Article Summary: Having clean ductwork is essential to good health. Unfortunately, most people don’t give clean ductwork the attention it deserves. For those that suffer from Chronic Inflammatory Response Syndrome (CIRS), making sure the air is relatively free of the numerous types of biotoxins produced by mold is critical. If a building has had some level of water damage, these biotoxins invariably have been deposited inside the ductwork. Clearly, for those with CIRS, this source of biotoxins must be remediated for the home to be livable. Even for those for whom CIRS isn’t an issue and haven’t had a water issue in their homes, I contend that regularly cleaning the ductwork is also important to good health. In this article, I’m going explain why clean ductwork is important for good health and how to properly clean it. January 6, 2016
I’m going to begin by explaining in some detail what ductwork is. I think it’s important to have this general understanding so you can appreciate some of the points I’m going to make later on. I’m doing this because my sense in talking with others is that most people have very little understanding of this mechanical system. Given the importance of clean indoor air, I think this oversight is costing a lot of people optimal health.
So let’s get started. Ductwork consists of array of rectangular and round passageways that transfer the warmed air from the furnace and cold air from the Air Conditioning (A/C) coil to the rest of the house. Air is constantly being circulated through this ductwork. This “conditioned” air typically enters our rooms through registers in the floor. It exits our living space to be re-heated or re-cooled via return registers mounted on centrally located walls in the house. Some registers supply conditioned air and some return air to be re-heated or re-cooled.
Average Home Makes 40 lbs. of Dust Yearly
So we all know how dusty the tops of tall cabinets get over time. Fibers from furniture and carpeting, dead skin, dirt from outside, pet hair, and the like build up quickly. Recently I was washing the floor on my hands and knees and commented to my wife that I was surprised at how much dust I was picking up given that the house had just be thoroughly cleaned a few days before. She just laughed and said I needed to wash the floor more often implying that this was normal build up.
So now imagine what the top of a cabinet would look like after a year or more! In the case of ductwork, it’s much worse because there is a very large blower drawing in over 1,000 cubic feet per minute through the return ducts pulling in a large volume of dusty air into the ductwork. The ductwork sees way more dust than the tops of cabinets ever will. Believe me, I’ve pulled apart old ductwork. It’s really filthy.
OK, I’m getting ahead of myself here. Let’s take a minute to back up and look at the typical ductwork layout in single story home with a basement. It all starts at the furnace. For most, the furnace is essentially a 4-foot tall metal box that contains a burner assembly and a large blower motor mounted below. When the thermostat calls for heat, a large flame similar to what you see on top of a gas range is ignited. The heat that is generated is pumped to the various floor registers through out the house via a large blower motor that sits below the hot flames in the furnace and drives the warmed air into the ductwork.
The heated air blasts out of the top of the furnace enclosure. When we worked through the winter in Wisconsin building houses, we would sometimes hook up an old furnace and just let it dump heat into the building without any ductwork. If you got too cold, you could just go over to the running furnace and warm your hands by holding them in the hot air blasting out the top of the furnace. It’s the contractor’s equivalent of a campfire on steroids. Of course without ductwork to distribute this heat, it makes for a really toasty basement and much cooler upper levels, but it definitely took the chill out of the air. In finished homes, the furnace is connected to all the floor registers and return registers via a system of ductwork.
Not surprisingly, this ductwork starts with a large rectangular metal passageway that connects to the top of the furnace called a “plenum”. The plenum is the start of directing the air through out the house. It runs vertically from the top of the furnace and then branches out when it reaches the basement ceiling. Just above the furnace and inside this large sheet metal plenum sits the air-conditioning (A/C) coil. Given that the A/C coil and burner assembly both sit directly above the blower motor, when the thermostat calls for cold air, the burner remains off while the A/C coil is cooled by Freon that comes from the condenser/compressor unit outside. When warm air is required, the A/C coil remains off while the burner assembly kicks in. It’s pretty basic. In either case, the blower operates to move the either warmed or cooled (conditioned) air from the furnace and plenum through out the house.
