York HFC-410A manual Smoke Purge Sequence, LOW-AMBIENT/HEAD-PRESSURE Control Operation

FORM 100.50-EG5 (108)

Controls (continued)

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.

SMOKE PURGE SEQUENCE

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%

Powered, Variable-Volume Exhaust-VFD Controlled

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.

LOW-AMBIENT/HEAD-PRESSURE CONTROL OPERATION

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 first 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.

•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.

VAV SPECIFIC SEQUENCES

SUPPLY FAN OPERATION

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 specified 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 efficiency for the unit by eliminating the losses (air-pressure drop) typical of inlet guide vane systems.

CV SPECIFIC SEQUENCES

COOLING OPERATION

Thermostat Control

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”

30 JOHNSON CONTROLS