Direct answer
This glossary defines the twenty terms that recur across DuctCleaning.ws: the parts of a residential duct system, the equipment a cleaning crew brings to the job, and the services that are frequently confused with duct cleaning. Each entry gives a one-sentence definition that stands on its own, a longer explanation, and a link to the guide where the term does real work. Definitions here are the ones used everywhere else on the site.
Why we define these terms once
Duct cleaning is sold in language the buyer cannot check. An advertisement that promises a “whole house” cleaning may or may not include the air handler. A quote priced “per vent” may count a return opening the same as a supply opening, or at double the rate. Two companies can describe the same job in words that sound unrelated.
Our answer is to fix the vocabulary. Every page on this site uses the definitions below, in these exact senses, and links back here rather than redefining a term locally. If you find a page that drifts, that is a defect and we would like to hear about it through our corrections process.
These definitions are written to be quotable. Each one is a single sentence that carries its own context, so it can be lifted out of the page without becoming misleading. Other publications are welcome to cite them.
Air duct
Definition: An air duct is the sheet-metal, fiberglass, or flexible passage that carries conditioned air through a building.
Residential duct systems are usually a mix of materials. Rigid sheet metal trunks run from the air handler through a basement, crawlspace, or attic; flexible duct branches off to individual rooms; some systems use rigid fiberglass duct board or sheet metal lined with fiberglass on the inside. The material matters more than most homeowners expect, because it determines what can be cleaned and what has to be replaced. EPA is explicit that insulation which becomes wet or moldy cannot be effectively cleaned and should be removed and replaced.
Related: supply duct, return duct, flexible duct, plenum. See also: what actually happens during a duct cleaning.
Air handler
Definition: An air handler is the cabinet containing the blower and coil that moves and conditions the air distributed through a home’s ducts.
The air handler is the center of the system and the single most important thing to ask about when comparing quotes. A cleaning that covers the ducts but not the air handler leaves the blower and the evaporator coil untouched, and those components are where moisture and accumulated debris tend to concentrate. The National Air Duct Cleaners Association (NADCA), the industry trade body, warns that very low-priced whole-house specials often exclude exactly these parts. In a furnace-based system, the equivalent cabinet is the furnace itself.
Related: blower, evaporator coil, plenum. See also: the questions that reveal what a quote actually covers.
Air whip
Definition: An air whip is a compressed-air tool with flexible lashes that agitates debris off the interior walls of a duct.
The whip is fed into the duct on a hose; compressed air spins the lashes, which strike and disturb settled dust so it can be carried away by the airflow the vacuum equipment is producing. It is one of two common agitation approaches, the other being a rotary brush. Agitation on its own accomplishes nothing useful and can make matters worse, which is why it is meant to be paired with negative pressure in the duct being worked on. A whip is the less forceful of the two, which matters on flexible duct, since flex is more easily damaged than rigid duct. Editorial assessment
Related: rotary brush, negative pressure, negative-air machine. See also: how the equipment fits together.
Blower
Definition: The blower is the fan inside the air handler that moves air through the duct system.
Because every cubic foot of air in the house passes across it, the blower wheel collects a film of dust that a duct-only cleaning does not touch. Blower cleaning is often quoted as a separate line item rather than being included by default. We have found published prices for it in only one trade source, so we do not present a figure as a market rate; see the cost guide for how we handle thinly sourced numbers.
Related: air handler, evaporator coil. See also: the sequence of a full-system cleaning.
Dryer vent cleaning
Definition: Dryer vent cleaning is the clearing of lint from a clothes dryer’s exhaust duct, and it is a fire-safety service unrelated to the HVAC duct system.
The dryer exhaust is a separate duct with a separate purpose: it carries hot, moist, lint-laden air from the dryer to the outdoors. It shares no air with the heating and cooling system. Duct cleaning companies commonly sell it as an add-on, and bundling it is usually cheaper than buying it standalone. Treat it as its own decision with its own justification, because the reasons to do it (restricted airflow, longer drying times, lint accumulation) have nothing to do with the reasons anyone gives for cleaning HVAC ducts.