So now we’ve got massive amounts of conditioned air pouring into our main plenum. At the point where the plenum reaches the basement ceiling, it typically branches off into a couple main “supply trunks”. These supply trunks split the volume of conditioned air from the plenum and direct it to the ends of the house. They are rectangular in shape, roughly 10”x20” in size, and are attached to the basement ceiling. Along side these metal supply trunks are matching “return trunks” that are bringing air from the return registers in the house back to the furnace so it can be reconditioned in a circulating cycle that is all driven by the blower motor. These return trunks connect into their own plenum (return air drop) that connects to the side of the furnace near the bottom where the blower sits.
Returning air enters the side of the furnace near the bottom where the blower motor awaits to grab it and propel it up through the furnace again, into the supply air plenum, across the A/C coil, and back into the supply trunks so it can be sent back into the house. Between the “return air drop” and furnace sits the furnace filter. You know, it’s the cheap 1” filter that most people replace on a yearly basis – at best.
OK, I know this is probably way too much detail and you’re wondering why you are wasting your time reading about ductwork. Hang in there; it matters. So with the main trunks running down the length of the building, the air is then distributed to each room via round 6” diameter tubing that connects from these supply trunks to registers in each room. The supply registers are generally located below windows in order to bath the poorly insulating glass in conditioned air. Every room gets at least one supply register.
The return registers are generally fewer in number and are located on centrally located walls in the house. Unlike the supply registers that must be made of fire resistant material, the return registers are typically connected to the trunks by using wall and floor cavities. In other words, between each pair of wood wall studs that holds up a wall or wood floor joist that support the floor, there is an open space. Given that carpenters frame homes so that wall studs and floor joist align, it’s a simple matter to cut openings between these cavities in order to create a chaseway back to the return trunks. It saves a lot of work using these internal cavities. It’s important to the discussion that follows to realize that the return passageways consist mostly of raw wood, the back of unfinished drywall, and the underside of sub-flooring.
Getting back to supply ducts, it used to be that these round air supply ducts were “hard piped”. This means that they were made from round, 6-foot long, straight sheet metal sections and adjustable metal elbows that were fitted together and screwed tight. In instances where this metal ductwork passed through unconditioned spaces like attics, it would also be wrapped in insulation. Now-a-days, this smooth metal piping is being replaced more and more with corrugated “flex duct”. Can you guess why? That’s right, money. In fact, even the larger trunks are being replaced with flex duct in some cases.
Installers like to argue that flexible ductwork is quieter. They’ll also say that when it’s sized and installed properly, it will deliver the same air flow as smooth walled galvanized metal pipe. OK, so I won’t go into how often flex duct is so often improperly installed because it’s so easy to kink and crush – something that can’t happen with hard pipe. After all, buyers love the fact flex duct reduces the mechanical cost of the home. Bigger is better, right? If you’re spending many times more money hard piping your supply ductwork, it cuts into square footage. What everyone fails to realize is that just like in washing machines, when you use corrugated piping, all manner of debris collects.
Likewise, I won’t get into the issues with trying to clean this stuff well. However, suffice it to say that it takes real care to prevent damaging this flimsy material when it is cleaned. Punching holes in flex duct isn’t a good idea. Granted, if you do damage your ductwork, you’ll be losing some of your conditioned air. What’s worse is that cold air dumping into hidden places creates the potential for condensation issues that may not be realized until everyone starts getting sick from the hidden mold that results. Suffice it to say that as a retired General Contractor, I don’t like flex duct.
OK, enough ranting about the insanity of using flex duct. If you have it in your home, it can be cleaned. It just takes greater care.
Now that you have a general understanding of ductwork, my question for you is how much of it do you think a smallish 1,500 square-foot ranch house has? My impression is that most folks don’t really give this much consideration. It’s the “out of sight – out of mind” mentality. Well, the answer is about 200 feet! It’s a lot.
So if you’re remediating a moldy home, there is a lot of hidden area that needs to be cleaned well for the house to be made safe. After emptying the house, resolving the water source, and removing moldy material, some of the later steps to proper remediation involve cleaning the walls, floors, and ceiling so they can pass a “white glove” test. All dust has to go. If you don’t get a good duct cleaning service, then don’t be surprised if the home is still unlivable for those with CIRS even if all the other steps were done well.
