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Chapter 5 Sequence of operations
50 CNT-SVX07C-EN
Filter-maintenance timer
The filter-maintenance timer tracks the amount of time (in hours) that
the fan is enabled. The Maintenance Required Timer Setpoint (config-
urable using the Rover service tool) is used to set the amount of time until
maintenance (typically, a filter change) is needed. If the setpoint is config-
ured to zero, the filter-maintenance timer is disabled.
The controller compares the fan-run time to Maintenance Required Timer
Setpoint. Once the setpoint is reached, the controller generates a Mainte-
nance Required diagnostic. When the diagnostic is cleared, the controller
resets the filter-maintenance timer to zero, and the timer begins accumu-
lating fan-run time again.
Freeze avoidance
Freeze avoidance is used for low ambient temperature protection. It is ini-
tiated only when the fan is off. The controller enters the freeze-avoidance
mode when the outdoor air temperature is below the Freeze Avoidance
Setpoint (configurable using the Rover service tool). The controller dis-
ables freeze avoidance when the outdoor air temperature rises 3°F (1.7°C)
above the Freeze Avoidance Setpoint.
When the controller is in freeze-avoidance mode:
• Valves are driven open to allow water to flow through the coil
• Fan is off
• Face-and-bypass damper (when present) is at full bypass
• Economizing is disabled
• The outdoor/return air damper is closed
• DX cooling is off
• Electric heat stages are off
Freeze protection (discharge air temperature low limit)
The controller monitors the discharge air temperature with a 10 kΩ ther-
mistor wired to analog input AI2. The freeze-protection operation is initi-
ated whenever the discharge air temperature falls below the Discharge
Air Temperature Low Limit. The Discharge Air Temperature Low Limit
is configurable using the Rover service tool. During freeze protection, the
controller increases the heating capacity or decreases the cooling capacity
in order to raise the discharge air temperature above the low limit. If the
discharge air temperature remains below the low limit for 3 minutes, the
controller generates a Discharge Air Temp Limit diagnostic. See Table 16
on page 59 for the effects of this diagnostic on outputs.