Duct cleaning has taken off like wildfire over the past decade. The advertising for it is near ubiquitous. Customers far and wide are singing its praises. And duct cleaners are making a tidy profit. So everyone is happy, right? Perhaps not. Some believe the four billion dollar indoor air quality industry to be based on lies.

If you want the complete picture then read on. If you want to skip the setup and get down to scientific brass tacks then scroll down to Pinocchio.

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

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

"Billions of dollars are spent annually to treat the symptoms of dirty air."
"Indoor air can be up to ten times more polluted than outside air."
"We spend up to ninety percent of our day indoors."
"Health organizations, scientists and medical practitioners say that
airborne dust may be one of the most common causes of health problems."

"I saw it with my own eyes and part of it fell on my head!"

Then they pull out the big guns. Chicken Little had an acorn. You have much worse. Prepare yourself...

My mockery might be a bit over the top, but if you pay attention to your weekly junk mail then you've seen pictures like those. The visual impact combined with quotes from official sounding entities make for a persuasive argument. If that's not enough, just look up at your ceiling 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."

I'm not a king and you're not a fearful little chicken. But the comparison between the story of Chicken Little and the phenomenon of duct cleaning is surprisingly accurate. Much of the duct cleaning industry relies on taking an "acorn of data" and persuading you that it means something it doesn't. Let us debunk piece by piece.

Most duct cleaning advertisements I've seen don't actually state that the air will be cleaner as a result of their service. Instead, they tell you all sorts of facts about dirty air and illness. And oh, by the way, your ducts are dirty too. Then they allow you to make the connection. Those ads rely on the joint effect fallacy. That's when one effect is said to be caused by another when in fact both are caused by the same thing. More simply put, some duct cleaners want you to believe that dirty ducts cause dirty air. In reality if your air is dirty then it is so for the same reason as the duct, not because of the duct. Ducts are typically made of plastic and/or metal. They have no inherent ability to add dirt to the air.

The duct behind the first door above is indeed very dirty. But let me put that duct in a different light. In my experience most ducts that look like that are in commercial systems that get constant use. And most of the time that would be a return duct, which doesn't usually have the advantage of having a filter in front of it. Take a look at the supply ducts of that same system and you'll probably see little more than a thin film of dirt. This would be an example of what I'm talking about. The air filter does exactly what it's supposed to do.

Add to that this bit of (what will seem to be at first) warped logic. We can all agree that the duct itself is incapable of producing dirt. Therefore the duct must be collecting dirt. Some will suggest that collected dirt eventually gets released. But look at the grille of an old floor fan and think again. Whether it's the grille of a floor fan or your duct, neither one will release dirt unless you bump them. Ducts only collect very small amounts of dirt. Logically, the more your ducts collect from the air the cleaner your air is. So when you see a filthy duct, take heart. The ducts are cleaning your air!

The dust mite behind the second door above takes the cake. With little monsters like that running around your ducts who can blame you for cleaning them? The reality is that those monsters probably aren't running around your ducts. To suggest otherwise is to ignore a basic fact of biology. Virtually all living creatures require water to live. Dust mites can get by on about 70% relative humidity, sometimes a little less. Turn your furnace on for a few minutes and those ducts become hotter than a desert and even more arid. Even when you run your air conditioner you're drying out your ducts. A significant percent of your air conditioner's capacity is spent removing moisture from the air.

Your typical neat freak just hates the dirt that collects near ceiling vents. It's unsightly and it must be a sign of dirty air coming from the system, right? Not necessarily. Often times it can be explained by aspirated dirt. The picture above does well to explain exactly how it happens. Circular currents cause deposits to form near the vent. It's not automatically a sign of dirty air coming from your ducts.

Years ago I attended an HVAC trade show in San Francisco. Much to my delight there was a representative from the National Air Duct Cleaners Association. I asked him bluntly to show me just one bit of evidence that cleaning my customer's ducts would do them any good. His response? "Oh, we don't just focus on the ducts. We also recommend coil and blower cleaning." He could not come up with one bit of evidence that cleaning a typical system of ducts will produce any gains in system performance, air quality, etc.

The NADCA rep was smart to answer as he did. Everyone knows that a dirty coil and/or dirty blower can reduce performance. More importantly, not only can it happen it does happen - and often. In other words the problem is real. Repairing and sealing leaky ducts also has very real benefits. In fact, duct sealing is one of the few things some duct cleaners do that actually improves air quality... sometimes. However, those real repairs aren't the focus of what duct cleaners do. Nor did they need to contrive the duct cleaning industry to address those real issues. HVAC technicians were cleaning coils and fixing ducts long before duct cleaners came along.

Some duct cleaners actually have courage enough to show the customer what they pulled out of the ducts. I witnessed this working for a big HVAC contractor that bundled furnace tune-ups with their duct cleanings. Sometimes I'd arrive to do the tune-up just as the duct cleaners were showing the customer what they removed from the ducts. More often than not the customer responded with silence. Sometimes the customer asked "That's it?". In either case the well trained duct cleaner immediately went into sales mode and assured the customer of the significance of that quarter cup of dirt. However, once in a while there was an impressive amount of dirt and debris. And boy was that customer grateful.  