Even for those without CIRS, it just makes good sense to routinely clean the ductwork, blower motor, and A/C evaporator coil. A considerable amount of material in dust is organic – 16-20%. In other words, some kind of bacteria or fungi is going to make lunch out of it depending on the conditions. The general consensus seems to be that it’s just dry dust that is completely inert – meaning it can’t hurt you. Really? If bacteria can live in the 380 degree Fahrenheit hot springs at Yellowstone National Park, I think we can safely assume there is a whole microbial village playing itself out in the dust trapped in ductwork. Let’s look at this closer.
Biotoxins in Ductwork
We know from the work of Dr. Shoemaker, that it’s not the larger particles in dust that makes people with CIRS very sick and taxes the immune systems of everyone else. When it comes down to it, I could care less about mold spores and mold hyphae. It’s the super small toxins produced by mold and bacteria that I really care about – that cause the massive inflammation associated with CIRS. Now granted, measuring mold levels with ERMI and HERTSMI-2 testing is reasonable way of figuring out if there is strong likelihood that those minuscule inflammagens like endotoxins, mycotoxins, VOCs, and the like are also hanging around. It’s just sometimes I think folks get fixated on mold and this can lead to mistaken thinking.
Specific to the topic at hand, it’s a fact that many of the inflammagens associated with mold that make people sick are so small that they pass right through even the best of HEPA filters. They’re that small. Dr. Keith Berndtson writes, “Mold toxins are classified as nanoparticles. They are far too small to be filtered by HEPA systems“. So even if your house hasn’t had a mold issue that you’re addressing in part through duct cleaning, it doesn’t take much stretch of imagination to suppose some of the toxins produced by bacteria and perhaps fungi growing in the dust is producing a similarly microscopic plume of toxins that is deleterious to health – regardless of whether there is visible mold or not.
Given this, I just shake my head incredulously when I see duct cleaning services that bring the vacuum unit inside the building. I don’t care how good the filter is. There is a whole soup of super small toxins that we are just beginning to understand in terms of adverse health effects. To get a sense of the complexity of toxins associated with mold alone, Dr. Ritchie Shoemaker writes “Trying to separate which of the potential initiators of inflammation, with the inflammatory responses showing no differences, is not possible. So-called, “specific causation” simply doesn’t exist as discussed previously. However, there is a significant data base that says that even if not measured specifically, that the multiple inflammagens and toxigens that can cause illness will be found in WDB (Water Damaged Building).“.
When the vacuum unit is brought indoors, a whole cadre of lesser known inflammagens are all being dumped into the building using this method. Ugh. I would argue that even if you don’t have CIRS, simply using HEPA filtration to capture particles over 0.3 microns isn’t enough when it comes to duct cleaning. HEPA filtration is a good start but we can do a lot better by simply keeping the vacuum unit outside. Likewise, duct cleaning studies that look at fungal and bacterial counts only are a nice start but they fall way short in terms of predicting health benefits. We’ve got a lot to learn yet about the relationship of these super small toxins to ill health.
Having said this, the one place I do think smaller portable vacuums make sense for duct cleaning is for condominiums. Generally speaking, condos each have their own independent Heating-Ventilating-Air-Conditioning (HVAC) systems. If the vacuum can be sealed outside on a balcony with the vacuum hose attached to a main trunk, then a good unit that can provide at least 3,000 cubic-feet-per-minute (cfm) seems like the perfect solution. Of course, you’ll want to make sure to keep the unit isolated outside during operation and have the technician blow off the balcony with compressed air as a final step.
The bottom line is that we’re still learning about the pathogens produced by mold and bacteria that can live in dusty ductwork. What we do know is that some of them are way smaller than can be captured with a HEPA filtering and that they can damage health. If there is a thick caked on layer of dirt/dust inside ductwork even without visible signs of mold, it’s given that the organic component of that dirt/dust is feeding some types of bacteria. When it comes down to it, it’s about exercising a little common sense.