Related: air duct. See also: how add-on services are priced.
Duct sanitizing
Definition: Duct sanitizing is the application of an antimicrobial product inside ductwork, and it is subject to federal pesticide registration rules that most homeowners are never told about.
The regulatory position is narrower than the marketing suggests, and the details matter. A small number of products are registered by EPA specifically for use on the inside of bare sheet metal air ducts. No products are registered by EPA as biocides for use on fiberglass duct board or fiberglass-lined ducts. A general-purpose disinfectant labeled for hard, non-porous surfaces does not thereby authorize use inside an HVAC system unless the label specifically says so, and using a pesticide product inconsistently with its label directions violates federal law. EPA also notes that little research has been done on the effectiveness of most biocides and ozone inside ducts. So an “antimicrobial fogging” upsell on a fiberglass-lined system has no EPA-registered basis. This is worth knowing before a technician offers it in your hallway.
Related: mold remediation, flexible duct. See also: what the evidence supports on mold in ducts, and how upsells are sold on site.
Duct sealing
Definition: Duct sealing is the closing of leaks at duct joints and seams, and it is a different service from duct cleaning even though the same companies often sell both.
Sealing addresses air escaping from the duct system into unconditioned space. Cleaning addresses material inside the ducts. They have different justifications, different equipment, and different outcomes, and a quote that blurs them is a quote you cannot compare against another one. There is a second, separate meaning to guard against: sealants used to encapsulate contaminants on the inside of ducts. EPA, NADCA, the North American Insulation Manufacturers Association, and the Sheet Metal and Air Conditioning Contractors’ National Association all decline to recommend the routine use of sealants for that purpose in any type of duct, and EPA adds that little is known about the toxicity of such products in typical use or if they catch fire.
Related: air duct, duct sanitizing. See also: services commonly confused with cleaning.
Evaporator coil
Definition: The evaporator coil is the cooling coil inside or near the air handler, and condensation on it is a key source of moisture in an HVAC system.
In cooling mode the coil runs below the dew point of the air passing over it, so water condenses on its surface and drains away. EPA identifies condensation on cooling coils as a major factor in moisture contamination of the system. That makes the coil and the area around it the first place to look when there is a genuine moisture or biological growth concern, and it is a reason to be skeptical of a mold diagnosis that focuses on duct interiors while ignoring the coil. Coil cleaning is usually quoted separately from duct cleaning.
Related: air handler, blower, mold remediation. See also: where moisture problems actually start.
Flexible duct
Definition: Flexible duct is a wire-helix duct with a plastic inner liner and an insulating jacket, and it is more easily damaged than rigid duct.
Flex is common on branch runs to individual rooms because it is fast to install and turns corners without fittings. The fragility is the point for cleaning purposes: the inner liner can be torn by aggressive mechanical agitation, and a torn liner is a repair, not a cleaning. It is a fair question to ask a company how it handles flex, and what it does if a run is damaged during the work. Flexible duct is also a case where the fiberglass registration limits on sanitizing products may apply.
Related: air whip, rotary brush, air duct. See also: damage that can happen during a cleaning.
Grille
Definition: A grille is a fixed vent cover without a damper, usually found on a return opening.
The practical use of the word is in reading a quote. Return openings are larger, fewer, and generally priced above supply openings in published cost tables, so a “per vent” price is ambiguous until you know which kind of opening is being counted and at what rate. If a company quotes one flat per-vent number, ask what it charges for returns before you sign.
Related: register, return duct. See also: the per-opening pricing model, applied.
Heating, ventilation, and air-conditioning (HVAC)
Definition: HVAC is the collective term for the equipment and distribution system that heats, cools, and circulates air in a building.
