30 November 1998
(12) Zone space temperature control. On a rise in temperature, each zone thermostat
modulates the zone dual-duct box damper actuator to maintain its setpoint.
(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) Time clock CLK-XX-01 has two independent sets of contacts, which between them determine
the mode of system operation. 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 current-to-pneumatic transducer
IP-XX-01. Thus, the dampers remain in their normal positions, with outdoor 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 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, energizing the return fan, and, with
the contacts of relay R-XX-01, energize relay R-XX-06. The contacts of relay R-XX-06 put the cold duct
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 time clock CLK-XX-01 open to end the ventilation
delay mode of operation, relay R-XX-04 is de-energized and pilot light PL-XX-02 is turned off. 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 return air
dampers between minimum position and fully open, while modulating the return air damper in the
opposite direction to maintain the mixed air temperature controller setpoint.