Demonstrate visible evidence of microbial growth in your duct work. Some service providers may attempt to convince you that your air ducts are contaminated by demonstrating that the microorganisms found in your home grow on a settling plate (i.e., petri dish). This is inappropriate. Some microorganisms are always present in the air, and some growth on a settling plate is normal. As noted earlier, only an expert can positively identify a substance as biological growth and lab analysis may be required for final confirmation. Other testing methods are not reliable.
I am a service technician [since '92].....and as to double filtering.....I have never recommended it. would be noticeably helpful if the fiberglass filters are the main ones used [which I also never long as system can deliver appropriate air flow with a pleated cotton filter...and any decent service tech can check the air flow for proper cfm]. Any particles small enough to pass through a pleated cotton filter are going to pass through a second filter as well....unless the second filters are so tight that the system is starving for air flow. Has anybody ever accidentally put two filters in their automatic drip coffee maker? What if cars used two fuel filters or air filters? I have told some homeowners to try using second filters at each register [the black thin filter material cut-to-fit....same type used in many window air conditioners].....but I only suggest this when I know they will still get at least 400 cfm per ton of air flow [350 absolute minimum] some homeowners know others who say it has helped and it gives them a peace of mind knowing they are doing something to help contribute to a solution, so it HAS to help [even though I think it falls in the "one-born-every-minute" category.....similar to duct cleaning].
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Did not realize cost had gone up so much for this service. Two years ago I had furnace and ducts cleaned at my old house and it was under $400. Now the cost is $700 for this house. I will keep looking for someone to offer a lower price, check them out, and have it done. Or will find the paperwork to see who did it before. I liked this article and did learn some things. Thanks.
Once i received a call from a Person he Named Steve and He told me that he is from some like GTA Home services and providing Air duct Cleaning in around 105$ and i booked an appointment because he convince me in very will mannered so i agree the services i placed an order and i don't think that they guyz do a great job as he describe but they do same as he told me on phone i'm just surprised that they do a great job and they provide me free inspections of furnace and A/C as well and they really do a great job and they don't waist my money and time. thanks Adam Stevenson and GTA home service Team.

Equipment used to clean ducts varies widely. The cheaper the equipment used the more you are just wasting money. This is why it's typically not worth doing. In some situations it may be beneficial, but only after determining and investigating the duct system. This should be done by a licensed HVAC contractor and not a duct cleaner that typically do not hold and HVAC license. (Duct cleaners aren't required to be licensed in HVAC in many areas.)
If your non metal ductwork is that dirty you should just have it replaced AND sealed. Also most duct systems are not very well designed. Look for proper sizing and do not go cheap with one or two intakes (return grills) You need them through-out the house. Here in Austin the duct cleaners are carpet cleaners and chimney swifts. (this is who the duct cleaning equipment mfgs call on when the hvac people do not get on board. They DO clean other types of ductwork and we (the hvac guys) have to fix them. I am not aware of any of these non hvac guys cleaning and servicing the actual equipment. Here in Texas it is against the law for them to do so. Treat the entire house as a system, not just the ductwork and /or hvac equipment. Home performance really does work to increase comfort and air quality while reducing operating costs.

Quality and price should come hand in hand, a too-good-to-be-true price for an efficient air duct cleaning will not give you the quality that you wanted. To make sure you are getting your money’s worth, check reviews of air duct cleaning companies, certifications from industry since this work requires continuous learning due to new techniques and research breakthrough from time to time. Make sure to shop around and get written estimates first.

Air duct cleaning service providers may tell you that they need to apply a chemical biocide to the inside of your ducts to kill bacteria (germs) and fungi (mold), and prevent future biological growth. Some duct cleaning service providers may propose to introduce ozone to kill biological contaminants. Ozone is a highly reactive gas that is regulated in the outside air as a lung irritant. However, there remains considerable controversy over the necessity and wisdom of introducing chemical biocides or ozone into the duct work.
When these aren't properly maintained and sanitized, they can build up dust, pollen, and even mold. This means when the air conditioner or heater turns on, the spores are released into the air for you to inhale. While some are less harmful, people who have allergies or sensitivity to certain spores will react negatively. Plus, mold is dangerous for anyone to ingest and should be avoided by regular maintenance.
Good article, but it fails to point out that leaky return-side ducts could be drawing dust and allergens into the duct system from the enclosed spaces in which they run, e.g. between walls, below floors, attic. So duct cleaning may only be a temporary solution. The homeowner can get a home energy audit -- free in many states -- to determine if they have significant duct leakage. An Aeroseal dealer may be able to effectively seal up the duct system without needing to tear into walls etc to get to the ducts.
If the system is sealed air-tight and a really good filtering system is in place then you should NEVER have to clean your ductwork. Most dust people see in their homes (on beds furniture etc....) is broken down insulation that is being blown into the home from the attic (venturi effect) where the duct bucket (box etc...) is not sealed at the sheetrock ceiling. On the return side, ANY leaks in the ductwork will pull hot attic air and dust into the system, clogging up everything.

Although i love to burn candles, depending on the type your asking for problems big ones! At work one day i had been talking with co workers about these black shadows that were appearing on my ceilings and higher wall areas. They seemed to in the corners and along the stud supports , i could even see what i thought to be where all had been nailed. There would be dark shadows perfectly straight across my ceiling and then real dark circels every few inches within the lines. I thought it was the propane heat but learned quickly that oil heat will do that but not normally propane. A co worker brought me a magazine with a huge article about candles and what actually burns off them when lit, an ashy soot that is attracted to the areas on walls and ceilings that omit the most heat or warmth, of course where the drywall and studs meet and corners of the room. Its been about 8 years since i painted the entire house. I started bending to my love for candles again in the last 3 years, surprise i will be spending part of my summer re painting again!! No more candles ,well maybe at the holidays LOL

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.
fact is most homes have bad filter racks. when you insert the filter it doesn’t seal well to the sides so some of the air and dust bypass it and end up in the supply ductwork and into the house. Most of the dust in the supply ductwork settles down and doesn’t get blown in the house but some of it does when the blower starts up or when you put a new filter. If you have allergies or other health problems put in an electronic air cleaner, get your ducts cleaned and run the fan as much as you can afford. you can also get an ECM motor installed which is going to be worth it if you want to run the fan 24/7. If you have uninsulated metal ducts take down the ones you can and wash them. Then put them back and seal the joints. Cleaning the ductwork also improves airflow which is important to AC and Heat pump systems for efficiency. An electronic air cleaner will also improve airflow and static pressure because they usually have a much larger filter surface area. An electronic air cleaner also keeps the ac coil clean like new which means better efficiency and no cleaning needed. AC furnaces sometimes grow mold around the coil because of the condensation. Electronic air cleaners should be able to filter mold spores but the best remedy is installing a UV light next to the coil which will kill the mold.
Vinick calls air duct cleaning an essential part of home maintenance, akin to mopping and vacuuming. "It’s like changing the oil in your car,” he says. “If you don’t change the oil, you’re going to have a problem. When your components are loaded up with debris, the system has to work harder. When you remove that debris, you get energy consumption savings.”
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: