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Zone day/night control requires one zone night thermostat
for tw o or mor e unit v entila tor s tha t mak e up a z one. Dur ing
night oper ation, the coil v alve opens, the outdoor air damper
closes, and the night ther mosta t cycles the fans to maintain the
setpoint of the z one night ther mosta t. For da ytime oper ation, a
centr al switch deacti vates the z one night ther mosta t and
pr ovides contin uous fan oper ation.
Unit Ventila tor Dig ital Contr ol
Digital contr ol offers several enhancements to unit v entila tor
control, including PID control, enhanced digital control, and
graphic displa y.
Figur e 27 is of a DDC ASHRAE Cyc le III basic system.
Unless the space temper atur e is below set point the f an disc har ge
temper atur e is constant a t 13 C. The space sensor modula tes
the HW valve for comfort control. Other than PI control and
fr iendl y displa y of values and setpoints, this figur e is the
standar d cycle III.
Figur e 28 utilizes the dig ital system ca pa bilities to pr ovide
enhanced Cycle II operation.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
INDIVIDUAL ROOM CONTROL APPLICATIONS
ON
OA
RA
13
VALVE OR
OPEN
FACE DAMPER
1/2
OUTDOOR
AIR DAMPER
CLOSED
COOL
ROOM TEMPERATURE
Fig. 27. Cycle III Digital Control
415
77
PERCENT
OPEN
WALL MODULE
TEMPERATURE
22.5
13
COOLING
22.5
SETPOINT
13
CONTROL
PROGRAM
77
PERCENT OPEN
TO OUTSIDE AIR
OA AT 10 C
OA AT -1 C
OA AT -7 C
OA AT -18 C
NORMAL
WARM
M15314
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