Your hvac duct system is the heart of your home's heating and cooling system. Without a healthy system you could be wasting a lot of money. Like the human body, if the heart isn't healthy you are in trouble. A duct system provides a controlled path for air flow throughout the home.he two main types of supply systems are extended plenum and central plenum. A central plenum system is a system where all of the branch ducts start at the same location called a plenum. The extended plenum system consists of a supply air plenum which is connected to the outlet of the hvac equipment.