The answer is almost always never. This article won't make me very many friends in the duct cleaning business but the topic is important considering the amount of air duct cleaning advertisements homeowners are bombarded with.
In some situations where metal ductwork is installed, duct cleaning might be appropriate. In San Antonio residential new construction, duct systems are almost always made of fiberglass ductboard and/or flexible duct not metal. With fiberglass ductboard and flexible duct systems there are many problems that can arise with duct cleaning.
- Fiberglass ductboard has a rough porous surface that makes it much harder to brush clean with a duct cleaning machine
- To access the entire duct system many times access holes must be cut in several places in the duct system. Sometimes the duct cleaning companies don't do a really good job of resealing these access holes or using code approved tapes and sealants. Duct leakage can lead to wasted energy and greater indoor air quality problems.
- The motorized duct cleaning brushes can sometimes damage ductwork or cause flexible ducts to come loose. This is a big problem if the ducts are not in an accessible place where they can be repaired, especially if the ducts are run between two floors of a house.
- Many times the allergen and particle count is higher after a duct system is cleaned than before. The dirt and dust that has settled in a duct system is made up of heavier particles that stay in place even with air blowing over them. You won't breath these particles unless they are disturbed enough to float back into your home.
Many of these problems also apply to metal ducts that are internally insulated instead of being externally insulated.
There are homes in San Antonio that have metal ductwork or partial metal ductwork. If ductwork is run exposed on a roof then it needs to be metal. We also recommend using metal duct anytime you have ductwork in a crawl space under the house so that it won't be easily damaged by animals that might get under the house. Other than these two reasons if you have a metal duct system in your house chances are that you live in an older house that was built before fiberglass ductboard and flexible duct were readily available. These older homes were built in a time when people weren't woried about energy costs as much and most of these metal duct systems weren't sealed well and are very leaky. Also many of these homes had central heating before they had central air conditioning. In these cases the metal duct system is undersized for air conditioning because ductwork for heating only doesn't need to be nearly as big. Despite being undersized sometimes the air conditioning systems still function connected to these ducts because they are leaky enough to keep from restricting the air too much. If you were to seal up these older smaller duct systems you might find that your air conditioning system won't work anymore because it can't move enough air and will freeze up. If there is space available and you can afford it, it's better to replace these systems with new properly sized duct systems.
If you have a fiberglass duct system and you get mold in your system for some reason cleaning the ducts won't fix the situation. The only EPA approved mold remediation for fiberglass insulation is to replace it.
It's normal for people to want to clean their duct systems as a way to improve their indoor air quality, especially if they have asthma or allergies. A better solution for air quality problems is to get a reputable air conditioning contractor to take a look at your duct system and eliminate any air leaks (especially return air leaks). Once duct leakage has been addressed then there are many available whole house air cleaners that can further help improve indoor air quality.
We're not usually big fans of the EPA but here is an excellent article about air duct cleaning that further explains some of the things I've mentioned above.
More interesting articles on heating and air conditioning here.