I just stand dumb-founded when I hear suggestions that dirty ducts don’t’ matter. At the extreme end of this viewpoint are folks that suggest dirt in the ducts is good because it means some of that dirt isn’t getting into the house air?! 😕 The politically correct EPA view is that you should only clean your ducts if dirt is visibly blasting out of the registers, they have “substantial” mold growth, or small animals are running around inside them. Setting aside Dr. Shoemaker’s citations and ignoring the fact that it’s not uncommon for one square inch of actively growing mold to produce one million or more spores thereby rendering the word “substantial” useless, these views seem to be lacking just plain old common sense. If you were really sick, would you lie next to a supply duct with all manner of non-descript dustiness and let the air bath over you? I think not. And why; because we all intuitively know it may not be too healthy. Let’s look into duct cleaning methods and then take a closer look at duct cleaning studies.
Duct Cleaning Methods
There are a multitude of duct cleaning methods. For the purposes of trying to be consistent with duct cleaning studies, these methods can be divided into three categories. These categories are Contact Vacuum, Mechanical Brush, and Compressed Air. In order to evaluate the following statements by the EPA and DOHS regarding the duct cleaning, it’s important to understand the various methods used to clean ducts.
According to the Environmental Protection Agency (EPA), “Duct cleaning has never been shown to actually prevent health problems. Neither do studies conclusively demonstrate that particle (e.g., dust) levels in homes increase because of dirty air ducts or go down after cleaning“. According to the Division of Occupational Health and Safety (DOHS), “Despite more than two decades of research, there is still not enough evidence to draw solid conclusions about duct cleanings benefits on IAQ (Indoor Air Quality), occupant health, HVAC system performance, or energy savings, according to a 2010 review of scientific studies on duct cleaning…Yet, even when duct cleaning was extremely efficient at removing contaminants within ducts, the effectiveness of reducing indoor air pollutants was highly variable, and, in many cases, post-cleaning levels of contaminants were higher than pre-cleaning levels“.
I’ve already discussed health related concerns. It should be of no surprise that there isn’t any “scientific” evidence to support the importance of duct cleaning when there is such poor understanding related to the types of pathogens in ductwork that cause ill health. On top of this, when you dig into the studies like the cumulative review done by M.S. Zuraimi and sited by the DOHS, it becomes clear that similarly poorly defined parameters are being used to conclude that duct cleaning has variable results in terms of Indoor Air Quality (IAQ). Having said this and before we dig into these statements, to its credit the EPA does make some good points on their page entitled Should You Have the Air Ducts in Your Home Cleaned?. They are:
- If family members have unexplained illnesses, think about the home environment including the ductwork.
- Take care in selecting a duct cleaning Service Provider.
- Service Providers that don’t use proper procedures can make the IAQ worse and damage HVAC components.
- Only cleaning the ducts while ignoring the A/C coil and blower motor has little effect on system performance.
- In addition to cleaning the A/C coils, make sure the drip pan and drain line are clean too.
- Failing to clean all the components of the HVAC system (ducts, coils, blower, etc.) may lead to worse Indoor Air Quality (IAQ).
- You should think long and hard about misting chemicals of any kind into your ductwork (i.e. don’t do it).
- Fiberglass insulation in ductwork that has gotten moldy can not be cleaned.
- Steam cleaning and other methods that use water should not be used to clean ductwork (although steam cleaning A/C coils can be effective).
So let’s take a look at the three main categories of duct cleaning and, in the process, learn about what constitutes a quality job along with understanding why the often sited cumulative review of duct cleaning studies done by M.S. Zuraimi is meaningless. We’ll begin with the “Contact Vacuum” method. Although these terms in this report are never clearly defined, the Contact Vacuum method appears to refer to anyone that physically goes in by hand to clean the ducts and uses a “conventional” vacuum in some part of the process.
