30 November 1998
(2) During occupied and ventilation delay modes the supply fan and return fan shall operate
continuously. During unoccupied mode the supply fan and the return fan shall cycle according to the
night setback schedule. The fans 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 the return fan and initiate a high static
alarm when the static pressure exceeds the setpoint. When the fans are off, the inlet vanes shall be
(4) When the return fan starts, the DDC system shall modulate the return fan inlet vanes from
the signals of an air flow measurement station and transmitter in the return air ductwork, in combination
with an air flow measurement station and transmitter in the supply air ductwork, to maintain a constant
difference between supply air and return air flow rates as shown.
(5) A differential pressure switch across each filter shall initiate a filter alarm when the pressure
drop across the filter reaches the setpoint as shown.
(6) A freezestat, located as shown, shall stop the supply and return fans, 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.
(7) Smoke detectors in the supply air and return air ductwork shall stop the supply fan and the
return fan and initiate a smoke alarm if smoke is detected at either location. Restarting the supply fan
and the return fan shall require manual reset at the smoke detector.
(8) 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.
(9) Minimum Outside Air Flow Control. When the fans are 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.
(10) 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.
(11) The control damper of the VAV box shall modulate in response to the signal from a flow
sensing element at the discharge or inlet of the VAV box to a microprocessor based VAV box velocity
controller. The velocity controller shall control the box damper from the minimum flow position to the full
flow position from the signal of a space temperature sensing element located as shown. When the space