The heart of your heating and cooling system if the HVAC duct system. Having a healthy system is a prerequisite, or else you’d end up just wasting your money. If compared to a human body, having an unhealthy heart gets you in serious trouble.

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. Before setting up your system, make sure to contact the right  ducting supplier in your area. 

The main function of the duct system is to provide a path of airflow throughout your home. Depending upon your specifications, this system can be made out of fiberglass, duct board, cloth, or insulated plastic. There could also be an option of a combination of these.
Comprising this system are three main parts, each that requires to be in good condition for a healthy system.

The first is the so called return air. This section gives a path between the inlet of the furnace to the individual rooms. This section is supposedly used for cleaning air. In some cases, addition of moisture is done in this part of the system. In an ideally healthy system, fresh air is to be introduced in this section. On the second part, air traverses into the furnace/handler. In this section of the duct system, it’s either heat is to be removed or added, depending upon the temperature requirement on making your home comfortable. The third part, the conditioned air will be distributed to the individual rooms from the supply section. 

Finally, this conditioned air enters the supply section of the system. The purpose of this part of the system is to deliver conditioned air to the individual rooms. These supply sections could either be classified as central plenum or extended plenum. Extended plenum systems has a separate supply air plenum connected directly to the output of the hvac equipment, and is attached to the main supply ducts. To maintain adequate air flow, size of this type is usually reduced as required. Brach ducts attach to the main ducts and carry the air to the individual rooms. On the contrary, in a central plenum supply system, all branch ducts start at the same spot referred to as the plenum 

Basically, this is how a duct system works. Maintaining your system is not only beneficial in providing your home clean air or making it comfortable, but it also makes you save more money by maintaining its efficiency as well. Reference.