INDIVIDUAL ROOM CONTROL APPLICATIONS
ENGINEERING MANUAL OF AUTOMATIC CONTROL
413
Fig. 23. Unit Ventilator in Standby/Warmup Stage.
CYCLE I–FIXED MAXIMUM PERCENTAGE OF OUTDOOR AIR
When the space is w ar med to the lo w end of the ther mosta t
thr ottling r ange, Cycle I begins. As shown in F igur e 24, contr ol
components ar e the r oom ther mosta t, hea ting coil valve, damper ,
and modula ting low-limit contr oller. The outdoor air damper
and hea ting coil valve oper ate in sequence in accor dance with
the demand of the ther mosta t. The hea ting valve is full open.
As space temperature rises, the thermostat modulates the
damper to its maxim um open position. The fixed maxim um
position is nor mally 100 percent open. As space temper atur e
contin ues to r ise, and after the damper mo ves to its maxim um
position, the hea ting coil valve modula tes closed. If a cooling
coil is installed in the unit, the cooling coil v alve opens as space
temper atur e rises fur ther. The low limit contr oller is necessar y
to pr event fr eezing conditions tha t occur w hen occupants lo wer
the r oom contr ol setpoint w hen it is belo w freezing outside ,
plus it pr events the discomf ort tha t would r esult in disc har ging
air into the room below 13 °C.
Fig. 24. Unit Ventilator Cycle I Control.
CYCLE II–FIXED MINIMUM PERCENTAGE OF OUTDOOR AIR
Cycle II contr ol (Fig. 25) provides a fixed minim um
percentage of outdoor air (usually 10 to 33 percent, adjustable
from 0 to 100 percent).
At low space temper atur es, the outdoor air damper is c losed
and the hea ting coil valve and f ace damper ar e full open. As space
temper atur e rises, the outdoor air damper mo ves to its minim um
position. On a fur ther r ise in space temper atur e, the f ace damper
modulates closed, and on a further rise in space temperature, the
coil valve closes. When the space temper atur e is at setpoint, the
valve is full closed. As space temper atur e rises above setpoint,
the outdoor and return air dampers modulate to 100 percent
outdoor air a t the top of the thr ottling r ange. The low-limit
contr oller can limit the c losur e of the coil valve, face damper , and
the opening of the outdoor air damper to pr event the disc har ge air
temper atur e from dr opping belo w its setpoint.
C3040
CONVECTION
AIR
OUTDOOR
AIR
OA, RA
DAMPERS
OPENCLOSED
RETURN
AIR
FILTER
HEATING
COIL
VALVE
FAN
FAN
INTERLOCK
THERMOSTAT
DAMPER
ACTUATOR
C3029
DISCHARGE
AIR
OUTDOOR
AIR
OPEN
OA, RA
DAMPERS
RETURN
AIR
FILTER
HEATING
COIL
VALVE
FAN
FAN
INTERLOCK
LOW-LIMIT
TEMPERATURE
CONTROLLER
THERMOSTAT
DAMPER
ACTUATOR
OPERATION CYCLE
HEATING
VALVE
OUTDOOR
AIR
DAMPER
FINAL
CONTROL
ELEMENT
POSITION
CLOSED
LOW
SPACE TEMPERATURE
HIGH
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