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• When the current saturated liquid line tem-
perature does not rise above the stage down
setpoint within 60 seconds of initial start-up,
a condenser fan is stopped
• This process repeats every 60 seconds un-
til the saturated liquid line temperature rises
above the stage down setpoint, or all con-
denser fans have been turned off
• When the saturated liquid line temperature
falls below the low critical value for more than
2 seconds, all condenser fans are immediate-
ly de-energized
After the saturated liquid line temperature returns to a
value between the stage up and stage down setpoints,
normal condenser fan operation resumes.
When the saturated liquid line temperature sensor val-
ue becomes invalid, the condenser fans operate in the
fail safe mode.
Fail Safe Mode
One condenser fan is started per circuit for every com-
pressor started on that circuit.
• When the OAT rises to 115.0°F or greater, all con-
denser fans on a circuit are started regardless of
the number of compressors operating
• When the OAT drops below 110.0°F, condenser
fan operation returns to one fan per compressor
per circuit
• When a VFD-controlled fan is installed, it is the
first ON and last OFF and operates at 100%
Low Ambient Operation
This sequence is only available during saturated liquid
line temperature control (see section on page 48).
Low ambient operation must be user enabled for every
compressor system that has a condenser fan VFD to
be operational.
During operation of any compressor system that has
a low ambient option, the unit controller continuously
monitors the saturated liquid line temperature.
After the saturated liquid line temperature reaches
80.0°F, the unit controller sends a signal to the con-
denser fan VFD to turn ON the fan at 15% (nonadjust-
able).
After the saturated liquid line temperature reaches
110.0°F, the condenser fan VFD speed is 100%.
YORK
Start-Up & Operation Guide | 5587001-YSG-A-520
At any saturated liquid line temperature between 80–
110.0°F, the condenser fan VFD speed is somewhere
between 15% and 100%.
When low ambient operation is NOT enabled on a cir-
cuit, the lower range of the OAT cooling enable set-
point is 45.0°F.
When low ambient operation IS enabled on a circuit,
the lower range of the OAT cooling enable setpoint is
-10.0°F.
Low ambient operation cannot be enabled on a circuit
unless there is a variable speed condenser fan and liq-
uid line pressure sensor installed on that circuit.
NOTE: When the OAT is below 40.0°F, the modulat-
ing condenser fan may be commanded OFF.
When the OAT rises above 45.0°F and a com-
pressor is running, the modulating condenser
fan is commanded ON. When the OAT requires
the modulating condenser fan to be ON and no
other condenser fans are operating, the modu-
lating condenser fan cannot be allowed to go
below 15%.
Dehumidification (Hot Gas Reheat)
The rooftop unit can have an optional hot gas reheat
(HGRH) system factory installed. HGRH is utilized to
help dehumidify the interior space.
• This is accomplished by controlling the compres-
sors to the evaporator leaving air setpoint, which
is typically set lower than the active DAT cooling
setpoint
• The colder air allows for more moisture removal
from the discharge air
• The discharge air then passes through the HGRH
coil, which adds heat to the discharge air
• Reheating the air allows dehumidification of the
space while preventing over-cooling, which keeps
building personnel comfortable
HGRH operation is not permitted below 55.0°F OAT.
The HGRH system is always on refrigeration circuit
#2. The HGRH system uses a 3-way modulating valve
controlled by a stepper motor.
The HGRH coil is located after the evaporator coil in
the airstream.
There is a solenoid installed on the outlet of the HGRH
coil. This HGRH bleed solenoid valve allows excess
refrigerant and oil to bleed back into the suction line
when HGRH operation is not needed.
49
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