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Single-Duct Units
The air damper is controlled between
minimum and maximum flow setpoints
proportionally. When the zone tempera-
ture is approximately 0.5°F above the
setpoint, the air damper will begin to
modulate to a maximum position,
providing more cool air to the zone until
the zone temperature reaches the zone
setpoint. Once satisfied, the air damper
will start to modulate to the minimum
position. When the zone temperature is
0.5°F below the setpoint, the damper
begins to modulate to a minimum
position. When the damper is at the
cooling minimum and the zone tem-
perature is still below setpoint (cold), up
to three stages of heat may be ener-
gized on a reheat unit. The first stage is
activated at approximately 2°F below
the setpoint, the second stage is
activated approximately 3°F below the
setpoint, and the third stage is activated
approximately 4°F below the setpoint.
VAV-SVX03A-EN
Sequence of
Operation
Series Fan-Powered Untis
The series fan-powered unit is a
constant volume, variable temperature
device. The fan is energized whenever
the zone is occupied. It delivers design
cfm regardless of the load. The air
damper controls primary air propor-
tionally between minimum and
maximum flow setpoints. When the
zone temperature is approximately
0.5°F above setpoint, the air damper
will begin to modulate to a maximum
position, providing more cool air and
less plenum air to the zone until the
zone temperature reaches the zone
setpoint. When zone temperature is
0.5°F below the setpoint, the damper
begins to modulate to its minimum
position, supplying less cold air and
more warm plenum air. When the air
damper is at minimum and zone
temperature is still below setpoint
(cold), up to two additional stages of
electric heat may be energized. On an
electric reheat unit, the first stage is
activated approximately 2°F below the
setpoint. The second stage is activated
approximately 3°F below the setpoint.
On Hot Water Reheat just one stage of
heat, in addition to the fan, will be
activated approximately 2°F below
the setpoint.
Parallel Fan-Powered Units
The parallel fan-powered unit is a
variable-volume, constant-temperature
device at high cooling loads and a
constant-volume, variable-temperature
device at low cooling and heating loads.
The air damper controls primary air
proportionally between minimum and
maximum flow setpoints. When the
zone temperature is approximately
0.5°F above setpoint, the air damper
will begin to modulate to a maximum
position, providing more cool air and
less plenum air to the zone until the
zone temperature reaches the zone
setpoint. When the zone temperature is
approximately 0.5°F below the setpoint,
the damper will begin to modulate to a
minimum position, reducing the cooling
capacity. If the cooling capacity is
reduced and zone temperature is still
below the heating setpoint, the fan will
be energized at approximately 2°F
below the cooling setpoint acting as the
first stage of heat. The second stage of
heat (electric or hot water) is activated
at approximately 3°F below the cooling
setpoint and the third stage (electric) is
activated at approximately 4°F below
the cooling setpoint.
9
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