30 November 1998
(1) Ventilation delay mode timing shall start prior to the occupied mode timing. During
ventilation delay mode, the dampers shall remain in their normal positions as shown, except when under
economizer control. At the time shown, the DDC system shall place the system in the occupied mode.
At the expiration of the ventilation delay mode timing period, the DDC system shall place the minimum
outside air damper under minimum outside air flow control and shall place the economizer outside air,
return air, and relief air dampers under mixed air temperature and economizer control. At the time
shown, the DDC system shall place the control system in the unoccupied mode of operation and all
dampers shall return to their normal positions as shown.
(2) During occupied and ventilation delay modes the supply fan shall operate continuously.
During unoccupied mode the supply fan shall cycle according to the night setback schedule. The fan
shall start and stop at the setpoints as shown.
(3) When the supply fan starts, the DDC system shall modulate the inlet vanes from the signal
of a static pressure sensing element and transmitter to maintain the setpoint as shown. A high limit static
pressure switch in the fan discharge shall stop the supply fan and initiate a high static alarm when the
static pressure exceeds the setpoint. When the fan is off, the inlet vanes shall be closed.
(4) A differential pressure switch across each filter shall initiate a filter alarm when the pressure
drop across the filter reaches the setpoint as shown.
(5) A freezestat, located as shown, shall stop the supply fan, cause the outside air, return air,
and relief air dampers to return to their normal position, and shall initiate a low temperature alarm if the
temperature drops below the freezestat's setpoint as shown. The DDC system shall monitor the
freezestat through auxiliary contacts and shall indicate an alarm condition when the freezestat trips.
(6) Smoke detectors in the supply air and return air ductwork shall stop the supply fan and
initiate a smoke alarm if smoke is detected at either location. Restarting the supply fan shall require
manual reset at the smoke detector.
(7) During occupied and ventilation delay modes, the cooling coil control valve shall be
modulated by the DDC system from the signal of a temperature sensing element and transmitter located
in the coil discharge air to maintain the setpoint as shown. During unoccupied mode, cooling coil control
valve shall remain closed.
(8) Minimum Outside Air Flow Control. When the fan is on, with the control system in the
occupied mode, and with the ventilation delay mode off, the minimum outside air damper shall be
modulated to maintain the minimum outside air flow at setpoint, as sensed by an air flow measurement
station located in the minimum outside air duct.
(9) The DDC system shall accept the signal of an outside air temperature sensing element and
transmitter and the signal of a return air temperature sensing element and transmitter. When the return
air temperature is above the economizer setpoint, and the outside air temperature is sufficiently below
the return air temperature to be effective for cooling, the DDC system shall place the AHU in the
economizer mode by modulating the economizer outside air, relief air, and the return air dampers to
maintain the mixed air temperature at setpoint. As the economizer outside air and relief air dampers
open, the return air damper closes. When the system is not in economizer mode, the economizer
outside air and relief air dampers shall remain closed and the return air damper shall remain open.