ASHRAE Journal - September 2022 - 33

TECHNICAL FEATURE
paper. It covers the retrofi t design, demonstration methodology,
performance analysis, applicability insights
and a suite of technology transfer support tools.
Background
Constant volume multizone (MZ) HVAC systems were
common in the 1950s and '60s prior to the introduction
of more energy effi cient variable air volume (VAV) systems
in the '70s. MZs have parallel paths (decks) inside
the air handler for heating and cooling airfl ows. The airstreams
are mixed at the outlet of the air handler based
on zone demands and then delivered to each zone via
dedicated ducts, making them quick to respond to the
needs of the space.
Attempts at improving MZ effi ciency over the years
included adding setpoint reset to the originally fi xed
deck temperatures based on outside air temperature
or shutting off decks seasonally to allow only heating
in the winter and cooling in the summer. New designs
emerged that removed the hot deck coil and added heating
coils to the zone ducts resulting in a bypass confi guration*
or confi gurations that added a third (neutral)
deck, but both still retained the constant volume feature.
(See the next section, " Multizone Confi gurations. " )
One retrofi t solution consisted of installing VAV box
terminal units in the branch duct serving each zone
along with a VFD on the supply fan with capacity control
based on static pressure sensed at the fan discharge. A
similar packaged retrofi t product provided a " slide-in
retrofi t terminal unit " for each zone where variable air
volume was achieved through terminal unit adjustment
of airfl ow. This retrofi t also included the addition of a
VFD with capacity regulation again based on duct static
pressure at the fan discharge. Another marketed product
was a self-adjusting diffuser. Use of this product was
more sophisticated as it required modifi cation of the
existing damper control signals such that the dampers
supplied either full heating or full cooling based on a
(modifi ed) thermostat signal. This retrofi t also called for
installation of a control damper in each zone's branch
duct, a VFD and duct static pressure sensors at the fan
discharge as well as in each zone's branch duct.
Retrofi ts also emerged that varied fan speed based
on measurements of damper position or discharge air
temperature. Yet another approach for conversion to
VAV broke the mechanical linkage/shaft connecting each
zone's hot and cold damper and then used separate actuators
for the hot and cold dampers. This approach allowed
for sequencing of the dampers so only one deck was open
at a time and modulated fan speed based on static pressure
in the open deck or discharge air temperature.3 -7
The collective observation of the project team, based on
three decades of working with dozens of DOD and state
government sites that have multizones, is that most of
these facility energy professionals aspire to upgrade their
MZs by replacing them altogether with VAV systems, but
that this expensive option is not practical with limited
budgets. To address the needs of these government coworkers,
the team developed a simple, low-cost remedy
to the multizone energy ineffi ciencies and fi eld tested
the proposed solution. This low-cost approach minimized
the physical changes to the systems and attempted
to reign in the complexity of the solution to allow staff,
including those with limited time or controls experience,
to implement and manage the resulting system.
Multizone Confi gurations
Multizone AHU systems have three typical confi gurations:
conventional two-deck, bypass and neutral deck.
Common features of multizone systems include serving
multiple spaces, a constant volume fan, multiple parallel
air decks and dedicated sets (assemblies) of zone
dampers (one set for each zone) to mix air at the air handler
for a customized blend of air at the right temperature
for each zone. A summary of each is in Table 1.
Each of the typical confi gurations has a supply fan that
operates at a single speed delivering a constant volume
of air. Each uses multiple sets of dedicated zone dampers
to mix parallel airstreams in the desired proportions
for delivery to each zone. Zone damper pairs are two
dampers mechanically linked and offset by 90° such
that as a damper opens to one deck, it closes its linked
deck damper proportionally in reciprocal fashion. (See
Figure 1 for zone damper sequencing of the assorted MZ
units in a proportional only control scheme.†
two decks have one pair of dampers (with a common
*This confi guration was especially popular in hot/humid climates where primary heating was seldom required. It was also often called the
" Texas Multizone. " 8
†Although the retrofi t uses proportional-integral (PI) control, the graphical representation of proportional-only control is shown to depict
the relative position of the dampers as they actuate.
S E P T E M B E R 2 0 2 2 ashrae.o rg ASHRAE JOURNAL
33
) AHUs with

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