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7.4.2.1.2.
Staging Off
When the current supply air temperature drops
below the active supply air cooling setpoint minus the
Cool Stage Deadband, cooling stages will begin to
stage off. The highest stage that is currently on will be
turned off first. Additional stages will be turned off
according to the Cool Off-Delay parameter. A cooling
stage will not be turned off unless it has satisfied its
minimum run time.
7.4.2.2.
Modulating Cooling
Up to two cooling stages may be defined as
variable capacity (VS or digital scroll). Variable
capacity stages should be assigned the lowest
numbered cool stages. As with fixed capacity staging,
variable capacity stages will be activated if the supply
air temperature is above the active supply air cooling
setpoint. However, the next higher stage after a
variable capacity stage will start its On-Delay only
after the variable stage is at its maximum capacity.
Likewise, the next lower stage before a variable
capacity stage will start its Off-Delay only after the
variable stage is at its minimum capacity.
When a variable capacity compressor is activated,
it will modulate to maintain the supply air
temperature at the active supply air cooling setpoint.
A PID loop will be used to determine the capacity
output.
If the first two stages are both variable capacity
compressors, the lower stage will turn on first and
ramp up. When the second modulating stage is
activated, the first modulating stage will remain at its
maximum percent while the PID loop controls the
second modulating stage. Once the second
modulating stage has turned off, the first modulating
stage will be allowed to modulate again.
The variable capacity compressors have both
minimum and maximum capacity setpoints. In the
case of multiple modulating stages, all stages will use
the same minimum and maximum settings.
7.4.2.3.
Cooling OAT Lockout
Cooling will only be allowed when the outside air
temperature is above the cooling lockout temperature
setpoint. If the outside temperature falls below this
setpoint, cooling will be deactivated. Cooling will
only be re-enabled when the outside temperature rises
back above the lockout setpoint plus 2°F.
Discharge Air Controller (DAC) Manual
7.4.3. Heating Mode
The DAC supports both staged and modulating
mechanical heating devices. A modulating device,
such as a hot water valve, is treated as a heating stage
in the case of a unit having a mixture of fixed and
modulating heating sources.
7.4.3.1.
Staged Heating
Up to two stages of primary heating and two stages
of secondary heating can be defined for a total of four
heating stages. Lower operating cost heat stages, e.g.,
reclaim heat, should be connected to the primary heat
outputs on the controller. Higher operating cost heat
stages, e.g., gas or electric heat, should be connected
to the secondary heat outputs. The controller will
always activate all primary heat sources before
activating secondary heat sources.
7.4.3.1.1.
Staging On
When heating is required, the heat stages will
stage on if the supply air temperature is below the
active supply air heating setpoint. The lowest primary
heat stage that is off will be turned on first. Additional
primary stages will be brought on according to the
Heat On-Delay setpoint as long as the supply
temperature remains below the active supply
temperature heating setpoint. A heating stage will not
be turned on unless it has satisfied its minimum off
time. Once all primary heating stages are on,
secondary heating stages will begin to stage on in the
same manner as the primary heating stages.
7.4.3.1.2.
Staging Off
When the supply air temperature rises above the
active supply air heating setpoint plus the Heat Stage
Deadband, heating stages will begin to stage off. The
highest active secondary heating stage will be turned
off first. Additional secondary stages will be turned
off according to the Heat Off-Delay setpoint as long
as the supply temperature remains above the supply
temperature heating setpoint plus the Heat Stage
Deadband. A heating stage will not be turned off
unless it has satisfied its minimum run time as
specified by the parameter Heat Min On. Once all
secondary heating stages are off, primary heating
stages will begin to stage off in the same manner as
the secondary heating stages.
Software Overview • 17
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