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Pressure Dependant and Pressure
Independent Control
Static pressure variations in an air handling system can affect
terminal unit operation. Pressure-dependent terminal units are
affected by changing duct static pressures because their
damper position is determined by space temperature only.
They may have mechanical or electric minimum and
maximum air flow limits. Pressureindependent terminal units
can automatically adjust to duct pressure changes because
they contain air flow sensors and the controllers compensate
for pressure changes in the main air distribution system. The
damper position in pressure independent terminal units is
determined by both space temperature and air flow volume.
Variable Air Volume ATUs (VAV)
Variable air volume (VAV) ATUs are commonly called VAV
boxes. Each VAV box has a controller that controls the
temperature of a room or zone by modulating a damper in the
VAV box to vary the amount of conditioned air supplied to the
zone rather than changing the temperature of the conditioned
air. They are used in larger buildings that have many zones
along with a central air handling Unit (AHU) that supplies
conditioned air via a pressurized main air duct system.
Single Duct Variable Air Volume (VAV)
Systems
Single duct VAV systems are used in over 80 percent of the
VAV applications and employ one main supply air duct from
the central air handling system. The air handling unit supplies
cool air virtually one hundred percent of the time, with the only
exception being a morning warm-up cycle that is used in
buildings that are not continuously occupied, that temporarily
raises the discharge air temperature of the central air handling
system to quickly warm the building from its unoccupied zone
temperatures to the occupied zone temperatures. Since the
central air handling system is usually supplying cool air, single
or multiple electric reheat coils or a modulating hot water
(hydronic) reheat coil are often added in the VAV box
discharge air duct to reheat the cool air when the zone
becomes too cold. VAV boxes with reheat coils typically have
a series or parallel fan in the VAV box to ensure air flow across
the coil in the heating mode.
Pressure Dependent Throttling VAV Boxes
Pressure dependent throttling VAV boxes are the simplest and
least expensive ATU. A controller modulates a damper
actuator according to the temperature in the zone. The
pressure dependent VAV box usually has minimum and/or
maximum damper position setpoint stops in the controller for
limiting air volume. Because the unit is pressure dependent,
the volume of air distributed to the zone at any given space
temperature varies with the supply duct static pressure at the
inlet of the VAV box. Maintaining a stable duct static pressure
is important for proper operation and proper setting of the
minimum damper position setpoint stop is essential for
adequate circulation. When reheat coils and/or finned tube
radiation are used the controller will set the damper position at
a minimum position during the heating mode to ensure some
air flow into the space and optimize heat transfer from the
reheat coil. Pressure dependent VAV boxes are used in
smaller buildings or in areas of larger buildings where the
supply duct static pressure is low and stable.
31-00083—02
Pressure Independent VAV Boxes
Pressure-independent or variable constant-volume VAV boxes
are essentially air flow control devices that deliver a constant
volume of air to a conditioned space at a given temperature
despite a varying supply duct static pressure. An air flow
sensor in the inlet of the VAV box is used to measure the
volume of air and the VAV box controller resets the air flow
volume setpoint as the thermal load changes in the space.
Therefore, a pressure independent VAV box controller
provides two control sequences; zone temperature control
and terminal unit air flow control. The controller usually has a
minimum air flow setpoint to maintain air flow at light load
conditions and a maximum air flow setpoint to limit the air flow
to meet the design conditions for the zone. A single zone
sensor can be used to control multiple VAV boxes with
differing volume ratings. When reheat coils and/or finned tube
radiation are used with this unit the controller will lower the air
flow setpoint during the heating mode to ensure air flow into
the room and optimize heat transfer from the reheat coil.
Pressure independent VAV boxes are used in buildings with
larger air handling systems that have constant duct static
pressure fluctuations due to the large number of zones.
Series Fan Powered VAV Boxes
Series Fan-powered VAV boxes are similar to pressure
dependent and/or pressure independent VAV boxes, except
they include an integral fan in series with the VAV box
discharge duct that recirculates space air at constant volume
and enhances the air distribution in the zone. Primary air is
modulated by the VAV box damper to meet space demand for
cooling and as primary air modulates down, more plenum air
is drawn in by the fan to maintain a constant discharge volume
to the zone. Typically the series fan is on continuously during
occupied hours, or it can be programmed to be activated as
primary air decreases to ensure adequate air circulation. In
addition to enhancing air distribution, the units serving the
perimeter area of a building usually include a reheat coil that
is sequenced with the primary air damper to supply heat when
required. When the primary air system is not operating
(nighttime or unoccupied control mode), the night operating
mode of the controller enables the fan and the reheat coil to
maintain the lower unoccupied temperature setpoint in the
space. Series Fan Powered VAV boxes can be pressure
independent or pressure dependent.
Parallel Fan Powered VAV boxes
Parallel Fan Powered VAV boxes or Bypass Fan Induction
Terminal Units are similar to Series Fan-powered VAV terminal
units, except the fan is located in the return plenum and does
not run continuously during occupied hours. When the zone
temperature is low and the need for primary air decreases, the
controller modulates the primary air damper to a minimum
and enables the fan, which recirculates warm air from the
return plenum into the zone acting as the first stage of reheat.
If a reheat coil is used the fan is cycled on when the reheat
coil is enabled. As the space warms, the reheat valve closes
and the fan cycles off as the primary air damper opens to
allow delivery of conditioned air from the air handling system.
When heating is required in the unoccupied mode, the fan at
the central air handling system remains off, the VAV box fan
and the reheat coil are enabled and the zone is heated to a
reduced night setback temperature using air from the return
air plenum. Parallel Fan Powered VAV boxes can be pressure
independent or pressure dependent.
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