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The controller will calculate the active OA damper
minimum position between the user-selected setpoints
based on the supply fan speed command. The range
for the design minimum setpoints at minimum and
medium fan speed will be 0-100% while the range for
the design minimum at maximum fan speed setpoint
will be 0-50%.
Space Pressure Control
For units configured with an exhaust fan with or
without Statitrac™, the new schemes implemented for
economizer minimum position handling require
changes to the existing space pressure control scheme
in order to prevent over/under pressurization. The
overall scheme will remain very similar to non-SZVAV
units with space pressure control utilizing the user
selected exhaust enable setpoint to calculate a dynamic
exhaust enable setpoint.
Discharge Air Setpoint (ReliaTel
™
™
Controlled Not User Adjustable)
For occupied cooling, heating (modulating only), and
dehumidification the unit will utilize a varying
discharge air setpoint (DASP) that is calculated based
on zone conditions. DASP will be used as the active
control point for controlling cooling (DACSP), heating
(DAHSP), and dehumidification capacity. In general, as
the ZT rises above the zone cooling setpoint (ZCSP),
DACSP will be calculated down and as the ZT falls
below the zone heating setpoint (ZHSP), DAHSP will be
calculated upward.
For cooling, heating, and reheat output control the
DASP calculation will be limited between user
selectable setpoints and will be calculated higher and
lower based on ZT.
If the controller determines that a discharge air
temperature higher than the user selected upper limit,
or lower than the user selected lower limit, is required
for space conditioning, then fan output will be
increased.
Economizer Cooling
Economizer Enabled for Cooling
Once the unit has a request for economizer cooling and
the unit has met all the cool mode transition
requirements, the economizer will open beyond
minimum position in order to meet the calculated
DACSP value. If the economizer reaches 100% before
DACSP falls below the DACSP lower limit (LL) and the
air temperature is greater than the DACSP, the control
will create a temporary DACSP LL that will be used to
determine fan speed output control. The temporary
DACSP will not be allowed to be less than the user
selected DACSP LL. As DACSP falls below the
temporary DACSP LL, the supply fan speed will be
increased proportionally up to 100% as necessary.
Once the supply fan speed reaches 100% with the
economizer at 100% for 3 minutes (5 if non V-3),
compressors will be allowed to energize to meet the
calculated DACSP value as necessary. Once
compressors have been energized, the supply fan
speed and economizer will be forced to 100%.
Economizer Enabled to Disabled
If the unit is actively cooling with the economizer and
then the economizer becomes disabled, the
economizer will be closed to the active minimum
position and compressors will be allowed to stage. At
the point in which the economizer becomes disabled,
the temporary DACSP LL will be set back to the user
selected DACSP LL. The supply fan speed will be
adjusted accordingly based on active cooling stage and
DACSP value.
Economizer Disabled to Enabled
If compressors are energized for cooling and the
economizer was disabled, but becomes enabled, the
economizer and supply fan speed will be forced to
100%.
Compressor (DX) Cooling
Cooling Sequence
If the control determines that there is a need for active
cooling capacity in order to meet the calculated
DACSP, once supply fan proving has been made, the
unit will begin to stage compressors accordingly.
Once the DACSP calculation has reached its bottom
limit and compressors are being utilized to meet the
demand, as the DACSP value continues to calculate
below the DACSP LL, the supply fan speed will begin to
ramp up toward 100%.
As the cooling load in the zone decreases, the
compressors begin to de-energize and the supply fan
will fall back to the cooling stage's associated
minimum speed.
Cooling Stages Minimum Fan Speed
As the unit begins to stage compressors to meet the
cooling demand, the following minimum supply fan
speeds will be utilized for each corresponding cooling
stage.
Table 21.
Minimum supply fan speeds
Unit Mode
VFD, BC
Plenum, non-17
SEER Minimum
Fan Speed
17 SEER
Minimum Fan
Speed
Fan Only
33%
57%
Economizer
Cooling
33%
57%
Stage 1
33%
57%
Summary of Contents for ReliaTel
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