30 November 1998
(10) Unoccupied mode of operation. During the unoccupied mode throughout the heating
season, the supply fan is cycled by the night thermostat to maintain its low limit space temperature
setpoint, the outside air and relief air dampers remain closed, and the return air damper remains open.
During the unoccupied mode in the cooling season, the unit remains de-energized.
(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. Until the
ventilation delay mode ends, return air is circulated, to bring the building to comfort conditions using a
minimum of energy.
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 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, 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.
(3) When the ventilation delay contacts of time clock 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, 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 contacts of relay R-XX-03 close between high signal selector TY-XX-01
and current-to-pneumatic transducer IP-XX-01; this allows the economizer dampers to assume the
position set on minimum position switch MPS-XX-01 unless relay R-XX-06 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-06 is energized and pilot
light PL-XX-03 is turned on. When relay R-XX-06 is energized, its contacts connect the output of space
temperature controller TC-XX-02 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 controllers
TC-XX-01 and TC-XX-02. Controllers TC-XX-01 and TC-XX-02 are proportional only controllers.
Temperature setpoint device TSP-XX-01 provides the means for adjusting the setpoints of space zone
temperature controllers, TC-XX-01 and TC-XX-02. On a rise in temperature during the heating season
when hot water is being supplied to the dual-temperature coil, space temperature controller TC-XX-02
modulates the dual-temperature coil valve VLV-XX-01 toward closed. On a further rise in space
temperature through the temperature deadband, the controller is inactive. On a still further rise in space
temperature, economizer controller permitting, controller TC-XX-02 modulates the outside air and relief
air dampers from the minimum position set on minimum position switch MPS-XX-01 toward fully open,
and simultaneously modulates the return air damper toward closed.