Pressure-Dependent VAV systems and two-pipe fan coils require changeover detection in order to switch seasonal operation between heating and cooling modes. The TEC supports the following methods for changeover: automatic changeover using an analog sensor (thermistor), automatic changeover using a binary switch, or remote changeover from a BAS and manual changeover.
For automatic changeover, a supply temperature sensor or switch must be connected to the Changeover Sensor (COS) input of the TEC. Changeover Mode must be set to Auto, and Supply Temp Type must be set for Analog Sensor, Cooling N.C. (cooling when switch is closed), or Heating N.C. (heating when switch is closed). When you use an analog sensor, you can adjust the changeover setpoint. The changeover logic applies a 10°F differential to the setpoint. The system switches to cooling mode when the temperature drops below the changeover setpoint and remains in cooling mode until the measured temperature has risen 10 degrees above the changeover setpoint.
Ensure that the Supply Temp type is set to Analog Sensor. The Changeover Mode is also exposed to the BAS through the CGOVR-MODE and can be commanded from the BAS.
On two-pipe or VAV systems without an automatic changeover, or on four-pipe systems, you can use RSEN or COS as a monitor-only point for reading an analog sensor. By setting the thermostat in four-pipe mode, or selecting Heating or Cooling for Changeover Mode, the thermostat defaults to monitor-only mode for RSEN or COS and exposes the value to the network as the supply temperature.