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It’s a fair question. It stands to reason that using powerful equipment to blow and suck dirt in order to move it from inside your ducts to an industrial containment off-site might result in some residual debris or airborne dust in your house. But how much is acceptable, and how much is normal? A good tech from a reputable company should do everything they can to minimize this nuisance

Preliminary precautions to keep your home clean

The first thing any tech should do when entering your home is either remove their shoes or place booties over them to prevent depositing onto your floor whatever was on their soles. They’ll also likely lay down tarps to protect your flooring from the large vacuum hose that will run from their truck down to your furnace (unless they’re using portable equipment). Some technicians will up the meticulousness factor further by placing corner guards to protect wall corners from any potential damage. 

In order to position the vacuum hose in your main trunk line, they’ll need to cut access into it, and for this task they’ll probably use tin snips. To prevent those tiny metal shavings from falling onto your floor (ouch!), there are products, such as Chipmaster, that collect the shards before they fall. Any technician not taking this simple precaution should cleanly sweep them up. 

Another precaution they may take, depending on how large your home is and what level of cleaning you’ve selected, is to cover the vents with adhesive or magnetic vent covers. These aid in keeping the dust level down when each of the other vents are being serviced with the cleaning tools. The covers can also help to create extra suction power. On a smaller system (under 2500 sq ft), they may not be needed and in fact may be counterproductive because of the sheer force of the vacuum.

During the service

Most air duct cleanings work by utilizing the power of negative pressure. An industrial-strength vacuum is hooked into the trunk line near the furnace. Then, with the system under negative pressure, the technician services each vent, room by room, with various tools designed to move debris from the branch lines to the trunk, where it's captured by the vacuum.

This negative pressure is the key component to minimizing the blow-back of debris into the house: all of the force or pressure exerted by the tools is directed toward the vacuum, increasing the negative air flow. Any vent covers placed over the registers will help in this process, both by creating more suction and by preventing debris from exiting the vents that aren't being serviced. 

In some cases, especially when the vents are particularly dirty, a small amount of dust from the corner of the vent will go airborne briefly in a small poof when the tool is initially engaged, usually being quickly sucked back into the system shortly thereafter.

When things go wrong

Things don't always go according to plan, of course, and sometimes despite the best intentions more dust than we'd like escapes the system and enters the living space. There are a few scenarios under which this could happen. 

In some cases when additions are constructed onto houses or retrofitting has been carried out, new vents are installed too close to the existing ones, or two vents are installed on the same branch. This unconventional installation is one of the most common reasons for dust exiting the vents during a cleaning. 

Excessive dust can also occur when a basic duct cleaning is performed on a system that begs for a higher-level cleaning, since large volumes of dust are present and being agitated but the tools used are insufficient to handle it all. Sometimes budget-conscious customers opt for a lower level of cleaning than needed, reasoning that a basic cleaning is better than none at all. It is arguably better to simply wait until the appropriate level of cleaning can be purchased. 

Finally, a cheap, ill-fitting, or bent furnace filter could easily allow dust to bypass the filter during the cleaning of the returns, causing dust to enter the supply line and escape from the supply vents. When this occurs, it is usually quickly apparent to anyone in the house, and a diligent technician will specifically check for these things before beginning the cleaning. 

In the event any of these problems occur, the technician should immediately halt work, investigate, correct the problem, and finally—clean up.

After the fact

With the proper precautions, many on the part of the tech but some (like air filter issues) on the part of the homeowner, excessive dust entering the home during routine air duct cleaning can easily be avoided. Small amounts of residual dust are normal and difficult to avoid, but large amounts of airborne debris settling onto furniture and into the carpet are not. 

A diligent technician will not only take precautions to prevent the accumulation of debris inside the home but will also perform any cleanup necessary. A diligent homeowner will inform the technician if they are unsatisfied with any aspect of the job, including residual debris. The cleaning company will want to be made aware of these occurrences as well, so that they can ensure technicians receive proper or additional training in this regard, and so that they can ensure the homeowner is satisfied with the final outcome.

Wanna dig deeper? Download our free tipsheet: 10 Questions to Ask Before Hiring an Air Duct Cleaning Company. 

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Many thanks to our technicians Ben S and Roy S for lending their expertise to this article.