50
CNT-SVX05A-EN
Sequence of operation
Table 29 — Relationship between outdoor temperature sensors and damper
position
Note 1:
The low ambient damper lockout setpoint is a configurable temperature setpoint used to close the outdoor air
damper, regardless of occupancy, when extreme outdoor air temperatures are present.
Note 2:
The Tracer
™
AH.540 controller disables economizing if the mixed air temperature sensor is not present or is not
valid.
Low ambient damper lockout
The controller closes the outdoor air damper during any heating, cooling,
or economizer mode of operation or occupancy when extreme outdoor air
temperatures exist. This condition disables outdoor air damper ventilation and
economizing functions, but low ambient damper lockout does not affect other
unit operations.
The outdoor air temperature must rise 5
°
C (9
°
F) above the low ambient damper
lockout setpoint before economizing and ventilation become possible again.
Exhaust fan operation
The exhaust fan/damper is coordinated with the unit supply fan and outdoor
damper operation. The exhaust output is energized only when the unit supply
fan is operating and the outdoor damper position is greater than or equal to the
configurable exhaust fan start setpoint. The exhaust fan output is disabled when
the outdoor air damper position drops 10% (configurable) below the exhaust fan
start setpoint. If the enable point is less than 10% (configurable), the unit turns
on at the start setpoint and off at zero.
The controller logic commands the exhaust fan to be energized/de-energized
based on the target position of the economizing damper. Because of device
stroke time, the state of the exhaust fan may change before the economizing
damper reaches its target position.
If the exhaust fan start setpoint is set at or lower than the outdoor air damper
minimum position, the exhaust fan will be on continuously when the outdoor
air damper is at minimum position.
If the exhaust fan start setpoint is set higher than the outdoor air damper
minimum position (minimum ventilation) the exhaust fan will be off during
periods of minimum ventilation. During economizer cooling operation the
exhaust fan start setpoint can be selected to compensate for the increased
outdoor ventilation.
The exhaust fan status binary input is present to detect operation of a
belt-driven exhaust fan. An
Exhaust Fan Air Flow
diagnostic is detected when
the control starts the exhaust fan and the exhaust fan status binary input does
not indicate operation after two minutes. This is an exhaust fan latching
Outdoor air damper
Outdoor air temperature Occupied or
Occupied bypass
Occupied standby
Unoccupied
No or invalid outdoor
air temperature
Open to occupied
minimum position
Open to occupied
standby minimum
position
Closed
Failed outdoor air
sensor
Open to occupied
minimum position
Open to occupied
standby minimum
position
Closed
Outdoor air temperature
less than the low ambi-
ent damper lockout set-
point (Note 1)
Closed
Closed
Closed
Outdoor air temperature
present and economiz-
ing feasible
Economizing, damper
controlled between
occupied minimum
position and 100%
Economizing, damper
controlled between
occupied standby mini-
mum position and 100%
Open and economizing
during unit operation,
otherwise closed
Outdoor air temperature
present and economiz-
ing not feasible (Note 2)
Open to occupied
minimum position
Open to occupied
standby minimum
position
Closed