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
cleaning often read like this:
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
"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...
through the doors to reveal the horror (and
turn on your speakers)!
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
"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.
Those "factual" quotes.
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.
Those hideous pictures.
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.
would be an example
of what I'm talking about. The air filter does exactly what it's supposed
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.
The dirt on
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
What about system performance?
Years ago I attended an HVAC
trade show in San Francisco. Much to my delight there was a representative
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.
NADCA rep was smart to answer as he did.
Everyone knows that a
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.
But the duct cleaners
pulled out so much.
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
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.
So forget the dirt. I'm
worried about mold!
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
of a connection between mold and widespread illness some
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
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
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.
still don't believe you!
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.
"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.
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.
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.
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.
"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.
Indoor Air 2002 (1C6o5 on page
"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.
"The results of the particle mass measurements suggest that...
airborne concentrations before and after cleaning were not substantially
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
Washington Post story talks about that same study and more.
But I saw a study that
the benefit duct cleaning!
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
cleaning". Such a combination of measures proves nothing about any one
measure in particular.
bit of obvious logic,
many duct cleaners have
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
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.
what's really going on?
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
business. What makes it even more attractive is that almost anyone can be trained to clean ducts in a
matter of hours.
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.
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.
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!
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
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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