You probably wonder how you can know if a duct system is losing large amounts of energy. Although it is often difficult to be sure without testing, some telltale signs should help you convince your customer that testing would be a good idea.
It will help to make a simple diagram of the system. This can be a rough sketch. Start with the central heating unit, which may be in a basement, garage, crawl space, attic, or in the living space itself. Then work forward into the supply duct system and backward through the return system. If it isn’t obvious which are supply and which are return ducts, follow them to the registers and notice whether air is being sucked in or blown out when the fan comes on.