The APS-C series systems are designed to turn on and off, as the weather requires heat to deal with snow or icing conditions. Although a thermostatic dry contact switch can be used to override the system on for temperature only, is not recommended since it will reduce the efficiency of the system. Running the heaters on cold days that do not have snowfall will not hurt anything but your budget.
If you must do so choose a thermostat with dry contacts that close when the temperature is below 40F and connect it to the override-on terminals of the EMC section of the Class 2 terminal block.
