30 November 1998
modes to current-to-pneumatic transducer IP-XX-02. The pneumatic signal of IP-XX-02 modulates the
cooling coil valve VLV-XX-02 to maintain the space temperature or humidity setpoint.
(10) Space humidity transmitter RHT-XX-02 signals the space relative humidity to relative
humidity controller RHC-XX-02. The output of relative humidity controller RHC-XX-02 is transmitted to
low signal selector RHY-XX-01 and inverter INV-XX-01. Low signal selector RHY-XX-01 also receives a
signal from unit discharge high limit humidity controller RHC-XX-01, (must be proportional only) which
receives unit discharge relative humidity signals from unit discharge relative humidity transmitter
RHT-XX-01. The output of RHY-XX-01, during the occupied mode only, is received by current-to-
pneumatic transmitter IP-XX-03, and the pneumatic output of IP-XX-03 modulates humidifier steam
valve VLV-XX-04. The signal from controller RHC-XX-02 to inverter INV-XX-01 is reversed and sent to
high signal selector TY-XX-01. The signal from space temperature controller TC-XX-02, (previous
paragraph) and the signal from space relative humidity controller RHC-XX-02, after inversion are
compared, and the higher signal, during the ventilation delay and occupied modes, is sent to current-to-
pneumatic transducer IP-XX-02. The pneumatic signal of IP-XX-02 modulates the cooling coil valve
VLV-XX-02 to maintain the space temperature or humidity setpoint.
(11) Temperature transmitter TT-XX-03 in the heat exchanger discharge, signals the hot water
supply temperature to temperature controller TC-XX-03. Whenever the pump runs, relay R-XX-10 is
energized, and, through the contacts of R-XX-10, the output of TC-XX-03, through current-to-pneumatic
transducer IP-XX-05, modulates heat exchanger valve VLV-XX-05 to maintain the TC-XX-03 temperature
setpoint. When the pump is de-energized, the contacts of relay R-XX-10 open, and the valve closes.
(12) When the occupied contacts of CLK-XX-01 open to end the occupied mode and index the
system to the unoccupied mode, relay R-XX-01 is de-energized and pilot light PL-XX-01 is turned off.
The contacts of relay R-XX-01 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 outside air damper shall remain closed. 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 open the outside air damper. At the time shown, the DDC system shall
place the control system in the unoccupied mode of operation and the outside air damper shall close.
(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 damper to
close, 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