The cleanliness of furnace ductwork has very little to do with energy efficiency. Dirty or plugged furnace filters and heat exchangers have a major impact on efficiency. In extreme cases, the only way to get improvement in efficiency is to have all three thoroughly cleaned, and then keep filters changed on a regular basis to keep the duct work and heat exchanger clean.
The most prominent reason for having duct work cleaned is health related.
There are a variety of methods and equipment used to clean ducts. Some are a dry vacuum and some use liquids to also disinfect.
Price is not necessarily an indicator of value. As with any service related business, there are a number of private contractors who vary in their skills, equipment used and thoroughness of work. Check references, get a free estimate, and understand exactly what will be done before hiring a duct cleaning service.
801 Hanover Drive, # 700
Grapevine, TX 76051
Telephone: 682-223-5111, 800-535-3878
Go to the Rotobrush web site at www.rotobrush.net
Source: Text Bob Fegan 12/2008; images from Rotobrush web site linked above 9/2003;