Cycle Iii-Variable Outdoor Air; Day/Night Setback Control - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

INDIVIDUAL ROOM CONTROL APPLICATIONS
UNIT
VENTILATOR
FAN
HEATING
COIL
DRAIN
PAN
FILTER
OUTDOOR
MIXING
AIR
DAMPER
FACE AND
BYPASS
OPERATION CYCLE
DAMPER
OPEN
FINAL
CONTROL
ELEMENT
POSITION
CLOSED
LOW
SPACE TEMPERATURE
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
is basicall y a r ehea t cycle wher e a constant cooling air
temperature (usually 13 C) is heated to maintain room
temper atur e. Accor ding 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
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 f an inter lock causes the
outdoor and r etur n air damper s to move 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 v entila tor 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.
DISCHARGE
AIR
THERMOSTAT
LOW-LIMIT FAN
TEMPERATURE INTERLOCK
CONTROLLER
VALVE
FACE AND
BYPASS
DAMPER
DAMPER
ACTUATOR
RETURN
AIR
HEATING
VALVE
OUTDOOR
AIR
DAMPER
HIGH
C3034
atur e. This cycle
HEATING
COIL
FAN
FILTER
OA, RA
DAMPERS
OUTDOOR
AIR
VALVE OR
OPEN FACE DAMPER
FINAL
CONTROL
ELEMENT
POSITION
OUTDOOR
AIR DAMPER
CLOSED
LOW
Fig. 26. Unit Ventilator Cycle III Control.
Day/Night Setbac k Contr o l
Unit ventila tor s can be oper ated a t lower setpoints dur ing
unoccupied hour s to save ener gy. Two commonl y 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.
Indi vidual r oom da y/night contr ol uses a da y/night r oom
thermostat that operates at a higher day temperature setpoint and
C.
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 pr ovide a man ual overr ide tha t
allows occupants to r estor e the unit v entila tor 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.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
414
DISCHARGE
AIR
VALVE
THERMOSTAT
MIXED AIR
TEMPERATURE
CONTROLLER
FAN
INTERLOCK
DAMPER
ACTUATOR
RETURN
AIR
OPERATION CYCLE
OA AT 10 C
OA AT -1 C
OA AT -7 C
OA AT -18 C
HIGH
SPACE TEMPERATURE
M15313