However, in most cases what the customer didn't notice amidst the hustle and bustle of the operation was the guy that went around to each floor vent with a small shop vacuum. Most houses more than a few years old have a decent amount of debris in their floor vents. It doesn't come from the HVAC system. It falls in from the floor above and settles in the first couple feet of the duct. A lot of it is too heavy for a duct cleaner's vacuum to pick up from the furnace side of the system. So with shop vacuum in tow they took care of it at the source, something the customer could have easily done. Then before the duct cleaners displayed what they pulled out they opened the lid of the big vacuum and dumped the shop vacuum's contents into it.

As an aside, I ended up talking with all eight duct cleaners. At the time that company had four crews of two. To a man they all told me privately that they thought duct cleaning was a waste of time. They looked at it as a way to bide their time until an opening in the installation department (of the heating and air conditioning side of the company) opened up.

Mold is gold. Or at the very least it is for some lawyers. They've stirred up a frenzy and are laughing all the way to the bank. A good many duct cleaners are all too happy to sit at the foot of their proverbial table and wait for the crumbs to drop. In reality there's no solid scientific proof of a connection between mold and widespread illness some have claimed. Trial Lawyers Inc documents that well.

However, for argument's sake let's say that mold is a bigger problem than we know. Though rare, a particular variety of black mold has been shown to cause serious health problems. And at the very least we can agree that certain molds can aggravate those with asthma and weak immune systems. If so, the question becomes one of the connection between mold and your ducts. The answer to that question is simple: water. If you don't have water, you don't have mold growth. Most molds require a least a few days of liquid water (tiny amounts will do) to establish themselves. Once established most molds still need at least sixty to seventy percent relative humidity to survive. Most of the SF Bay Area simply doesn't see humidity high enough to sustain ongoing mold growth. Nor can your ducts sustain mold growth unless you have an external source of water. Even in humid parts of the country your ducts are constantly being dried out by both your furnace and your air conditioner. Plumbing leaks, roof leaks, poor ventilation, condensation issues, bad ground drainage, excess irrigation and other sources of unwanted water are the real enemies. Legitimate reporting on mold virtually never mentions calling your local duct cleaner.

As you may know, a huge percentage of homes have mold growing somewhere. It's likely that mold spores are fluttering about you at this very moment. Humans have been living with mold for thousands of years. Some of you pay extra to eat it in your cheese. That's not to say that mold isn't a real problem for a relative few. But it is to say that many so called mold problems are really trial lawyer problems. Mold has been called "the next asbestos". It's ironic then that the overreaction to mold and other contaminants may be causing the exact problems that people are trying to avoid. It's widely believed that exposure to a variety of allergens, germs, etc. at a young age enhances the immune system. That's no surprise given our understanding of vaccines. In fact, some of the lowest asthma rates are amongst farm kids who breathe in enough junk to give Richard Thalheimer nightmares. Conversely, a lack of exposure may increase the chances of asthma and allergy problems as an adult. Children are often done a great disservice when their homes are turned into Lysol impregnated clean rooms.

Some people are hard to convince, most of all duct cleaning salesmen and people that just paid to have the service done. So for arguments sake let's assume for a moment that dust and other contaminants are actually spewing out of your ducts and affecting your home's air negatively. The question then becomes: Will the air be cleaner after a visit from your local duct cleaner? Thus far I have seen no major studies on the topic. Major studies require major money. However, there have been several small studies. The results of those small studies may very well explain why no one has funded a major study. Below you'll find quotes from and links to those small studies.

From HealthCentral.com:
"But 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."
In other words, dust mite allergens could not even be measured in 25 of 27 duct systems. And the other 2 didn't have enough to qualify as even a minor problem. All of this was without any duct cleaning.

From Canada Mortgage and Housing:
"In the 1990s duct cleaning research conducted by Canada Mortgage and Housing Corporation and the US Environmental Protection Agency tested house and duct performance before and after cleaning. Researchers observed little or no discernible differences in the concentrations of house airborne particles or in duct airflows due to duct cleaning."
Duct cleaning had virtually no effect on air quality or airflow.

From a related publication:
"Measurements of the dust in house air showed that there was no significant reduction after duct cleaning. Some houses showed a temporary increase in dust levels for several hours after of duct cleaning. This was probably due to dust loosened up by the cleaning process."
Duct cleaning made things temporarily worse.

From Indoor Air 2002 (1C6o5 on page 25):
"Duct cleaning had no measurable effect on supply air quality."
Dust was removed from the ducts but the effect on the air was nil. Interestingly the occupants perceived an improvement in air quality.

From the EPA:
"The results of the particle mass measurements suggest that... the airborne concentrations before and after cleaning were not substantially different..."
Again duct cleaning had virtually no effect on air quality.

What makes the EPA study especially interesting is that it was performed in conjunction with NADCA. Talk about a conflict of interest! Surely NADCA saw to it that the very best methodology was used. Yes, the duct cleaners did pull out dirt. And no, doing so did nothing to improve air quality. Since the sampling was so small the EPA refused to draw a conclusion. But despite that the numbers are there for all to see. When you average the numbers and factor out the effect of background particulate levels, the air in those homes was nine percent dirtier after duct cleaning. This Washington Post story talks about that same study and more.

