When the company arrives at your house, the rep tries to sell you on a bunch of other services, including carpet cleaning or mold remediation, to improve the air quality inside your home. If you don’t bite at these invoice-increasing items, he gives your ducts a cursory cleaning before moving on to the next mark somewhere in your area.

That lowball offer should have been a red flag to avoid that firm. A reputable company will charge $400 to $1,000 to clean the ducts in a typical 2,000-square-foot house, says the National Air Duct Cleaners Association (NADCA).

• Have the ductwork cleaned if it contains visible mold buildup or has debris coming out of supply registers, or if the ducts are infested with vermin. Check for problems by removing the registers and shining a flashlight into the ductwork.

• Consider having your ductwork professionally inspected if someone in your home is suffering from unexplained allergies or illness. Before you do that, however, a doctor should rule out other causes.

• Find NADCA member contractors in your area on the association's Web site, www.NADCA.com. Be sure the company cleans the entire HVAC system, including the ducts, heat exchanger, and air handler.

• Consumer Reports recommendes that customer's "insist on a thorough visual inspection...to ensure all components of the HVAC system arefree of visible dust and debris" (consumerreports.org). At Mr. Duct, we provide detailed before and after video inspections to do just that.