On this site, HVAC means the whole assembly: the air handler or furnace, the coil, the blower, the plenums, and all the supply and return ductwork. “Duct cleaning” is a narrower term that in ordinary usage covers only part of that assembly, which is precisely where scope disputes come from. We expand the acronym on first use on every page.
Related: air handler, air duct. See also: how the pieces of the system relate.
Mold remediation
Definition: Mold remediation is the controlled removal of mold contamination together with correction of the moisture source that caused it.
The second half of that sentence is the half people skip. EPA’s guidance on duct cleaning is consistent on the point that underlying causes must be corrected or the problem returns. Remediation is a distinct trade from duct cleaning, priced differently and scoped differently, and the published dollar figures that circulate under headings like “mold in ducts” are almost always whole-house remediation figures rather than a duct-cleaning add-on. Two further facts are worth carrying: visual inspection alone cannot confirm that a substance is mold, and once fiberglass duct liner is contaminated, cleaning is not sufficient to prevent regrowth.
Related: duct sanitizing, evaporator coil, flexible duct. See also: mold in air ducts and why the published mold prices vary twentyfold.
National Air Duct Cleaners Association (NADCA)
Definition: NADCA is a trade association for the duct cleaning industry, not a government regulator, and its certification is a voluntary industry credential rather than a legal requirement.
NADCA publishes standards, cost guidance, and consumer warnings, and its warnings are often blunt about its own industry: it cautions homeowners against “blow-and-go” operators, against being told mold is present without a proper inspection, and against extremely low-cost whole-house specials that quietly exclude the air handler, blower, coils, or registers. It also warns that some companies use its logo or claim membership without being members, which is why verification is worth the two minutes it takes. Because NADCA is an industry body, we attribute its guidance as industry guidance and pair it with a government source wherever a claim carries real consequences. DuctCleaning.ws is not NADCA-certified, affiliated, or endorsed.
Related: air handler, duct sanitizing. See also: what NADCA certification does and does not guarantee.
Negative-air machine
Definition: A negative-air machine is the vacuum unit, either truck-mounted or a portable HEPA-filtered model, that creates negative pressure in a duct system during cleaning.
It is the collection half of the job. The unit connects to the duct system, usually near the air handler, and pulls air out of it so that everything an air whip or brush knocks loose travels toward the machine instead of into the house. Where the exhaust goes and how the equipment is filtered are reasonable things to ask a company about before work starts.
Related: negative pressure, air whip, rotary brush. See also: how duct cleaning works.
Negative pressure
Definition: Negative pressure means holding the pressure inside a duct below the pressure of the surrounding room so that dislodged debris is drawn into collection equipment rather than pushed into living space.
This is the single concept that separates a controlled cleaning from an expensive way of redistributing dust. Agitation without collection has a documented failure mode: the Michigan Attorney General’s consumer warning about duct cleaning scams describes dirt being blown onto drapes, carpets, and furniture. Negative pressure is what prevents that. When a company describes its process, the part to listen for is not the brand of brush but whether and how the system is placed under negative pressure while the agitation happens.
Related: negative-air machine, air whip, rotary brush. See also: the mechanics of a controlled cleaning.
Plenum
Definition: A plenum is the large air chamber attached directly to the air handler where the supply or return ducts connect.
Most systems have two: a supply plenum on the discharge side and a return plenum on the intake side. Plenums matter commercially as well as physically, because connecting vacuum equipment to a system generally requires an access opening that has to be created and then closed again. NADCA warns that very low-priced whole-house specials may exclude the necessary access openings. If a quote does not mention them, ask.
Related: air handler, supply duct, return duct. See also: the steps of a full cleaning.
Register
Definition: A register is a vent cover with adjustable louvers or a damper, usually found on a supply opening.
The everyday word for these is “vents,” and that word is doing a lot of work in duct cleaning advertising, because the most common consumer pricing model in this industry is per vent. Counting the registers in your home before you call anyone is the cheapest preparation you can do: it converts a per-vent quote into a total you can compare. Remember to count the return grilles separately, since they are typically priced higher.
