30 November 1998
controller permits, the controller, through loop driver LD-XX-02, modulates the outside air and relief air
dampers from minimum position toward fully open and simultaneously modulates the return air damper
toward fully closed. On a still further rise, the controller, through loop driver LD-XX-01 and sequencer
SQCR-XX-01, successively energizes the stages of cooling. The reverse occurs on a fall in temperature.
(6) On a fall in temperature to its setpoint, low temperature protection thermostat TSL-XX-01
opens a set of closed contacts in the supply fan circuit, de-energizing the supply fan. A set of open
contacts in low temperature protection thermostat TSL-XX-01 closes, energizing relay R-XX-08 and
lighting low temperature pilot light PL-XX-05. The contacts of R-XX-08 energize relay R-XX-10. One set
of the contacts of relay R-XX-10 locks in relay R-XX-10, and the other set de-energizes the supply fan.
To restart the fans after a low temperature shutdown both the low temperature thermostat TSL-XX-01
and the control panel must be manually reset. The control panel is reset by momentarily depressing
manual switch HS-XX-02.
(7) When smoke detector SMK-XX-01 or smoke detector SMK-XX-02 detects the presence of
smoke, its normally closed contacts in the supply fan starting circuit open, de-energizing the supply fan.
The normally open contacts of the smoke detector(s) close, energizing relay R-XX-09 and lighting smoke
pilot light PL-XX-06. The contacts of relay R-XX-09 energize relay R-XX-10. One set of the contacts of
relay R-XX-10 closes to lock in R-XX-10, 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.
(8) 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.
(9) When the occupied contacts of time clock CLK-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.