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Duct Cleaning

By Arnie Katz
Building Science Specialist
Advanced Energy Corporation

Question: Lately I’ve been seeing a lot of ads for duct cleaning. A few of my friends have spent from a few hundred to over a thousand dollars for this. One says it’s made a big difference in her house and in her family’s health. The others haven’t really noticed any difference. Is this worthwhile, or just another scam?

Answer: My colleagues and I have looked into, taken apart, crawled into, destroyed, repaired, and otherwise been intimate with thousands of duct systems. We’ve seen slime molds growing inside ducts that looked like they belonged on X-Files, along with the more common beer and soda cans, candy wrappers, half-eaten burgers, saw-dust, mouse carcasses, construction debris, and shredded insulation, all of which often has mold or other unidentifiable stuff growing on it.

So the notion that there may be nasty, and potentially unhealthy, stuff inside your ducts is certainly no scam. In order to decide whether or not to spend hundreds of dollars to have your ducts cleaned, however, there are some very specific pieces of information you need to know: What kind of ducts do you have? Can they be effectively cleaned by the techniques used by a particular contractor? What are your ducts contaminated with? What’s the source? If you have them cleaned today, will they be contaminated again next week, or next year?

Most ducts in houses are made of one of three materials: sheet metal (rectangular or round); duct board (a fiberglass board usually covered with aluminum foil on the outside); or flex duct (a spiral metal spring layered with fiberglass insulation and plastic liners). Sheet metal ducts are sometimes lined on the inside with fiberglass or other material to insulate the duct and to deaden sound. Sometimes they are uninsulated, or insulated on the outside, with only bare metal exposed to the air passing through. In most houses, the duct system is made up of two or more of these types of ducts.

Research now underway by the Environmental Protection Agency has demonstrated that some of the techniques currently in use can be very effective in cleaning bare sheet metal ducts. I haven’t seen any evidence to date that the other types of ducts can be effectively cleaned without damaging them and risking causing more problems, such as putting fiberglass fibers into the air stream, or creating breeding places for future contamination.

So if you have all unlined sheet metal duct work, and you know it’s contaminated, it may be well be worth it to have it cleaned. If all or most of your system is made of other materials, cleaning is of questionable value, at best. Even if you do have bare sheet metal, you still need to determine the cause of the contamination. If the problem is mold growing on construction debris, clearing out the trash and cleaning of the mold should certainly help.

But if the ducts aren’t really sealed – and most aren’t – it’s quite possible that more moisture and mold will come in, and you’ll be back where you started. If you have your ducts cleaned, make sure the air handler, blower, and coil are also cleaned. You should also have the ducts tested and sealed by a certified contractor who has been trained to identify other duct-related health and safety problems as well. This will also usually improve the efficiency of the system, reduce energy bills and improve comfort.

If you have reason to believe that little nasties in your ducts are affecting your health, cleaning the ducts may be an option. Installing a good filtration system, such as a two- to six-inch pleated media filter, may be a better option. If your ducts are really bad, or of a non-cleanable material, replacing them may make more sense than trying to clean them.

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