30 November 1998
(12) Zone space temperature control. On a rising space temperature, each zone space
thermostat modulates the zone mixing damper toward closed to the hot deck and open to the cold deck.
(13) Unoccupied mode of operation. Throughout the unoccupied mode, the outside air and
relief air dampers and the cooling coil valve remain closed, and the return air damper remains open.
The supply and return fans are cycled by the system's night thermostat to maintain its low limit space
(14) Ventilation delay mode of operation. During the ventilation delay mode, the dampers
remain as they were throughout the unoccupied mode, and the supply and return fans run continuously.
Until the ventilation delay mode ends, the HVAC system circulates return air to bring the building to
comfort conditions, using a minimum of energy.
(15) Occupied mode of operation. The supply and return fans run continuously, and the outside
air and relief air dampers are at minimum position or are under mixed air temperature control as
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 reaching current-to-
pneumatic transducer IP-XX-01. 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, relay R-XX-01 is energized. 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. The contacts of relay R-XX-06
place the cold deck temperature controller in control of cooling coil valve VLV-XX-01. The outside air
and relief air dampers remain closed and the return air damper remains open.
(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,
in series with the now closed but normally open contacts of relays R-XX-01 and R-XX-05, energize relay
R-XX-03. The normally open contacts of relay R-XX-03 connect the output signal of high signal selector
TY-XX-01 to current-to-pneumatic transducer IP-XX-01. The pneumatic output of transducer IP-XX-01
places the dampers at the minimum position set on minimum position switch MPS-XX-01, unless relay
R-XX-07 is energized. Economizer EC-XX-01 controls the action of relay R-XX-07. The economizer
receives signals from outside air temperature transmitter TT-XX-02 and from return air transmitter
TT-XX-03. The difference between the return air temperature and the outside air temperature controls
the DEV contacts, and the outside 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 mixed air controller TC-XX-01 to high signal selector
TY-XX-01 to modulate the outside air and relief air dampers between minimum position and fully open
while modulating the return air damper in the opposite direction to maintain the mixed air temperature