A study published in the Annals of Allergy, December 1993 ostensibly supports the notion that HVAC sanitation reduces airborne fungi in residences. The overwhelming length and seeming complexity of the study summary might convince you that duct cleaning works as advertised. But there's one key sentence in that report that tells all: "A permanent, washable electrostatic air filter was installed in each system."
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. Hence they used the term "HVAC sanitation" and not "duct cleaning". Such a combination of measures proves nothing about any one measure in particular.

Despite that bit of obvious logic, many duct cleaners have glommed onto that study as proof of duct cleaning's effectiveness. This guy in particular is typically comedic in his half truths. He declares triumphantly that "The Proof is Out There!" He rips the EPA study that I quote above for its supposedly poor methodology, yet fails to mention a very relevant part of his vaunted study's methodology: high efficiency filters were installed in place of cheaper filters! The funniest omission of all? The study he praises showed an increase in some kinds of fungi as a result of the so called HVAC sanitation. This industrial hygienist (you know... a real IAQ professional) talks about that study and more.

This is the kind of intellectual dishonesty that just frosts my Frigidaire. It's the same tactics used by diet pill hustlers. Virtually all of them recommend adding diet and exercise to their pill regimen. Everyone knows that IF any weight is lost then it was the diet and exercise that did it, not the pill. The pill serves only to separate you from your money. The same goes for the study mentioned above. The fact that a better filter improves air quality is as obvious as the fact that diet and exercise cause weight loss. The duct cleaning serves only to separate you from your money.

Measuring airborne contaminants isn't hard. If duct cleaning actually improves air quality then we'd be buried hip deep with studies that prove it. Instead were left with half baked propaganda from 1993. IAQ has become a sixteen billion dollar industry, with four billion of that going to duct cleaners. NADCA, certain HVAC contractors, some so called IAQ specialists and the manufacturers of duct cleaning equipment have a huge vested interest in proving the validity of duct cleaning. Yet in more than a decade it hasn't happened. The feeble attempts that I've seen thus far are nothing but quackery.

The motivation:

Many HVAC contractors are barely making it. It's understandable that they should want to provide a profitable service to their customers. Given that profitability it becomes easy for them to twist logic in order to justify what they're doing. I can tell you from personal experience that some duct cleaners get paid half of what a lot of HVAC repairmen make. Yet often times a team of two duct cleaners can pull in twice the money doing duct cleaning as a single technician can doing repairs. It's not always that way of course. But duct cleaning can be much more profitable than the repair side of the HVAC business. What makes it even more attractive is that almost anyone can be trained to clean ducts in a matter of hours.

The work:

It depends on the company. Companies that just hook up a truck mounted vacuum to your system are accomplishing little. 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 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 branch ducts progress into trunk ducts and get bigger, 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 they eventually come to a point where the brush will no longer go in. At that point they're done. And it's usually well shy of the end of the duct run. Very few duct systems get all their ducts cleaned. Some are lucky to get half of them cleaned.

Then comes the worst problem of all. Many homes, probably the majority in northern California, have what's referred to as wire flex duct. Imagine a big stretched slinky with plastic bonded to it. Wrap that with insulation and then wrap it again with one more layer of plastic and you have wire flex duct. If it's installed well and if it's in good shape then you shouldn't have any problems. But unfortunately wire flex ducts are famous for being installed poorly. Often times the connections are loosely held together with dried out duct tape. And what's worse, there are often cuts and other imperfections in the inner plastic liner. The duct cleaners feed their motorized brush down the duct and RIP! Part of the duct comes apart. Naturally they don't know what's happened and neither do you. Most of what they do is sight unseen. The lack of airflow due to the damage won't be noticeable until it gets especially hot or cold. That's usually long after they're gone. Good luck getting them to cop to it.

The result:

What you often end up with is lost time out of your day, two to seven hundred dollars out of your pocket, possibly damaged ductwork, a partially cleaned duct system, temporarily dirtier air, unchanged or worse system performance and a lot of smoke and mirror assurances from the duct cleaners.

Conclusion

Can duct cleaning be beneficial in residential applications? Under certain unusual circumstances, maybe. But in over fifteen years of experience in the field I've found that it's extraordinarily rare. The science and plain old common sense simply don't back the claims made by many duct cleaners. If you just like the idea of duct cleaning and are willing to risk the damage, fine. But don't do it believing that your system will run better or that your air will be cleaner. The odds are not only against that but they're in favor of exactly the opposite. Save your money. Go out to dinner with family and friends. Stop worrying about what's going to kill you. You're not going to get out of this life alive anyway!

DISCLAIMER: Absolutely nothing in this essay should be interpreted as or taken as medical advice. If you believe poor air quality is affecting your health then you should seek the advice of your doctor or other medical professional immediately.

This essay is strictly the opinion of the author. It's intent is to focus on residential duct cleaning as it is typically advertised and implemented locally. It does not address commercial applications, specialized situations or other unusual circumstances.

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