30 November 1998
occupied mode, the ventilation delay contacts close, energizing relay R-XX-04 and lighting ventilation
delay pilot light PL-XX-02. The closed contacts of relay R-XX-04 open, preventing relay R-XX-03 and
electrically actuated pneumatic valve EP-XX-01 from being energized. The open contacts of relay
R-XX-03 interrupt the signal to IP-XX-03 keeping the humidifier valve closed. When EP-XX-01 is de-
energized, the damper remains closed.
(2) When the occupied contacts of timeclock CLK-XX-01 close, relays R-XX-01 and R-XX-02 are
energized and pilot light PL-XX-01 is turned on. The contacts of relay R-XX-01 energize the supply fan.
The auxiliary contacts of the supply fan starter energize relay R-XX-05, and since R-XX-01 is already
energized it also energizes relay R-XX-06. The contacts of relay R-XX-05 energize the pump starter, and
the contacts of relay R-XX-06 place the cooling coil valve under space temperature / humidity control.
(3) When the ventilation delay contacts of timeclock CLK-XX-01 open to end the ventilation
delay mode of operation, relay R-XX-04 is de-energized. The normally closed contacts of relay R-XX-04
close, energizing relay R-XX-03 and pneumatic valve EP-XX-01. Pneumatic valve EP-XX-01 opens the
outdoor air damper. The contacts of relay R-XX-03 place the humidifier valve under control as described
(4) On a rise in differential pressure to its setpoint, differential pressure switch DPS-XX-01 or
DPS-XX-02 lights the filter pilot light PL-XX-03.
(5) On a fall in temperature to its setpoint, the closed contacts of low temperature protection
thermostat TSL-XX-01 open, de-energizing the fan; its open contacts close, energizing relay R-XX-07
and lighting low temperature pilot light PL-XX-04. The contacts of relay R-XX-07 energize shutdown
relay R-XX-09. One set of the contacts of relay R-XX-09 closes to lock relay R-XX-09 in, and the other
set de-energizes the supply fan. To restart the fan, the thermostat and the control panel both must be
reset. Momentary switch HS-XX-02 is used to reset the panel.
(6) When either smoke detector SMK-XX-01 or 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-08 and lighting smoke pilot
light PL-XX-05. The contacts of relay R-XX-08 energize relay R-XX-09. One set of the contacts of relay
R-XX-09 close to lock in R-XX-09, and the other set of contacts, in the supply fan starter circuit, opens to
de-energize the fan. 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) Whenever the pump runs, the auxiliary contacts of the pump starter energize relay R-XX-10
and light heating pilot light PL-XX-06. The contacts of relay R-XX-10 place heat exchanger valve
VLV-XX-05 under control.
(8) Temperature transmitter TT-XX-01 signals the preheat coil discharge temperature to
temperature controller TC-XX-01. The output of TC-XX-01 is received by current-to-pneumatic
transducer IP-XX-01. The pneumatic output of IP-XX-01 modulates the preheat coil valve VLV-XX-01 to
maintain the setpoint of controller TC-XX-01.
(9) Space temperature transmitter TT-XX-02 signals the space temperature to temperature
controller TC-XX-02. The output of TC-XX-02 is received by current-to-pneumatic transducer IP-XX-04
and high signal selector TY-XX-01. The pneumatic output of current-to-pneumatic transducer IP-XX-04
modulates the reheat coil valve VLV-XX-03. High signal selector TY-XX-01 passes the higher of the
temperature signal and the humidity signal (next paragraph) during the ventilation delay and occupied