Related: grille, supply duct. See also: estimate a total from your vent count.
Return duct
Definition: A return duct carries air from rooms back to the air handler.
Returns are the intake side of the loop. They are usually larger and fewer than supply runs, and their covers are grilles rather than registers. Two consequences follow. First, published price tables put return openings above supply openings, so vent counts are not interchangeable. Second, a dusty return grille is the thing homeowners most often point at as evidence of dirty ducts, and EPA’s position is that dusty return registers on their own are normal and do not by themselves indicate contamination.
Related: supply duct, grille, plenum. See also: what actually indicates a problem.
Rotary brush
Definition: A rotary brush is a motor-driven brush pushed through a duct to dislodge debris mechanically.
A brush is the more forceful of the two common agitation methods. On flexible duct and fiberglass-lined duct that force is worth asking about, because flex is more easily damaged than rigid duct and the surface being brushed is the part that can be harmed. Editorial assessment As with an air whip, a brush is only half of a method: without negative pressure collecting what it loosens, it moves dust rather than removing it.
Related: air whip, negative pressure, flexible duct. See also: agitation and collection explained.
Supply duct
Definition: A supply duct carries conditioned air from the air handler to the rooms of a home.
Supply runs are the outbound half of the loop and the branch most homeowners picture when they think about duct cleaning, since they end at the registers in each room. They also carry the one symptom EPA treats as a genuine trigger for cleaning: debris clogging the ducts badly enough that particles are actually being released into the home from the supply registers. Visible dust on a grille is not that. Particles coming out of the supply side is.
Related: return duct, register, plenum. See also: the three conditions that justify cleaning.
Terms we avoid, and why
Some vocabulary common in this industry does not appear on our pages as fact, because we could not verify the underlying claim in a government, court, standards-body, or investigative source.
- “Sanitized,” “disinfected,” or “purified” air. We describe what a product is registered to do, not what a brochure says it achieves. See duct sanitizing.
- “Healthier air” as an outcome of cleaning. EPA states duct cleaning has never been shown to prevent health problems, and that statement applies to the whole service category, including work done well by reputable firms. We do not write around it.
- “Planted debris.” This appears in industry marketing writing but we found no regulator, court, or investigative source documenting it, so we treat it as folklore and do not publish it as a scam pattern.
Citing this glossary
Definitions on this page are written to be quoted directly. If you cite one, a link back to the specific anchor (for example /glossary/#negative-pressure) is appreciated and keeps the definition attached to its context. Anchors are stable and we do not renumber them.
What to do next
- How duct cleaning works — the equipment terms above, shown in the order a crew actually uses them.
- Do I need duct cleaning? — the three conditions EPA says justify the job, and the symptoms that do not.
- Duct cleaning facts and figures — the sourced numbers behind these definitions, with their evidence strength marked.
Sources & references
- U.S. Environmental Protection AgencyShould You Have the Air Ducts in Your Home Cleaned?Page last updated February 19, 2026Source for the definitions of system components, the three conditions justifying cleaning, biocide registration limits, and the position on sealants and moisture.
- U.S. Environmental Protection AgencyUse of Disinfectants and Sanitizers in Heating, Ventilation, Air Conditioning, and Refrigeration SystemsPage last updated July 17, 2026A hard-surface disinfectant label does not authorize HVAC use unless the label says so.
- National Air Duct Cleaners AssociationAvoid Air Duct Cleaning ScamsUndatedIndustry guidance. Source for blow-and-go, low-cost whole-house specials excluding the air handler, and misuse of the NADCA logo.
- Michigan Department of Attorney GeneralAG Nessel Warns Michigan Residents About Air Duct Cleaning ScamsDecember 12, 2023Government-voice description of dirt blown onto furnishings during uncontrolled cleaning.