CEMP-E
TI 810-11
30 November 1998
d. Sequence of operation for DDC applications.
(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 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 the
dampers shall return to their normal positions as shown.
(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) A differential pressure switch across the filter shall initiate a filter alarm when the pressure
drop across the filter exceeds the setpoint as shown.
(4) 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. Return to the normal mode of operation
shall require manual reset at the freezestat. The DDC panel shall monitor the freezestat through
auxiliary contacts and shall indicate an alarm condition when the freezestat trips.
(5) 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 fans shall
require manual reset at the smoke detector.
(6) The DDC system shall modulate the heating coil control valve from the signal of a
temperature sensing element and transmitter located in the discharge air of the coil to maintain the hot
deck temperature setpoint. A temperature sensing element and transmitter in the outside air intake shall
reset the hot deck temperature setpoint with respect to the outside air temperature signal in a linear
schedule as shown.
(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 cold deck temperature setpoint as shown. During the unoccupied
mode, the cooling coil control valve shall remain closed.
(8) 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. The DDC system
shall perform switch over between outside air economizer control mode and minimum outside air mode.
Until the outside air temperature rises above the setpoint, the DDC system shall hold the system in the
minimum outside air mode. When the outside air temperature rises above the setpoint, the DDC system
shall place the AHU in the economizer mode or in the minimum outside air mode as determined by a
comparison of the outside air and return air temperatures in accordance with the differential temperature
setpoints as shown. When the outside air temperature is low with respect to the return air temperature,
the AHU shall be in the economizer mode. When the DDC system places the control system in the
minimum outside air mode, the outside air damper shall be open to the minimum outdoor air setting.
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