Heating and Air Conditioning Advice

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Duct Cleaning

Sucking More Money Than Dust

a chicken stares fearfully at the skyDust is falling! Dust is falling!

Studies prove duct cleaning does NOT improve air quality. What’s worse, it might even make your home’s air quality worse and damage your ducts. If you’d like to skip straight to the studies, scroll down to Einstein. Of course a full understanding takes more than a few blurbs. So, if you have time, please join me for a wild ride through the wacky world of duct cleaning. By the time it’s over, you may also conclude that it’s nothing more than dust in the wind.


The science is in: Not only will your air NOT be cleaner following a duct cleaning, in many cases it’ll be dirtier! And in some cases your ducts will be damaged. Consequently, cleaning the ducts of a typical residential central heating and air conditioning system is a waste of money.

Table of Contents

Typical Ads for Duct Cleaning

Debunking The Ads’ Quotes

Debunking the Ads’ Pictures

The True Source of Dirt Near Vents

The True Source of Dirt in Their Vacuum

Will It Reduce Mold?

Will It Improve System Performance?

The Real Science

The Fake Science

What Really Happens During Duct Cleaning


Those Alarming Ads

danger sign

“The sky is falling! The sky is falling! I must go and tell the king!”

Chicken Little’s bump on the noggin is nothing compared to what you have to worry about. Advertisements for duct cleaning read like this:

“We spend as much as ninety percent of our day indoors.”
“Indoor air can be up to ten times more polluted than outside air.”
“Health organizations, scientists, and medical professionals tell us that
airborne dust may be one of the most common causes of health problems.”
“Billions of dollars are spent annually to treat the symptoms of contaminated air.”

“I saw it with my own eyes, and heard it with my own ears, and part of it fell on my head!”

Then they pull out the big guns. Chicken Little had an acorn to deal with. You have much worse. Open the doors if you dare…

scary looking door one scary looking door two

Not everyone appreciates my sarcasm. Nevertheless, peruse your junk mail and you’ll see pictures like those. The visual impact combined with seemingly factual quotes make for a persuasive argument. If that’s not enough, just look at your vents. Filthy, right? Surely your central heating and air conditioning system is spewing out vile, dust laden air.

“Go home in peace and do not fear because the sky cannot fall.”

Duct cleaners aren’t frightened little chickens, but they sometimes act like it. Duct cleaners use fear-based marketing to convince you that their “acorns of data” mean something they don’t. Let’s crack six of those nuts one at a time.

Those “Factual” Quotes

Dr. Evil doing air quotes

Most of the ads I’ve seen don’t actually state that your air will be cleaner or your health improved by way of duct cleaning. Instead, they quote all sorts of so-called facts about dirty air and illness. Then they tell you about duct cleaning. Then they let you make the connection between your supposedly dirty air and your ducts. What those ads rely upon is the joint effect fallacy. That’s when A is said to be caused by B, when in fact both A and B are caused by C.

Without actually saying so, duct cleaners want you to believe that dirty ducts cause dirty air. As you’ll see below, that’s almost never true. Most ducts are made of plastic and metal. Plastic and metal ducts have no inherent ability to contaminate the air. If your air is dirty, and/or if your ducts are dirty, they are almost certainly dirty for the same reason. Particulate from the outdoors, carpets, clothing, pets, etc. are the usual sources of airborne dust.

Those Hideous Pictures

frightened baby

Few residential ducts look like the one behined the second door above. Those that do are usually “return ducts”. Your HVAC''HVAC'' is an acronym for ''heating, ventilation, air conditioning''. system draws air from the house through one or more return ducts. If the air filter is near the HVAC equipment, then the air probably runs through the return duct(s) unfiltered. That may sound bad, but in most cases it just doesn’t matter. The “supply ducts” are downstream of the filter. They don’t normally accumulate any more dust than this. That thin film of dust is functionally meaningless.

Add to that this bit of warped logic that isn’t actually warped: Ducts are incapable of producing dust. Therefore, they can only collect dust. It’s rare that ducts release any dust. Generally speaking they only collect dust and most collect very little. So, to whatever extent ducts do collect dust, this much is probably true: The more dust your ducts collect from the air, the less dust that’s in the air. So when you see a dirty duct, take heart. Your ducts are cleaning the air for you!

But what about dust mites? With monsters like that in your ducts, who can blame you for cleaning them? The reality is that it’s unlikely those monsters are inside your ducts at all. Living creatures need water to live. Entomologists say that dust mites need 70% relative humidity to survive. Turn your heater on for a few minutes and those ducts become hotter and drier than the desert. Even running the air conditioner dries out your ducts. A significant percent of your AC’s capacity is spent dehumidifying the air.

