Most often the ducts that carry hot air from your furnace to your home leak. Average duct leakage is 30%, but leakage could be as high as 50%. That means that you could be leaking dollars out your ducts.
On average heat and cooling comprise 45% of your total energy bill
There is a fairly simple process to determine the amount of leakage your ducts may have. This process is called Duct Testing and should be performed by a HERS rater.
A HERS rater has been certified to perform the proper testing as well as authorized to certify a ducting system meets state codes.
The California Energy Commission required duct testing in certain climate zones when work is performed on a heating or cooling system. While it may not be required in your climate zone, it might be a good idea to test your ducts and have any leakage remediated. The pay back period could be as short as a year or up to 6 years dependent on your HVAC system and use.