30 November 1998
relay R-XX-09 closes to lock in R-XX-09, and the other set of contacts, in the supply fan starter circuit,
opens. To restart the fan, the smoke detectors must be manually reset and the HVAC panel must also
be reset by depressing momentary switch HS-XX-02.
(7) Differential pressure gauge DPI-XX-01 across the filter provides local indication of filter
loading. On a rise in pressure drop across the filter to its setpoint, differential pressure switch
DPS-XX-01 turns on pilot light PL-XX-04.
(8) When the occupied contacts of timeclock TC-XX-01 open to end the occupied mode and
index the system to the unoccupied mode, relays R-XX-01 and R-XX-02 are de-energized and pilot light
PL-XX-01 is turned off. The contacts of relay R-XX-02 open, de-energizing the supply fan and placing
the system's night thermostat TSL-XX-02 in control of the supply fan. On a fall in space temperature to
13 degrees C (55 degrees F), the contacts of TSL-XX-02 close, energizing the fan; on a rise in
temperature to 16 degrees C (60 degrees F), the contacts open, de-energizing the fan.
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 space 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 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) 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 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. 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
initiate a smoke alarm if smoke is detected at either location. Restarting the supply fan shall require
manual reset at the smoke detector.
(6) 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,