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6 VAV-PRC003-EN
Features and
Benefits
Overview
Changeover-bypass VAV is a comfort
system developed for light commercial
applications. A changeover-bypass VAV
system responds to changing cooling
or heating requirements by varying the
quantity or volume of air delivered to
each zone. Each zone has a thermostat
for individual comfort control. An
HVAC unit delivers a constant volume
of air to the system. As the volume of
air required by the zone changes,
excess supply air is directed to the
return duct via a bypass duct and
damper. (See Figure 3 for typical
system components.)
A changeover-bypass VAV system
combines the comfort benefits of VAV
with the cost effectiveness and
simplicity of packaged, constant-
volume unitary equipment.
How the System Works
A changeover-bypass VAV system
commonly consists of an HVAC unit
with a constant-volume supply fan, and
direct-expansion (DX) cooling. This
combined system has the ability to
“change” to the heating mode or
cooling mode, depending on individual
zone comfort requirements. A heating
coil or a gas-fired heater and an outside
air damper are possible options.
A
temperature sensor in each zone
communicates information to an
electronic controller on the VAV
terminal unit. The controller then
modulates the zone damper open or
closed, supplying heating or cooling air
to the zone.
The HVAC unit delivers a constant
volume of supply air to the system. In
order to maintain duct static pressure, a
bypass duct and damper are required
to bypass (detour) air not required in
the zones.
The VAV terminal unit controller
communicates zone temperature
information to a central control panel
(CCP). The CCP also gathers
information from the system, including
duct static pressure and supply-air
temperature. The CCP determines zone
heating or cooling needs using voting
(or polling) logic, then requests heating
or cooling from the HVAC unit. The CCP
directs the HVAC unit to provide
ventilation air to high-occupancy areas
(demand control ventilation) or free-
cooling when the outside air
temperature falls below the
temperature setpoint (economizer
control).
Auto Changeover
“Auto changeover” refers to the ability
of the system to automatically change
between the heating and cooling
modes.
In a changeover-bypass VAV system,
the CCP determines whether the HVAC
unit should heat or cool by polling the
temperature of the individual zones. It
then compares the zone temperatures
to the space temperature setpoints. If
the supply air does not meet the criteria
for the heat or cool mode called for, the
CCP sends a signal to the HVAC unit to
change the system to the opposite
mode.