Operating Instructions

Application 2522: VAV with Electric Reheat or Baseboard Radiation
Siemens Building Technologies, Inc. 3-29
It is only when the controller is in night mode that the override switch on the room sensor has
any effect on the controller.
Heating/Cooling Switchover
Based on Room Temperature (Internal Logic)
The heating/cooling switchover determines whether the controller is in heating or cooling
mode by monitoring the room temperature and the demand for heating and cooling (as
determined by the temperature control loops).
If the following conditions are met for the length of time set in SWITCH TIME (Point 86), the
controller switches from heating to cooling mode by setting HEAT.COOL (Point 5) to COOL:
• HTG LOOPOUT (Point 80) is less than SWITCH LIMIT (Point 85).
• CTL TEMP (Point 78) is greater than CTL STPT (Point 92) by at least the value set in
SWITCH DBAND (Point 90).
• CTL TEMP is greater than the appropriate cooling setpoint minus SWITCH DBAND.
If the following conditions are met for the length of time set in SWITCH TIME, the controller
switches from cooling to heating mode by setting HEAT.COOL to HEAT:
• CLG LOOPOUT (Point 79) is less than SWITCH LIMIT.
• CTL TEMP is less than CTL STPT by at least the value set in SWITCH DBAND.
• CTL TEMP is less than the appropriate heating setpoint plus SWITCH DBAND.
Based on Supply Air Temperature (External Control)
CAUTION:
The ATEC’s internal heating/cooling switchover mechanism is not affected by
the air temperature in the supply duct.
To change the value of HEAT.COOL (Point 5) based on the supply air temperature,
HEAT.COOL must be commanded through PPCL. This is required when the supply duct
delivers warm air for heat and cool air for cooling. In this case, the room-temperature-based
switchover must be disabled by commanding HEAT.COOL and the heat/cool mode setting
must be based on the supply air temperature. When the supply air temperature is warm, the
room is in the heating mode. When it is cold, the room is in the cooling mode.