Whole house dehumidifier- freestanding vs vented
I have installed dehumidifiers in the past. The only difference I saw was that free standing models have a catch basin.  

The built-in models require a drain or a pump in the tray, to pump the water to a drain, but otherwise, they work the same way. 

For ducted models using the pressure of your HVAC system, there is a flow switch to start the compressor when the blower fan is running in "heat mode".