30 November 1998
supply fan. To restart the fans after a smoke alarm shutdown, smoke detectors SMK-XX-01 and/or
SMK-XX-02 and the control panel must be manually reset. The control panel is reset by momentarily
depressing manual switch HS-XX-03.
(6) 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 filter pilot light PL-XX-04.
(7) Differential pressure gauge DPI-XX-03 across the outside air filter provides local indication of
filter loading. On a rise in pressure drop across the filter to its setpoint, differential pressure switch
DPS-XX-03 turns on filter pilot light PL-XX-08.
(8) Temperature controller TC-XX-02, with its temperature transmitter TT-XX-04 in the unit
discharge, through the contacts of relay R-XX-06 and current-to-pneumatic transducer IP-XX-02
modulates cooling coil valve VLV-XX-01 to maintain its temperature setpoint. The contacts of relay
R-XX-06 are closed during the ventilation delay and occupied modes, allowing control. The contacts are
open during the unoccupied mode, the cooling coil valve control signal is interrupted, and the valve
(9) A pressure sensing element and transmitter DPT-XX-01 located in the supply duct (the
location is determined by field conditions) signals duct static pressure to controller PC-XX-01. Whenever
the supply fan runs, the auxiliary contacts of the fan starter energize relay R-XX-07. The output of
pressure controller PC-XX-01 is sent, through the contacts of relay R-XX-07, to current-to-pneumatic
transducer IP-XX-03. The pneumatic output of the transducer modulates the supply fan inlet vane
actuator DA-XX-04 to maintain the pressure controller setpoint. When the fan is de-energized, relay
R-XX-07 is de-energized, its contacts open, and the inlet vane dampers remain closed.
(10) When the setpoint of high limit static pressure switch DPS-XX-02 in the supply fan
discharge is exceeded, its contacts close, energizing relay R-XX-11 and lighting high-static pilot light
PL-XX-07. One set of the contacts of relay R-XX-11 locks in relay R-XX-11, and another set of contacts
energizes relay R-XX-12. One set of the contacts of relay R-XX-12 locks in relay R-XX-12, and the other
set of contacts de-energizes the supply fan. To restart the fan, manual switch HS-XX-02 must be
momentarily depressed and then manual switch HS-XX-03 must be depressed.
(11) Temperature sensing element TE-XX-XX and air flow sensing element FE-XX-XX signal the
VAV box controller to modulate reheat coil valve VLV-XX-XX and VAV box damper actuator DA-XX-XX.
On a rising space temperature, the VAV box controller first gradually closes VLV-XX-XX and then, after
the space temperature passes through a dead band, gradually opens the VAV box beyond minimum air
flow position to maximum air flow position to maintain setpoint.
(12) 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 supply 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