30 November 1998
(13) Ventilation delay mode of operation. During the ventilation delay mode, the dampers
remain as they were throughout the unoccupied mode, and the supply air and return air fans run
continuously. Until the ventilation delay mode ends, return air is circulated, to bring the building to
comfort conditions using a minimum of energy.
(14) Occupied mode of operation. The supply air and return air 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 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, 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 energize relay R-XX-05 and, with contacts of relay R-XX-01, energize
relay R-XX-06. The contacts of relay R-XX-06 places 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 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 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 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 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 temperature controller setpoint.
(4) On a fall in temperature to its setpoint, low temperature protection thermostat TSL-XX-01
opens a set of closed contacts in the supply fan circuit, de-energizing the supply fan. The auxiliary
contacts of the supply fan starter de-energize relay R-XX-05, and the contacts of relay R-XX-05 in the
return fan starter circuit de-energize the return fan. A set of open contacts in low temperature protection
thermostat TSL-XX-01 close, energizing relay R-XX-08 and lighting pilot light PL-XX-05. The contacts of
relay R-XX-08 energize relays R-XX-10 and R-XX-11. One set of the contacts of relay R-XX-10 locks in
relays R-XX-10 and R-XX-11, and another set de-energizes the supply fan. The contacts of relay
R-XX-11 de-energize the return fan. To restart the fans after a low temperature shutdown, both the low