There are several situations when duct cleaning could make sense. If you have a problem with water in your ducts, this may result in mold growth in the duct. In this situation, solve the water problem first, clean and disinfect the ducts where mold growth has occurred, and have the rest of the ducting system cleaned as well. In some cases, it may be necessary or more cost effective to simply replace the affected duct areas.
If you are moving into a newly constructed house, and have doubts about the diligence of the construction crew, duct cleaning can be useful. Drywall dust, fiberglass pieces, and sawdust have no place in ducts. Duct cleaning will also catch the odd occurrence where lunch bags or soft drink cans have fallen or been swept into ducting. For similar reasons, duct cleaning may be advisable for older houses following major renovations. If you are having trouble with furnace airflow, a duct cleaning could reveal significant blockages. Have the ducts cleaned before you embark on more expensive duct replacement or rerouting to solve an airflow problem.
Cleaning of furnace and air conditioning components (e.g. furnace fan blower, air conditioning coil, humidifier, furnace heat exchanger, etc.) has been shown to make some difference in duct air flow. If you look in the return air registers and see an accumulation of debris in the duct below, you may want to have it removed. CMHC research showed that return air ducts, which are relatively big and have slow moving air, are generally far dirtier than the supply ducts. For example, households with hairy pets or young children may find it necessary to inspect their return air ducts more frequently.