Trane Tracer UC400 Installation, Operation And Maintenance Manual page 41

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BAS-SVX48H-EN
load decreases, the fan speed is reduced to maintain zone temperature at cooling setpoint,
while cooling capacity (and/or economizer) modulates or cycles to maintain DAT at the same
design setpoint as show below.
Figure 17. Single Zone VAV Control
When the fan reaches minimum speed and the zone cooling load continues to decrease,
the fan continues to operate at minimum speed while the DAT setpoint begins to reset
upward to maintain zone temperature at cooling setpoint. Cooling capacity (and/or
economizer) modulates or cycles to maintain this DAT setpoint. When the zone
temperature drops to heating setpoint, the fan continues to operate at minimum speed
and the DAT setpoint resets further upward. Heating capacity modulates or stages to
maintain this DAT setpoint. If the zone heating load increases to where the DAT reaches
the maximum limit, the fan speed again, increases while heating capacity modulates or
stages to maintain DAT at this maximum limit.
C C a a s s c c a a d d e e Z Z o o n n e e C C o o n n t t r r o o l l : : used in the occupied, occupied bypass, and occupied standby
modes. It maintains zone temperature by controlling the discharge air temperature to control
the zone temperature while minimizing the fan speed. The UC400 Controller uses the
difference between the measured zone temperature and the active zone temperature setpoint
to produce a discharge air temperature setpoint. In addition, it compares the discharge air
temperature setpoint with the discharge air temperature and calculates a unit heating/cooling
capacity accordingly (refer to the illustration below). The end devices (outdoor air damper,
valves, and so on) operate in sequence based on the unit heating/cooling capacity (0–100%).
If the discharge air temperature falls below the discharge air temperature low limit
setpoint, (configured using the Tracer TU service tool), and the cooling capacity is at a
minimum, the available heating capacity is used to raise the discharge air temperature to
the low limit (refer to the following section, Discharge Air Tempering.).
Figure 18. Discharge Air Comparison
S S e e q q u u e e n n c c e e o o f f O O p p e e r r a a t t i i o o n n
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