ENGINEERING MANUAL OF AUTOMATIC CONTROL
INDIVIDUAL ROOM CONTROL APPLICATIONS
414
Fig. 25. Unit Ventilator Cycle II
Control (Face and Bypass Model).
CYCLE III–VARIABLE OUTDOOR AIR
Cycle III contr ol (Fig. 26) pr ovides a var iable per centa ge of
outdoor air de pending on the outdoor air temper atur e. This cycle
is basically a r eheat cycle where a constant cooling air
temperature (usually 13 °C) is heated to maintain room
temper atur e. According to ASHRAE’s definition, ther e is no
minim um or maxim um O A damper position.
When space temper atur e is below the ther mosta t setpoint,
the outdoor air damper is closed and the return air damper and
coil valve (or face damper) ar e full open. As space temper atur e
rises, the mix ed air contr oller modula tes the outdoor and r etur n
air dampers to maintain a mixed air temperature of 13 to 16 °C.
On a fur ther r ise in space temper atur e, the valve or face damper
modula tes towar d closed.
Whenever the f an tur ns off, the fan inter lock causes the
outdoor and r etur n air damper s to mo ve to the 100 per cent r etur n
air position. The coil valve opens and the unit v entila tor
functions as a con vector.
Unit ventilator s with air conditioning can use an y of the
contr ol cycles with a mec hanical cooling sta ge. As space
temperature rises, the room thermostat controls the unit
ventila tor cooling ca pacity thr ough the r egulation of the cooling
valve or compr essor.
Fig. 26. Unit Ventilator Cycle III Control.
Day/Night Setback Control
Unit ventila tor s can be oper ated a t lower setpoints dur ing
unoccupied hour s to save ener gy. Two commonly used da y/
night systems ar e individual r oom da y/night contr ol and z one
day/night control. Pneumatic actuation is preferred for unit
ventila tor contr ol because it oper ates smoothl y and c hang es
modes of operation through a simple pressure change.
Individual r oom day/night contr ol uses a da y/night r oom
thermostat that operates at a higher day temperature setpoint and
a lower night temper atur e setpoint w hen r esponding to a call f or
heat. During night operation, the outdoor air damper remains
closed, the r etur n air damper and coil v alve remain open, and the
fan cycles to maintain the lo wered space temper atur e.
The r oom ther mosta t ma y provide a man ual override tha t
allows occupants to r estore the unit v entilator to da ytime
setpoint f or after -hour s occupanc y. Retur n to da ytime oper ation
is optional with the man ual override function de pending on the
type of thermostat.
C3034
OPEN
OPERATION CYCLE
HEATING
VALVE
FACE AND
BYPASS
DAMPER
OUTDOOR
AIR
DAMPER
FINAL
CONTROL
ELEMENT
POSITION
CLOSED
LOW
SPACE TEMPERATURE
HIGH
DISCHARGE
AIR
OUTDOOR
AIR
MIXING
DAMPER
RETURN
AIR
FACE AND
BYPASS
DAMPER
FILTER
DRAIN
PA N
HEATING
COIL
FAN
VALVE
FAN
INTERLOCK
LOW-LIMIT
TEMPERATURE
CONTROLLER
THERMOSTAT
DAMPER
ACTUATOR
UNIT
VENTILATOR
M15313
DISCHARGE
AIR
OUTDOOR
AIR
OA, RA
DAMPERS
RETURN
AIR
FILTER
HEATING
COIL VALVE
FAN
FAN
INTERLOCK
MIXED AIR
TEMPERATURE
CONTROLLER
THERMOSTAT
DAMPER
ACTUATOR
OPERATION CYCLE
FINAL
CONTROL
ELEMENT
POSITION
LOW
SPACE TEMPERATURE
HIGH
OPEN
CLOSED
VALVE OR
FACE DAMPER
OUTDOOR
AIR DAMPER
OA AT 10
°
C
OA AT -1
°
C
OA AT -7
°
C
OA AT -18
°
C
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