Contact Vacuum Method
On his blog, homeowner David Wirth describes how he uses his shop vacuum and a electric leaf blower to clean his ductwork. While I give Mr. Wirth credit for his ingenuity, this approach is guaranteed to make indoor air quality (IAQ) worse. First, both the vacuum and the dirty air it exhausts are inside the house instead of being outside well away from the building as previously discussed. Second, the small shop vacuum Mr. Wirth uses can not possible create enough suction to ensure dislodged debris is all drawn into the unit. Even larger shop vacuum types pull at best about 400 cubic-feet-per-minute (cfm). Third, a leaf blower will not come close to removing caked on dust deep inside ductwork. Even where it does have some effect, it will undoubtedly leave a thin residual layer of dust.
Clearly using a conventional vacuum is going to do a very poor job resulting in dislodged dust being launched into environment that diminishes air quality. Furthermore, failing to properly clean the A/C coil, HRV/ERV units, and the blower motor along with cleaning ductwork is a major oversight. Cleaning ductwork improves air flow. Even though the resulting improvement in air velocity is relatively small, if these other components aren’t cleaned at the same time as the ducts, you can plan on dust/dirt being dislodged from these still dirty units as a result of higher air flow.
Mechanical Brush Method
Again, I have not seen a clear definition of what this method entails but clearly it involves the use of a mechanical brush. Generally, these are rotary brushes that are sent down the ductwork. The bristle stiffness is selected based upon the type of duct (stiffer for metal and softer for flex duct). The rotating head is either driven by a small electric motor or a pneumatic (air driven) motor located at the end of a long flexible line that is sent into the ductwork. The brush bristles are oversized in order to ensure good contact when going around bends and moving through larger rectangular shaped ducts.
It’s unclear if pneumatic driven brushes fall into this category or the Compressed Air category below. Regardless, a vacuum is used that is generally much more powerful than the conventional vacuum employed in the Contact Vacuum method pulling somewhere between 1,000-5,000 cfm. These vacuums are generally much weaker than truck mounted units often used in the Compressed Air method. These vacuums are most often brought onto the premises and exhaust air through a filtration system into the building.
One of the better versions of this type of system is made by SpinDuct. Their vacuum units pull several thousand cubic feet per minute and can be equipped with an additional HEPA filtration system. As seen in this video, their intermediate systems use a rotary brush that is attached to a smaller drive line that is separate from the larger vacuum hose. This arrangement allows the brush to get around the many bends found in typical ductwork. This is a better arrangement than units where the rotary brushes are built into the end of a larger vacuum hose. The larger hose in these other systems can’t get around distant ductwork corners (elbows).
Unfortunately, the vacuum unit is almost always brought into the building. As discussed above, this is not a good practice. Even with repeated passes, I have concerns about the degree of material that gets left behind given that the vacuum source is often quite a distance from the rotating brushes. In Air Duct Cleaning Equipment: Agitation Tools Review, even though they are using high pressure air with a rotating brush and a much larger vacuum, material is still left behind. Granted the brush can be spun such that debris is pushed toward the end of the vacuum hose, but my sense is that the moderate vacuum source along with the fact that vacuum hose is often a good distance from the brushes means that there isn’t enough air flow to keep some particles from being re-deposited in the ductwork. I suppose repeated passes might go a long way in making up for the weaker vacuum source.
Also, there are cross-contamination issues that occur near the open registers. Some amount of dust will invariably escape the draw of the vacuum hose as the brush is being inserted and removed. Nevertheless, I do think that when extra care is taken, this approach makes sense for condominiums and other buildings where outdoor truck mounted services do not have enough vacuum hose to reach. Furthermore, condominiums are usually smaller in size wherein a moderately sized vacuum should work well. I especially like this type of approach when the vacuum can be sealed outside on a balcony or deck with a hose running to a temporary hole cut into a main trunk or plenum indoors.
Compressed Air Method
Also called the Air Sweep method, this approach uses air under pressure. You can watch a general overview of this approach at Air Sweep Duct Cleaning – from Modernistic. As noted previously and in the video, compressed air can be used to drive a rotating brush that is sent down ductwork. The brush assembly is attached to the end of a compressor air line that is pushed into the ductwork. In addition to the rotating brush, air can be blasted out near the head of the rotating brush to push debris either ahead or behind the brush depending on the location of the vacuum source. Besides the rotating brush, there is also a whole range of additional tools that the technician can elect to use. These consist of different types of “skipper balls” and “whips”.
