Hi...

What is DDC system in HVAC. What are the connections of this DDC.
I would like to know overall about this DDC system,it's performance & it's connections.

DDC bascically means Direct Digital Control. Building HVAC systems need a lot of variables to manipulate and a lot of things need to be turned on/off. Some examples of read variables might be: Inside temp(many locations), outside temp, outside humidity, inside humidity, CO2 levels. It might control chillers, outside air dampers, cooling water loops, boilers, variable speed drives etc.

You can't control that with a 4 wire thermostat. The Infinity Control for Carrier Heat/AC units comes close for residential applications.

Do a search for "Building Automation HVAC" for more info.