Bryant F - Manual (Page 68)

68
When free cooling is available as determined by the
appropriate changeover command (dry bulb, outdoor
enthalpy, differential dry bulb or differential enthalpy), a
call for cooling (Y1 closes at the thermostat) will cause
the economizer control to modulate the dampers open and
closed to maintain the unit supply air temperature at 50 to
55_F. Compressor will not run.
During free cooling operation, a supply air temperature
(SAT) above 50_F will cause the dampers to modulate
between Minimum Position setpoint and 100% open. With
SAT from 50_F to 45_F, the dampers will maintain at the
Minimum Position setting. With SAT below 45_F, the
outside air dampers will be closed. When SAT rises to
48_F, the dampers will re-open to Minimum Position
setting.
Should 100% outside air not be capable of satisfying the
space temperature, space temperature will rise until Y2 is
closed. The economizer control will call for compressor
operation. Dampers will modulate to maintain SAT at 50
to 55_F concurrent with Compressor 1 operation. The
Low Ambient Lockout Thermostat will block compressor
operation with economizer operation below 42_F outside
air temperature.
When space temperature demand is satisfied (thermostat
Y1 opens), the dampers will return to Minimum Damper
position if indoor fan is running or fully closed if fan is
off.
If accessory power exhaust is installed, the power exhaust
fan motors will be energized by the economizer control as
the dampers open above the PE-On setpoint and will be
de--energized as the dampers close below the PE-On
setpoint.
Damper movement from full closed to full open (or vice
versa) will take between 1-1/2 and 2-1/2 minutes.
Heating With EconoMi$er IV
During Occupied mode operation, indoor fan operation
will be accompanied by economizer dampers moving to
Minimum Position setpoint for ventilation. If indoor fan is
off, dampers will close. During Unoccupied mode
operation, dampers will remain closed unless a DCV
demand is received.
When the room temperature calls for heat (W1 closes), the
heating controls are energized as described in Heating,
Unit Without Economizer above.
Demand Controlled Ventilation
If a field-installed CO
2
sensor is connected to the
Economize IV control, a Demand Controlled Ventilation
strategy will operate automatically. As the CO2 level in
the space increases above the setpoint (on the EconoMi$er
IV controller), the minimum position of the dampers will
be increased proportionally, until the Maximum
Ventilation setting is reached. As the space CO
2
level
decreases because of the increase in fresh air, the
outdoor-damper will follow the higher demand condition
from the DCV mode or from the free-cooling mode.
DCV operation is available in Occupied and Unoccupied
periods with EconoMi$er IV. However, a control
modification will be required on the 580J unit to
implement the Unoccupied period function.
Supplemental Controls
Compressor Lockout Relay (CLO) -- The CLO is available
as a factory-installed option or as a field--installed
accessory. Each compressor has a CLO. The CLO
compares the demand for compressor operation (via a
24-v input from Y at CLO terminal 2) to operation of the
compressor (determined via compressor current signal
input at the CLO’s current transformer loop); if the
compressor current signal is lost while the demand input
still exists, the CLO will trip open and prevent the
compressor from restarting until the CLO has been
manually reset. In the lockout condition, 24-v will be
available at terminal X. Reset is accomplished by
removing the input signal at terminal 2; open the
thermostat briefly or cycle the main power to the unit.
Phase Monitor Relay (PMR) -- The PMR protects the unit
in the event of a loss of a phase or a reversal of power line
phase in the three--phase unit power supply. In normal
operation, the relay K1 is energized (contact set closed)
and red LED indicator is on steady. If the PMR detects a
loss of a phase or a phase sequence reversal, the relay K1
is energized, its contact set is opened and unit operation is
stopped; red LED indicator will blink during lockout
condition. Reset of the PMR is automatic when all phases
are restored and phase sequence is correct. If no 24--v
control power is available to the PMR, the red LED will
be off. Smoke Detectors -- Factory--installed smoke
detectors are discussed in detail starting on page 17.
RTU--MP Sequence of Operation
The RTU--MP will control the compressor, economizer
and heating outputs based on its own space temperature
input and setpoints. An optional CO
2
IAQ sensor mounted
in the space can influence the economizer minimum
position. The RTU--MP has its own hardware clock that is
set automatically when the software is installed on the
board. The RTU--MP’s default is to control to occupied
setpoints all the time, until a type of occupancy control is
set. Occupancy types are described in the scheduling
section. The following sections describe the operation for
the functions of the RTU--MP. All point objects that are
referred to in this sequence will be in reference to the
objects as viewed in BACview 6 Handheld.
580J