Carrier 50HCQ04---12 - Manual (Page 18)

18
Cooling
The compressor outputs are controlled by the Cooling
Control PID Loop and Cooling Stages Capacity algorithm.
They will be used to calculate the desired number of
stages needed to satisfy the space by comparing the Space
Temperature (SPT) to the Occupied Cool Setpoint plus the
T56 slider offset when occupied and the Unoccupied Cool
Setpoint (UCSP) plus the T56 slider offset, if unoccupied.
The economizer, if available, will be used for cooling in
addition to the compressors. The following conditions
must be true in order for this algorithm to run:
S Indoor Fan has been ON for at least 30 seconds.
S Heat mode is not active and the time guard between
modes equals zero.
S If occupied and the SPT > (occupied cool setpoint plus
the T56 slider offset).
S Space Temperature and supply air temperature values
are available.
S If it is unoccupied and the SPT > (unoccupied cool
setpoint plus the T56 slider offset). The indoor fan will
be turned on by the staging algorithm.
S If economizer is available and active and economizer
open > 85% and SAT > (SAT low limit + 5_F) and SPT
> effective set point + 0.5_F.
OR
Economizer is available, but not active
OR
Economizer is not available
S OAT > DX Lockout temperature
If all of the above conditions are met, the controller uses a
PID logic loop to energize or de--energize compressors to
satisfy the cooling demand.
There is a fixed 3--minute minimum on time and a
5--minute off time for each compressor output and a
3--minute minimum time delay between staging up or
down.
Any time the compressors are running the RTU--OPEN
will stage down the compressors if the SAT becomes less
than the cooling low supply air setpoint. After a
compressor is staged off, it may be started again after a
normal time--guard period and the supply air temperature
has increased above the low supply air setpoint.
Economizer
The Economizer dampers are used to provide free cooling
and Indoor Air Quality, if optional CO
2
sensor is installed,
when the outside conditions are suitable.
The following conditions must be true for economizer
operation:
S Indoor Fan has been on for at least 30 seconds.
S Enthalpy is Low if the Enthalpy input is enabled.
S SAT reading is available.
S OAT reading is available.
S SPT reading is available.
S OAT <= High OAT economizer lockout configuration
(default = 75).
S OAT <= SPT
If any of the mentioned conditions are not true, the
economizer will be set to its configured minimum
position. The minimum damper position can be
overridden by the IAQ routine described later in this
section.
If the above conditions are true, the Economizer control
will calculate a damper position starting with a minimum
vent position and based on current space temperature and
a setpoint that is halfway between the effective cool and
heat setpoints. If the SAT drops below the cooling low
supply air setpoint (+ 5_F), the economizer will ramp
down to minimum position.
Power Exhaust
If RTU--OPEN is also controlling an exhaust fan, it can be
enabled based on damper position or by occupancy. If
configured for continuous occupied operation, it will be
energized whenever the controller is in the occupied mode
and disabled when in the unoccupied mode. If configured
for damper position control, it will be energized whenever
the economizer exceeds the power exhaust setpoint and
disabled when the economizer drops below the setpoint by
a fixed hysteresis of 10%.
Heating
The heat outputs are controlled by the Heating Control
PID Loop and Heating Stages Capacity algorithm. They
will be used to calculate the desired number of stages
needed to satisfy the space by comparing the SPT to the
Occupied Heat Setpoint the T56 slider offset when
occupied and the Unoccupied Heat Setpoint plus the T56
slider offset if unoccupied. The following conditions must
be true in order for this algorithm to run:
S Indoor Fan has been ON for at least 30 seconds.
S Cool mode is not active and the time guard between
modes equals zero.
S If occupied and SPT <(occupied heat setpoint plus T56
slider offset)
S SPT and supply air temperature values are available
S If it is unoccupied and the SPT < (unoccupied heat
setpoint plus T56 slider offset). The indoor fan will be
turned on by the staging algorithm.
S OAT < High OAT lockout temperature.