In Air Duct Cleaning Equipment: Agitation Tools Review, you can see that skipper balls consist of an aluminum ball drilled out with many air holes. The ball is attached to the end of an air compressor line and blasts jets of air either ahead or behind the ball. Whips consist of thin rubber lines attached to a small aluminum block that is affixed to the end of an air compressor line. The rubber lines dance about wildly while blasting jets of air out their ends leaving behind only slight amounts of stuck on dust.
What’s important to note is that whips should only be used on metal duct work and that both whips and skipper balls leave behind a thin layer of stuck on dust. The picture to the right shows the residue left behind inside a section of ductwork on our house after a triple whip was used in conjunction with a truck mounted vacuum. To be thorough, a rotary brush in combination with an attach air compressor line should be used for the final cleaning to remove this residual material still clinging to the duct walls.
Although not noted in the studies I read, the Compressed Air method generally uses a very powerful truck mounted vacuum along with compressed air under high pressure – 175 pounds-per-square-inch (psi) is typical. Of course there are services that use smaller portable gasoline powered vacuums that draw somewhere between 5,000-7,000 cfm. The larger the home, the better off you are using a stronger vacuum.
We had our house cleaned by Ken at Mr. Power Clean. Ken uses a 12,000 cfm vacuum unit powered by the same diesel engine used in German Panzer tanks. The suction is powerful enough that if you walk within a foot or so of the end of the large 10” diameter hose, it’ll latch onto your leg! It doesn’t hurt; I tried it for fun. The main point is that these are powerful vacuums.
Before we get into coil and blower cleaning, I wanted to take a minute to talk about humidifiers attached to the HVAC system. I noticed in Mr. Wirth’s blog post that he has a humidifier attached to the plenum above his furnace. Putting water, even in the form of humidified air, in ductwork is never a good idea. In Evaluation of Fungal Growth on Fiberglass Duct Materials for Various Moisture, Soil, Use, and Temperature Conditions, the authors mention that mold growth occurred on dusty fiberglass lined ductwork when the relative humidity (RH) levels exceeded 50%. When the fiberglass was dust free, mold growth started to occur at 85% RH.
This is consistent with what we know about mold in general. Namely, when the humidity level starts getting around 50-60% and there is food present, some form of mold will grow. It’s guaranteed that the humidity levels in the ductwork nearby a humidifier attached to HVAC ductwork are very high. Given that all ductwork has some level of dust (food for mold) present, my guess is that there is some level of mold growth in nearly all HVAC systems that use an attached humidifier.
In addition, when you see How a Humidifier Works, the devices themselves are prone to mold growth. Regardless of whether the unit uses a foam lined drum suspended in a reservoir of water, or cascades water over a foam medium, the surfaces that evaporate the water are prone to dust accumulation just like the rest of the duct system. Remember, all it takes is food and a water source for mold to grow. There is plenty to feed mold in household dust.
Lately, they’re coming out with humidifiers that use steam. Again, humidity levels where the steam is piped into the HVAC ductwork is going to be very high. High enough to support mold growth. In general, humidifiers attached to ductwork are a bad idea. If you absolutely have to, use a portable humidifier that you can routinely disassembly and inspect for mold.
A/C Coil & Blower Cleaning
It’s imperative that the air conditioning (A/C) coils and furnace blower be cleaned at the same time as the ductwork. As mentioned earlier, cleaning the ducts improves airflow slightly. This change in air speed will invariably cause dirt and dust that remained stuck to these components when the system was dirty to break loose. Furthermore, even using powerful truck mounted vacuum, some debris dislodged during duct cleaning falls down the main plenums and ends up on the coils and the bottom of the cold air return plenum. Failing to clean the A/C coils and blower properly will result in poorer indoor air quality (IAQ), even if the ductwork is immaculate.
