York HFC-410A Manual Do Utilizador

JOHNSON CONTROLS

FORM 100.50-EG5 (108)

proportionally controlled open and the Exhaust Fan is 
controlled ON. If the Building Pressure falls, the Exhaust 
Damper is proportionally controlled closed and the Ex-
haust Fan is controlled OFF. The position of the Exhaust 
Damper in which the Exhaust Fan is controlled ON and 
OFF as well as the Building Pressure setpoint are user- 
selectable from the Rooftop Unit User Interface.

This optional variable-volume, powered-exhaust system 
consists of an Exhaust Fan driven by a Variable Fre-
quency Drive (VFD), which is controlled by the Rooftop 
Unit controller. The Rooftop Unit controller monitors the 
pressure within the building. As the pressure rises, the 
VFD is controlled to increase Exhaust Fan speed. As 
the pressure falls, the VFD is controlled to decrease 
Exhaust Fan speed. The Building Pressure Setpoint is 
user-selectable from the Rooftop Unit User Interface. 
On/Off control is maintained the same as Exhaust-Dis-
charge Damper control stated above.

The Rooftop Unit controller continuously monitors the 
outside air temperature to determine if mechanical cool-
ing should be allowed. As a safety, if the Outside Air 
temperature falls to or below the Low Ambient Lockout 
temperature, mechanical cooling is prevented from 
operating.

For units with economizers, the Low Ambient Lockout 
temperature is typically low enough that mechanical 
cooling will rarely be required. However, for some ap-
plications, mechanical cooling is required when the 
Outside Air temperature is lower than the Low Ambient 
Lockout temperature.

For these applications, the unit must be equipped with 
optional Low Ambient controls. For optional Low Ambi-
ent operation, the Rooftop Unit controller monitors the 
refrigeration-system discharge pressure and controls the 
speed of the fi rst stage condenser fan. If the discharge 
pressure falls, the speeds of the condenser fan is re-
duced to maintain acceptable condensing pressures in 
the refrigeration system. With the optional Low Ambient 
controls, mechanical cooling is allowed down to Outside 
Air temperatures of 0°F.

A contact closure input (PURGE) is provided to place the 
unit in smoke purge mode. When the contact is closed 
the unit will operate as follows:

  •  Turn off all heating and cooling operation

  •  Set the outdoor air damper output to 100%

  •  Close the return to 0%

  •  Turn the supply fan on

  •  On VAV unit set the supply fan output to 100%.

  •  Turn the power exhaust fan on

  •  On VFD driven exhaust fans set the exhaust fan 

output to 100%

Note that 24 volts terminal (R) on the Simplicity control 
board must be used as the 24 Volt AC source for switch 
the contact to the Unit Controller Smoke Purge (PURGE) 
input. Use of any power source external to the controller 
will result in damage to the Unit Controller.

For VAV units, the supply fan is controlled ON and OFF 
based on the occupancy state. When the unit goes 
into the Occupied mode of operation, the Rooftop Unit 
controller will monitor the static pressure within the
supply-duct system and control the speed of the supply 
fan to maintain a specifi ed Duct Static Pressure setpoint. 
A Variable Frequency Drive (VFD) is used on all VAV 
units to vary the speed of the supply fan. Note, the use 
of a VFD in lieu of inlet guide vanes provides for higher 
energy effi ciency for the unit by eliminating the losses 
(air-pressure drop) typical of inlet guide vane systems.

If a 7-wire thermostat (2 Cool/2 Heat) controls the unit, 
all zone-temperature setpoint-control is maintained at 
the thermostat. With this operation, the unit remains idle 
until it receives a stage call from the thermostat. If “G”