ENGINEERING MANULA OF AUTOMATIC CONTROL
INDIVIDUAL ROOM CONTROL APPLICATIONS
419
Zone day/night control requires one zone night thermostat
for two or more unit ventilators that make up a zone. During
night operation, the coil valve opens, the outdoor air damper
closes, and the night thermostat cycles the fans to maintain the
setpoint of the zone night thermostat. For daytime operation, a
central switch deactivates the zone night thermostat and
provides continuous fan operation.
Digital Unit Ventilator Control
Digital control offers several enhancements to unit ventilator
control, including PI control, enhanced digital control, and
graphic display.
Figure 27 is of a DDC ASHRAE Cycle III basic system.
Unless the space temperature is below set point the fan
discharge temperature is constant at 55F. The space sensor
modulates the HW valve for comfort control. Other than PI
control and friendly display of values and setpoints, this figure
is the standard cycle III.
Figure 28 utilizes the digital system capabilities to provide
enhanced Cycle II operation.
Fig. 27. Cycle III Digital Control
M10544
RA
OA
ON
CONTROL
PROGRAM
73
73
55
77
77
WALL MODULE
TEMPERATURE
COOLING
SETPOINT
55
55
PERCENT OPEN
TO OUTSIDE AIR
OPEN
CLOSED
VALVE OR
FACE DAMPER
OUTDOOR
AIR DAMPER
COOL
WARM
ROOM TEMPERATURE
1/2
NORMAL
OA AT 50°F
OA AT 30°F
OA AT 20°F
OA AT 0°F
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