The Dirt Near the Vents

a friendly ghost

Fastidious homeowners don’t need quotes or pictures to be sold on duct cleaning. Dirt near the vents is all the proof they need that their ducts need to be cleaned. However, as you have read, not everything in the world of indoor air quality is as it appears to be. The source of the dirt near the vents isn’t always what it appears to be either.

The easiest way to understand where that dirt may come from is to look at this picture. Circular currents cause deposits to form on and near the vents. In other words, the dirt near the vent may not have come from the vent. Dirt ghosting and streaking is a well-researched phenomenon. You can read more about it herehere, and here.

The Dirt in Their Vacuum

a pile of dirt

Some duct cleaners have courage enough to show customers what was removed from their ducts. I witnessed this years ago while working for a large HVAC''HVAC'' is an acronym for ''heating, ventilation, air conditioning''. contractor. Said contractor bundled so-called tune-ups with their duct cleanings. In some cases I’d arrive to perform the tune-up just as the duct cleaners were showing the customers what was removed from their ducts. The customers’ response was telling.

Most of the time customers responded with disappointed silence as they gazed upon the small amount of dirt sitting in the big vacuum before them. Unphased, the duct cleaners dispensed well-rehearsed mumbo-jumbo to assure those customers of the significance of that cup of dirt. But on other occasions customers saw a truly impressive amount of dirt. That made those customers very happy indeed.

Most of those happy customers had floor vents. The dirt directly under a floor vent doesn’t come from the HVAC system. It falls in from the floor above. Most of it is too heavy for their big vacuum to suck up from the other side of the duct system. So the duct cleaners vacuumed it out with a small shop vac and dumped it into the big vacuum. In most cases, the majority of the big vacuum’s “catch” came from the little vacuum.

What about mold?

blue cheese

The mold in your salad may be blue, but for lawyers mold is gold. Duct cleaners are all too happy to sit at the foot of the table judge’s bench and wait for the crumbles to drop. Despite the payouts, I have yet to find solid evidence of a connection between mold and widespread illness. Even so, it’s reasonable for those sensitive to mold to wonder if it’s growing in their ducts. The answer to that question may be found in the very nature of mold itself.

According to these folks, most molds need sustained “water activity” that’s equivalent 70% RH to grow. Under normal circumstances, the humidity inside a duct system is considerably less than that. Generally speaking, the cooling coil and its drain are the only parts of a properly functioning air conditioning system that may see that much sustained humidity. If mold is growing there, you’re better off calling an HVAC''HVAC'' is an acronym for ''heating, ventilation, air conditioning''. tradesperson than you are a duct cleaner.

Plumbing leaks, poor ground drainage, poor ventilation, and the like are the usual sources of excess moisture. Get rid of excess moisture and the mold growth will almost certainly follow. Then, if you really do have mold in your ducts that you want completely removed, you may just have to replace the affected ducts. It takes a lot more than the swipe of a duct cleaner’s brush to remove those embedded buggers.

What about performance?


Many years ago I attended a trade show in San Francisco. The National Air Duct Cleaners Association had a booth. I asked the gentleman manning the booth if there was any evidence that cleaning ducts did anyone any good. His answer went something like this: “Oh… We don’t just focus on ducts. We also recommend cleaning blower wheels and evaporator coils.” Much to my dismay, the NADCA rep couldn’t come up with any evidence that cleaning ducts produces any gains in air quality or anything else.

The booth boy’s answer was a clever dodge. Cleaning dirty coils and wheels may not improve air quality, but it can improve system performance. Repairing damaged and/or leaking ducts can also improve system performance and sometimes even air quality. However, implementing those improvements requires skill. As such, if your heating and air conditioning system needs those improvements, it’s probably best to hire a tradesperson who went to trade school, not a duct cleaner who went to duct school.

The Real Science

Albert Einstein

We’ve busted the myths. Now let’s take a look at the facts as borne out by studies. We’ll follow that with some comments from respected medical professionals and other experts. Click the blue links to see source material.

The Studies

From a study of fifteen office buildings:
“Duct cleaning had no measurable effect on supply air quality.”

From a study sponsored by the EPA:
…the airborne concentrations before and after (duct) cleaning were not substantially different…

From a study sponsored by the Canada Mortgage and Housing Corporation:
“…there was no significant reduction (of dust in the air) after duct cleaning. Some houses showed a temporary increase in dust levels…”

From a study performed by Indoor Biotechnologies:
“…there were no mite allergens going in because the air ducts are not a good environment for mite growth… there are no mites in there.”

From the same Indoor Biotechnologies study:
“…when the air ducts of these homes were tested, only 2 of the 27 samples showed dust mite allergen at the 0.5 microgram per gram of dust threshold of detection. An allergen level below 1 microgram per gram of dust is considered too low to cause allergic reactions.“

The Professionals

From Jeffrey Siegel, a University of Toronto professor who specializes in indoor air quality:
“There’s a… review paper… someone found 60 or 70 studies on duct cleaning… They found that duct cleaning doesn’t make a difference.”

