30 November 1998
(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 relays R-XX-12 and R-XX-13. One set of the contacts of relay R-XX-12 locks in relays
R-XX-12 and R-XX-13, and the other set of contacts de-energizes the supply fan. The contacts of relay
R-XX-13 de-energize the return fan. To restart the fans, manual switch HS-XX-02 must be momentarily
depressed and then manual switch HS-XX-03 must be depressed.
(11) Air flow measuring station AFMA-XX-01 and flow transmitter FT-XX-01, located in the
supply air duct, and air flow measuring station AFMA-XX-02 and flow transmitter FT-XX-02, located in
the return air duct, send flow signals to flow controller FC-XX-01. The output of FC-XX-01, through the
contacts of relay R-XX-07, is sent to current-to-pneumatic transducer IP-XX-04. The pneumatic output of
IP-XX-04 modulates the return fan inlet vane dampers to maintain the constant differential air volume
between the supply and return fans set on controller FC-XX-01. When the fans are de-energized, relay
R-XX-07 is de-energized, its contacts open, breaking the control loop, and the return fan inlet vanes
(12) 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.
(13) 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, and, through the supply fan
auxiliary contacts and relay R-XX-05, in control of the return fan. On a fall in space temperature to 13
degrees C (55 degrees F), the contacts of TSL-XX-02 close, energizing both fans; on a rise in
temperature to 16 degrees C (60 degrees F), the contacts open, de-energizing the fans.
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 minimum
outside air damper under minimum outside air flow control and shall place the economizer 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 all
dampers shall return to their normal positions as shown.