Duct cleaning is a band aid, like washing you hands is. Under your description, why wash your hands more than once? If it gets cleaned once, it shouldn't get dirty again. Well that is, unless they touch something dirty. Keeping your hands completely clean is virtually impossible. Same as keeping dust out of your home. You clean your ducts and over a 3-5 year span they get dirty again. Also, duct cleaners are not licensed but should be certified by NADCA(National Air Duct Cleaning Association).
There are many interesting comments from several informed individuals. The question that has not been answered is how do duct systems get dusty and dirty? As a RESNET Home Energy Rater, I perform pressure tests on homes and duct systems, and the one common thread is . . . most air delivery sytems have large amounts of air leakage. The solution is to seal the duct system, and pay particular attention to the return duct system and ductwork near the air handler (the 'blower'). As others have said, a captive (sealed) duct system with proper filter maintenance should be more then adequate for most occupants. For those who have allergy, asthma, or other breathing problems, a filter with a higher MERV rating (number should be listed on the package) or a HEPA filter are good choices. These filters will capture smaller particulates and should improve the indoor air quality. Again, make certain that the entire duct system is sealed, preferably with mastic, second choice UL-181 foil tape - never use cloth duct (also known as duck) tape.
I noticed a lot of negative feedback, though unfortunately there are many dishonest companies out there. I currently work with a company that cleans duct work and we DO NOT use any scare tactics or up sell services or equipment. Once every 10years of course there are exceptions. We clean supply, returns, air handler, housing, motor, and fan. Though be discerning when qualifying anyone to work in your home. I would ask for REAL REFERENCES to contact.

I believe a $3 [sic] pleated cotton filter is all that is ever needed to keep a system running top notch. A cleaning of the blower squirrel cage and of the evaporator [indoor] coil may be needed one time if fiberglass or no filters have been previously used. The return air grille [or grilles] need vacuumed off occasionally, as they are upstream from the filter. The $3 filter should be changed every 3 months [rule of thumb]....and most of them have a little white square on the cardboard frame to write install date.....and the arrow should point into the duct [if at filter grille]....or towards the furnace [if in ductwork slot]...or towards the blower [if in blower compartment]. My two cents.....
And if you have flex duct and/or fiberglass ductboard in your home- the last thing you want is a big brush or anything abrasive running through it. Even high-pressure air can erode ductboard so I wouldn’t clean it with anything. If it becomes contaminated, get rid of it and install real ductwork. If your ductwork is accessible in the basement – the best thing to do is take it down and manually clean it if it’s full of junk. It’s not going to cost a nickel more to do that than to pay $500-1000 for a ‘duct cleaning machine” AKA big blower and vacuum – to do the job.
Yes, ducts can look terribly dusty. However, that dust will not become aerosolized during the normal operation of your furnace. There is an easy way to test this. Rent a particle counter and test the air flowing out of your registers before and after cleaning. You will not see any improvement in the air quality or any reduction in the airborne dust level. Think of it this way. Duct cleaning companies tell you that you need to clean your ducts when you can see a layer of dust in your ducts. Yet that very layer of dust is proof that duct cleaning is unnecessary, because that dust proves that the air movement in your furnace is not fast enough to re-aerosolize the dust.

Concealed behind your walls and mostly ignored, your air ducts serve as vital pathways that deliver warm and cool air throughout your home. While they may be unseen, your air ducts are working whenever you have your air conditioner or furnace running–nearly year round. Attached to your HVAC, air ducts transfer the air from these systems throughout your home. What many homeowners don't realize is that your air ducts, in nearly constant use, continually accumulate dust, grime, debris, pet dander, and allergens throughout the year. A professional air duct cleaning is necessary for better indoor air quality and the health of your family.
Most people are now aware that indoor air pollution is an issue of growing concern and increased visibility. Many companies are marketing products and services intended to improve the quality of your indoor air. You have probably seen an advertisement, received a coupon in the mail, or been approached directly by a company offering to clean your air ducts as a means of improving your home's indoor air quality. These services typically — but not always — range in cost from $450 to $1,000 per heating and cooling system, depending on:
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