cient than Conventional Multizone (avoids simultaneous heating and
cooling).
* More coils to maintain.
NEUTRAL DECK MULTIZONE
* Two decks: hot, neutral, and cold.
* Neutral Deck has no coil and introduces mixed air (recirculated and outside
air)
* Uses unconditioned air (e.g., recirculated and outside air) in neutral deck for
partial " free heating " when not economizing, especially useful in hot/humid
climates that cool to 48°F - 52°F for dehumidifi cation needs.
* More effi cient than Conventional Multizone (avoids simultaneous heating and
cooling).
Supply Fan
SF
V
Hot Deck
H C
Neutral Deck
Cold Deck
CC
T
V
actuator) for each zone to mix the air from the two available
decks. AHUs with three decks may have two damper
pairs for each zone (with two actuators, one for each
pair) or an assembly of three linked dampers with a
single common actuator to orchestrate the mixing of the
three available decks. The key feature in the triple deck
arrangement is that the hot deck and cold deck are not
open at the same time, and the neutral deck is reciprocal
to the operating conditioned deck. Each of the damper
actuators receives its signal from its respective zone controller.
The various types of multizone air handlers differ
in the number of decks they have as well as the number
and placement of heating and cooling coils. Some units
also have return and/or exhaust fans.
Variable Volume Retrofi t Design
The Basic Retrofi t (Figure 2) reduces wasted energy
Temperature
Sensor
Digital Control
N.C.
Typical Damper Actuator
Digital Control
Typ. Zone Stat
through capacity control of the fan(s) and turning off the
heating and cooling coils when not needed. Where direct
digital control (DDC) hardware preexists, it requires
the addition of two pieces of equipment and controls
programming:
* A variable frequency drive (VFD) to allow for fan
operation at reduced speeds;
* An airfl ow measurement array (AFMA) to measure
and thereby control outdoor air intake to supply the
required amount of ventilation and/or makeup air;
* Controls programming that implements the new
functionality: variable fan speed control, outside airfl ow
control and on/off heating and cooling coil control.
The control is novel in that it uses the damper command
signals§
to establish the fan speed and inform
decisions about enabling or disabling the coils.
The building zone with the greatest need for space
§Attempts by others to use the zone dampers to control fan speed used positioning sensors.5 The proposed retrofi t simplifi es the control
scheme and reduces costs of the retrofi t by skipping the measurement of damper position and instead uses the damper command signals
from the zone controllers to the zone actuators. It polls the maximum of all the zone damper signals as the process variable by which
to set the fan speed.
34
ASHRAE JOURNAL ashrae.o rg S E P T E M B E R 2 0 2 2
N.O.
N.C
To Each
Zone
Digital Control
T
Temperature
Sensor
Digital Control
N.O.
Typ. Zone Stat
Typical Damper Actuator
Supply Fan
SF
Bypass Deck
Cold Deck
C
V
Digital Control
C
T
Temperature
Sensor
Typical Damper
Actuator
Digital Control
Typ. Zone Stat
N.O.
N.C.
V
HC
To Each
Zone
Typical Damper
Actuator
Digital Control
Typ. Zone Stat
C
N.O.
N.C.
To Each
Zone
Digital Control
T
Temperature
Sensor

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ASHRAE Journal - September 2022

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