CEMP-E
TI 810-11
30 November 1998
the cooling coil valve to maintain its setpoint temperature. During the unoccupied mode, the cooling coil
valve is not controlled and remains closed.
(13) Space temperature control. Each controlled space is equipped with a variable air volume
(VAV) box, which is controlled by a microprocessor based VAV box controller. The controller receives
temperature signals from a temperature sensing element in the space served and from a flow sensor
upstream of the VAV box. On a fall in space temperature, the controller modulates the damper toward
minimum position, to maintain the cooling mode setpoint. After the minimum position is reached, the
controller is inactive while the space temperature falls through a temperature deadband. On a further fall
in temperature below the temperature deadband, the heating coil valve modulates to maintain the
heating setpoint.
(14) 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 in unison by the system's night thermostat to maintain its low limit
space temperature setpoint.
(15) 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, return air is recirculated, to bring the building to comfort conditions
using a minimum of energy.
(16) Occupied mode of operation. The supply and return fans run continuously, the minimum
outside air damper is modulated to maintain minimum outside air flow, and the economizer outside air
and relief air dampers are either closed or are under mixed air temperature control as previously
described.
c. Detailed sequence of operation.
(1) Timeclock 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 transducers
IP-XX-01 and IP-XX-05. 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, and relay R-XX-07, and, with the
contacts of relay R-XX-01, energize relay R-XX-06. One set of contacts of relay R-XX-05 energizes the
return fan. The other set of contacts (line 4) is involved in the energizing of relay R-XX-03 when the
ventilation delay mode of operation is over. The contacts of relay R-XX-06 enable discharge
temperature controller TC-XX-02 to control cooling coil valve VLV-XX-01. The contacts of relay R-XX-07
enable control of the inlet guide vanes on the supply and return fans. 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 and pilot light PL-XX-01 is turned off. The
normally closed contacts of relay R-XX-04, in series with the now closed but normally open contacts of
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