First, find the central heating unit. It is often located in the basement, however, it may be located in an attic or crawl space. Once you have found your central heating equipment, you will notice ducts leading away from it. There is a supply duct and a return duct, but which is which? In one common type of installation, the return duct leads down from the basement ceiling to enter the furnace near the floor. The supply duct runs out from the top of the furnace.
There are many types of installations and it is not always easy to tell which is the supply and which is the return just by looking at the ducts. However, if you wait for the furnace to come on, you should be able to feel the supply duct getting warm.
Another method is following what you think are the supply ducts to a register. If this is the supply side, you will feel air coming out of the register.
After identifying supply and return sides of the duct system, follow the supply side. Most often you will see something like a long, box-like structure called a trunk sprouting smaller branch ducts that lead out toward the home's rooms. Another common installation has all the supply ducts branching right from the furnace like the arms of an octopus.
So far we have assumed that the duct system is completely separate from the other components of the home, which often is not true. To save money, builders sometimes use the building structure itself as part of the duct system. One common tactic is using the spaces between basement or ceiling joists as ducts. Joists are the horizontal-running boards generally 2" x 10" or 2" x 12" that support the floor above. Although this type of construction can be made to operate efficiently, it often leads to significant energy losses. One reason is that joist-space ducts are likely to be uninsulated. Another problem is that they may allow air leakage through the end of the joist cavity.
It is even more common to see portions of the building structure used as return ducts. Some homes have no return at all, the furnace simply has an intake grille through which basement air is drawn in to be warmed and distributed to the home.