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setpoint, the VS fan will increase in speed (reverse
acting). A user parameter will specify the minimum
speed that the VS fan is allowed to operate at when
active.
7.5.2. Fan Proofing
For both CAV and VAV modes, fan proofing may
be enabled. When enabled and a fan proof failure is
detected, heating and cooling loads will be shut down
until the proof failure goes away or the fan proof
alarm is reset or cleared. The fan may be configured
to shut down on a fan proof failure. In this case, the
fan proof alarm must be reset or cleared in order to
restart the system.
7.6. Dehumidification Control
The DAC dehumidifies by using either
mechanical cool stages or a separate dehumidification
device. When dehumidification is needed, the DAC
will generally turn on the dehumidification device or
activate stages of cooling up to a user-defined
maximum number of cool stages to be used for
dehumidification.
For units equipped with hot gas reheat, the supply
air setpoint reset analog output will modulate any
time the reheat relay is activated, which occurs when
the space temperature drops below the cooling space
temperature setpoint. Additionally, the unit's primary
heating outputs may be used to augment or replace the
hot gas reheat function.
7.6.1. Enabling Dehumidification
The DAC can use several methods to determine if
the controlled space requires dehumidification. If the
sensor needed for the method selected is not valid,
dehumidification will not be available until the sensor
operation is corrected or a different method is
selected. The methods supported are:
• Dehum by Inside RH
• Dehum by Outside RH
• Dehum by Inside Dewpoint
• Dehum by Outside Dewpoint
7.6.1.1.
Dehumidification Setpoint
The active dehumidification setpoint that the
controller uses is based on the occupancy state and the
method of determining when dehumidification is
required. Both occupied and unoccupied relative
humidity and dewpoint setpoints are available,
20 • Dehumidification Control
depending on the dehumidification method selected.
A dehumidification RH or dewpoint throttling range
will determine the range of humidity or dewpoint that
represents the full dehumidification load. The
dehumidification load can range from 0% (no
dehumidification needed) to 100% (full
dehumidification needed). Once the dehum load rises
above 20%, the controller will activate
dehumidification. Once activated, dehumidification
will remain active until the dehum load goes back to
0%.
Figure 7-4 - Dehumidification Graph
7.6.1.2.
Dehumidification by Dewpoint
If the selected dehumidification method is by
Inside Dewpoint or Outside Dewpoint, the controller
will calculate the dewpoint using the appropriate
temperature and humidity inputs. If either the
temperature or humidity input is not valid,
dehumidification will be disabled until the inputs are
corrected or the method is changed.
7.6.1.3.
Low Temperature Lockout
To preserve occupant comfort, dehumidification
will be disabled if the control temperature falls below
the minimum space during dehumidification setpoint.
Dehumidification will be allowed again when the
control temperature rises 2°F above this setpoint.
7.6.2. Dehumidifying Using Cool
Stages
The DAC activates the cooling stages specified for
dehumidification based on the calculated evaporator
coil temperature compared to the evaporator coil
temperature setpoint. The evaporator coil temperature
is calculated by converting the suction pressure to a
temperature based on the refrigerant type. For units
with a digital or VS compressor, the application will
modulate the compressor to maintain the evaporator
coil temperature setpoint and activate additional fixed
capacity compressors as necessary based on the
026-1727 Rev 3
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