Chapter 5 Sequence of operations
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.