From the American Academy of Allergy, Asthma and Immunology (AAAAI):
“There are no data to support the practice of duct cleaning as far as allergen environmental control is concerned, and an ultraviolet filter would not be helpful for aeroallergen filtration.”

From the American College of Allergy, Asthma & Immunology:
“Duct cleaning has not been proven to reduce exposure to furry animal allergens.”

From Consumers’ Checkbook:
The duct-cleaning industry promotes a solution in search of a problem.”

From the past president of the AAAAI:
“I would not recommend that someone get their air ducts cleaned.”

What makes the EPA study especially interesting is that it was co-sponsored by the National Air Duct Cleaners Association. Quite naturally, NADCA saw to it that the very best practices were used. Nevertheless, their best wasn’t good enough. When you factor out the effect of background particulate, the air in those homes was not cleaner after the duct cleaning. It was dirtier! This Washington Post story talks about that same study and more.

The Fake Science

Young Frankenstein

To be fair, the study I’ve heard duct cleaners talk about the most is not fake at all. You can get a copy of it here. It appears to have been conducted by real scientists and published in the Annals of Allergy decades ago. Many a duct cleaner has cited it as proof that their service is worthwhile. Ostensibly reputable trade magazines have done the same. So what’s the problem? This easily missed sentence from the study report is the problem:

“A permanent, washable electrostatic air filter was installed in each system.”

While that study may be real, how it’s used to prove duct cleaning’s efficacy is not. The researchers were NOT studying the effect of duct cleaning alone, but rather the effect of duct cleaning combined with coil cleaning and improved filtration. Such a combination of measures proves nothing about any one measure alone. If any one measure was responsible for the improved air quality, common sense suggests it was the upgraded air filter.

Almost twenty years later this study came out. It also appears to be legit and, at first glance, its concluding remarks seem supportive of duct cleaning. However, the actual results were disappointing: The ducts looked cleaner, but air quality was unchanged. The duct cleaning industry has been swinging and missing for decades, but apparently that’s not going to stop them from swinging some more!

What’s really going on?

Sherlock Holmes
The Motivation

In some cases a duct cleaner can generate the same revenue as an HVAC''HVAC'' is an acronym for ''heating, ventilation, air conditioning''. repairperson while costing half as much to employ. Given that profitability, it becomes easy for contractors to ignore the facts and embrace the fiction.

The Work

Duct cleaning methods vary wildly. Simply hooking up a truck mounted vacuum to your system is a particularly useless method. Your car travels down the road at over sixty miles per hour, yet the dust clings stubbornly. So does the dust in the ducts, what little there is, unless they agitate it. They have to knock the dust loose or there won’t be much to vacuum.

That’s where the big roto-rooter looking brush comes in. Companies that use one are at least removing some of the dirt. But they have a problem: The brush is of a fixed size. It has to fit comfortably down the smallest duct it passes through. As smaller branch ducts progress into larger trunk ducts, the brush scrubs less and less of the duct walls.

Then they have another problem: Ducts often make sharp turns. As they feed the rotating brush down the duct it eventually comes to a point where the brush will no longer go in. At that point they’re done with that duct whether it’s completely clean or not. Much of the time it’s just not possible to clean all of the ducts. Sometimes they don’t even clean half of them.

Then comes the worst of all potential problems: In some cases they can knock a duct loose or even collapse a duct’s inner liner. Since they do most of their work from inside the living space, and since they can’t see inside most of the ducts, they may not even know about the damage. You may be the one to discover it later when you notice the resulting lack of airflow.

The Result

What you could end up with is lost time out of your day, hundreds of dollars out of your pocket, possibly damaged ducts (damage that you may not become aware of for months), a partially cleaned duct system, temporarily dirtier air, unchanged or worse system performance, and a lot of smoke and mirror promises.


chicken dancing

Study after study has shown duct cleaning to be ineffective at improving air quality. Now that you know a few basics about duct cleaning and air quality, your own common sense probably shows the same. The odds of that ever changing are about as good as that of the sky falling.

If your ducts really do contain something awful like rodent droppings, then you may want to replace the affected ducts. But if your ducts are just a little dusty, it might be best to leave them alone. There are more important things to worry about than dusty ducts!


Nothing on this page should be taken as medical advice. If you believe poor air quality is affecting your health, then you should immediately seek help from a qualified medical professional. You may also consider hiring a true air quality expert such as a certified industrial hygienist.

The opinions expressed on this page refer to duct cleaning as it’s most commonly applied to local residential HVAC systems. They do not refer to commercial systems, or to systems serving those with medical conditions, or to dryer vent cleaning, or to anything else not explicitly stated.