It’s somewhat surprising to me, but nearly all ductwork cleaning Technicians I’ve come across really don’t have a deep understanding into how toxic ductwork can be. I guess it’s to be expected since the hazards of mold, let alone the whole slew of even tinier airborne inflammagens, isn’t really mainstream. For many people including Service Technicians, the consequences of not following a strict containment protocol isn’t as harmful – at least in the short run. However, for those that suffer from CIRS, or want a top notch job, it’s important to make every effort to ensure every little bit of dust is contained and removed from the building.
When it comes to cleaning the A/C coil and blower motor, along with the furnace housing and assemblies, it’s not uncommon for Technicians to use compressed air to blow off these components. If they’re more conscientious, they’ll hold a large vacuum hose where they’re working to suck up much of the debris. Needless to say, huge plumes of microscope particles are sent into the building. In other words, the furnace area needs to be tented off and put under negative pressure using a vacuum located outdoors in order to contain the dust. After the components are cleaned, the inside of the tented area should be “washed” with low pressure compressed air that is used to sweep the entire volume of tented air into a large vacuum hose.
A/C Coil Cleaning
It’s funny, but it wasn’t until I inspected a friend’s A/C coil a couple years ago and found a large growth of Stachybotrys on the A/C coils that I ever gave home cooling systems much thought. When it comes down to it, the A/C coils inside the ductwork directly above the furnace not only cools the air passing through them, but they also lower the humidity. Everyone knows that running the A/C drops the humidity in the building.
The reason the humidity lowers is because as the warm, moist air passes over the cold aluminum fins on the A/C coil, some water condenses on the fins. It’s the same as when you leave your ice-cold glass of lemonade sit outside on a hot summer day. In short order, the glass becomes loaded with beads of condensed water that drip into a puddle at the base of the glass. When it comes to A/C coils, a “drip pan” sits below the coils and catches the dripping condensate. This water exits through a flexible line that runs into the drain on the floor.
The point being that there is a lot of water. Remember that all of the air in the HVAC system passes through these wet coils. Even though they are somewhat self cleaning with the amount of water that drips off of them, and the air being pumped over them has been cleaned by the furnace filter, some amount of dirt invariably begins to build up. All it takes is water and a little dust before mold starts to grow. Personally, I inspect and clean all my cooling coils on a yearly basis. In addition, there is very strong evidence in support of using a UV-C light to keep the coils microbe free – more about this in an upcoming article. I love UV light for killing microbes. 😀
The trouble for most is that they don’t clean their A/C coils regularly. Dirt builds up. Mold starts to grow. The kids start coming down with “allergies” and everyone starts having to drag themselves out of bed in the morning. The list goes on and on.
This buildup is not easily cleaned. The aluminum fins on A/C coils are very tightly spaced and can be up to a couple inches deep. When mold and dirt take hold, it’s hard to get the fins back to their original shiny bright condition. This is important not only for the health of occupants but clean coils reduce energy costs. The layer of biological material acts as insulation reducing cooling effectiveness and decreasing airflow.
Other than containment, there are two main concerns related to coil cleaning. The first has to do with chemicals and the second with making sure dirt isn’t packed into the coils. Regarding cleaning, a quick Google search on A/C coil cleaning will bring up a long list of companies selling chemicals to help with the job. The article Carefully Choosing Coil Cleaners, covers the subject of selecting chemicals nicely.
I don’t recommend using chemicals, but if you do decide to go this route, make sure the coils are rinsed off regardless of whether the chemical is listed as “self-rinsing” or not. As stated in the article, these cleaners rely on condensate coming off the A/C coils to remove them. Clean the coils in fall and those chemicals are going to be baked on when the heat kicks in. Personally, I’d want to ensure the chemicals were removed as best as possible while contained in a tented space under negative air pressure and before the HVAC system was turned back on – as opposed to being slowly washed off by condensate dripping off the fins.
In terms of cleaning, in Dirty Air Conditioners: Energy Implications of Coil Fouling by UC Berkeley, the authors write “it is unclear whether coil cleaning always removes deposited material, rather than just pushing it deeper into the coil”. This is a major concern. Regardless of what method is used, it’s imperative that the fins are shown to be cleaned across their depth and not just the outer one-half inch or so. Personally, I’d use a combination of a bright flashlight, a mechanics mirror, and high resolution pictures to ensure the fins are shiny bright everywhere.
