Should You Get Your Heating Ducts Cleaned?
A hot air furnace heats and distributes air through its ducting system.The ducts are usually made of sheet metal and are most obvious in your basement, where they hang from the floor joists.The return, or cold air, ducts bring air to the furnace, usually collecting it centrally in the house. The return air trunk duct is the big rectangular duct along the basement ceiling that enters the bottom of the furnace. The supply, or warm air, ducting usually exits from the top of the furnace. It starts with a trunk duct on the basement ceiling.The individual supply ducts, in round or smaller rectangular sheet metal, branch off the trunk duct and go to each room, where they terminate in a floor or wall register. Over time, dust and debris will collect in these ducts, particularly in the return air ducts.You may be wondering whether it would be worthwhile to have these ducts cleaned.

Duct cleaning is a major industry. As a homeowner, you may be regularly solicited to have your heating ducts cleaned on a regular basis. Claims are made that duct cleaning will:

•provide you with better indoor air quality (or IAQ),
•reduce the presence of house molds and allergens,
•get rid of house dust,
•result in more airflow and better delivery of warm air, and/or
•reduce energy costs.
If you expect duct cleaning to make these improvements, you may be disappointed. It is difficult to find objective and independent research which substantiates these claims.

I found this on the CHMC   http://www.cmhc-schl.gc.ca/en/co/maho/gemare/gemare_011.cfm