Discharge Air Controller (DAC) Manual Software Overview • 15
7 Software Overview
The DAC maintains space temperature setpoints
by modulating heating or cooling stages to maintain a
desired supply air temperature. For this reason, a
supply air temperature sensor must be connected to
the controller. The desired supply air temperature is
primarily determined by a setpoint and can be
automatically adjusted based on changes in building
load.
7.1. Application Mode
The DAC supports packaged HVAC control in
either Constant Air Volume (CAV) or Variable Air
Volume (VAV) applications. The main difference
between the two modes is how the variable speed fan
will be controlled. When CAV is selected, the
variable speed fan is modulated based on the number
of heating or cooling stages that are currently active.
When VAV is selected, the variable speed fan
modulates to maintain a static duct pressure. See
Section 7.5., Fan Control for more details.
Also, VAV mode does not require a local space
sensor to be connected to the controller. Instead, the
system terminal load is calculated by the E2 and is
sent over the network to the DAC so the operational
mode can be determined. Note that the primary
control of heating and cooling stages using the supply
air temperature sensor is identical for either CAV or
VAV mode.
7.2. Operational Modes
The DAC will be in one of nine possible modes of
operation at any given time. All modes have different
priorities so if conditions warrant two modes to be
operational at the same time, one of the modes will
win and become active. In the table, the modes are
shown in ascending priority
with the exception of
Cooling and Dehum. For example, Idle is the lowest
priority and Emergency Shutdown is the highest
priority,
but if both cooling and dehumidification are
called for, the larger of the two calculated loads will
take priority and activate the proper mode of
operation.
Corresponding
Site Manager
Mode
Operating
Mode
Description Outputs Active
0 Idle Default mode if no other modes are active None
1 Ventilation Space air recirculation only Indoor Fan
2 CO2 Purge Max CO2 level reached and fresh air must be
brought in to building
Indoor Fan, Outdoor Air Damper
3 Economizing Outdoor air free cooling active Indoor Fan, Outdoor Air Damper
4 Make-up Air Exhaust Fan detected and make-up air must be
brought in to building
Indoor Fan, Outdoor Air Damper
5 Heating Heating is active Indoor Fan, Heat Stages, Modulat-
ing Heat
6 Cooling Cooling is active Indoor Fan, Cool Stages (Fixed or
Modulating),
7 Dehum Dehumidification is active Indoor Fan, Cool Stages or Dehum
Output, Reheat Output
8 Emergency
Shutdown
One of the following is active: Smoke
Detected, Phase Loss Active, Fan Proof
Failure
None
Table 7-1 - DAC Operational Modes with Corresponding Site Manager Mode
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