30 November 1998
(10) Unoccupied mode of operation. Throughout the unoccupied mode, the outside air and
relief air dampers remain closed, and the return air damper remains open. The heating coil valve
remains under space temperature control. The cooling stages are de-energized. The supply fan is
cycled by the system's night thermostat to maintain its low limit space temperature setpoint.
(11) Ventilation delay mode of operation. During the ventilation delay mode, the dampers
remain as they were throughout the unoccupied mode, and the supply fan runs continuously. The
cooling system is enabled. Until the ventilation delay mode ends, return air is circulated, to bring the
building to comfort conditions, using a minimum of energy.
(12) Occupied mode of operation. The supply fan runs continuously, and the outside air and
relief air dampers are at minimum position or are under space temperature control as previously
c. Detailed sequence of operation.
(1) Timeclock CLK-XX-01 has two independent sets of contacts, which between them determine
the mode under which the system operates. Five minutes before the scheduled beginning of the
occupied mode, the ventilation delay contacts close, energizing relay R-XX-04 and lighting pilot light
PL-XX-02. The normally closed contacts of relay R-XX-04 open, preventing relay R-XX-03 from being
energized. The normally open contacts of relay R-XX-03 prevent any signal from reaching electric
damper actuators DA-XX-01, DA-XX-02, and DA-XX-03. Thus, the dampers remain in their normal
positions, with outside air and relief air dampers closed and return air damper open.
(2) When the timeclock's occupied contacts close, relays R-XX-01 and R-XX-02 are energized,
and occupied pilot light PL-XX-01 is turned on. The contacts of relay R-XX-02 energize the supply fan.
The auxiliary contacts of the supply fan starter energize relay R-XX-05 and, with the contacts of relay
R-XX-01, energize relay R-XX-06, which places the space temperature controller TC-XX-01 in control of
cooling sequencer SQCR-XX-01.
(3) When the ventilation delay contacts of timeclock CLK-XX-01 open to end the ventilation
delay mode, relay R-XX-04 is de-energized and pilot light PL-XX-02 is turned off. The contacts of relay
R-XX-04 energize relay R-XX-03. The contacts of relay R-XX-03 close between high signal selector
TY-XX-01 and the electric damper actuators. This allows the economizer dampers to assume the
position set on minimum position switch MPS-XX-01 unless relay R-XX-07 is energized.
(4) Economizer controller EC-XX-01 receives signals from outside air temperature transmitter
TT-XX-01 and from return air temperature transmitter TT-XX-02. The difference between the return air
temperature and the outside air temperature controls the DEV contacts, and the return air temperature
controls the PV contacts. When both these contacts are closed, relay R-XX-07 is energized and pilot
light PL-XX-03 is turned on. When relay R-XX-07 is energized, its contacts connect the output of space
temperature controller TC-XX-01 to high signal selector TY-XX-01, to modulate the outside air and return
air dampers between minimum position and fully open while modulating the return air damper in the
opposite direction to maintain the temperature controller setpoint.
(5) Space temperature transmitter TT-XX-03 signals the space temperature to proportional only
space temperature controller TC-XX-01. The TC-XX-01 output through loop drivers LD-XX-01,
LD-XX-02, and LD-XX-03 controls the cooling sequencer, the economizer dampers, and the heating coil
valve respectively. On a rise in space temperature, controller TC-XX-01 through loop driver LD-XX-03
modulates heating coil valve VLV-XX-01 closed. On a further temperature rise, if the economizer