In addition, if the coils are particularly dirty or contaminated with mold, my recommendation is to have the coil removed from the ductwork and cleaned outside. This is more expensive than cleaning the coils in place. Before being able to remove the coils, the The Technician must pump out the Freon and cut the copper tubing connecting the coils to the condenser unit outside. After cleaning, the coils must be solder back into place, the refrigerant lines must be pumped down, and Freon reintroduced. However, it’s an expense well worth it as part of an overall remediation mold remediation project. In my mind, the risk of not completely containing the mold toxins is too great. This is especially true in light of all the other work and expense that goes into properly remediating a moldy building.
My choice for cleaning A/C coils is steam like that of PURE-Steam Coil Cleaning. The combination of steam, ample water to flush the fins, and an application head that can get into tight spaces along with direct water at a 90-degree angle to the face of the coil makes good sense. You want some amount of water pressure, steam, and a lot of water to flush the closely spaced fins clean.
The fact that Pure Air Control Services believes enough in their steam method to at least fund the modest study Evaluation of Steam Cleaning in Energy Conservation gives credence to this approach. In the study, their steam cleaning method was shown to be “… 99.99% effective in removing microbial contaminants and other surficial debris/lint….(and) increased airflow and energy efficiency, leading significantly towards optimal coil performance”. The friend I mention with moldy coils had them cleaned using steam with good results. As always, make sure to check references before hiring anyone and confirm that the work has been done properly before making payment.
As mentioned, the HVAC blower sits in the bottom of the furnace enclosure and pushes well over 1,000 cubic-feet-per-minute (cfm) of air. The blower consists of an electric motor attached to a set of smaller cupped shaped blades. The cupped blades are about 3/4″ wide, tightly spaced, and are set in a cylinder that measures roughly 10” in diameter and 10” deep. This blade assembly is called a “Wheel Blower”. The motor and wheel blower fit inside a matching cylindrical metal enclosure. As a unit, these types of blowers are often referred to as a “squirrel cage” blowers.
Of importance when it comes to cleaning, it’s very difficult to clean the blades and blower assembly well if they are left inside the furnace. The blower is turned so that it’s difficult to get at the blades and there is very little room. As such, it’s always recommended to remove the blower and take it outside for cleaning. This may sound like a major undertaking but it just a few bolts and a couple wire connectors before the whole unit can be pulled out.
Of course, the blower should only be removed after the furnace area has been tented and put under negative pressure using a vacuum. Once outside, the unit can be further disassembled to give good access to the blades and inside the metal housing. The electric motor can be blown out with high pressure air while the blades can be scrubbed with soapy water and washed off.
Disclaimer: I don’t have any association with any the service providers mention in this article.
What’s the Best Method?
We’re finally at a point where enough groundwork has been laid that we can look at the studies that have been done on duct cleaning with a critical eye. As noted earlier, the general consensus seems to be that duct cleaning isn’t typically warranted and could very likely make the indoor air quality (IAQ) worse. Let’s take a closer look.
I’ve already made the case above regarding the difficulty in establishing a direct causal relationship between any particular airborne toxin and health. However, this does not mean that bacterial and fungal toxins don’t damage health and aren’t present in the dirt that collects in dust. You can read a bit more about this in my article Mold Testing – Moldy Versus Bad for Your Health along with reviewing Dr. Shoemaker’s “Exhibit: WDB There are toxins and inflammagens.
Properly done, there is good evidence in support of duct cleaning. There are plenty of studies in support of this. In How Particle Resuspension from Inner Surfaces of Ventilation Ducts Affects Indoor Air Quality, the study focuses on how dust levels increase dramatically for the first 5 minutes or so every time the blower kicks back on. The authors write, “Some of the dust in ventilation ducts is originated from outdoor particles that have been proved to have adverse effects on human health by medical and epidemiological studies (Dockery et al. 1993; Samet et al. 2000; Pope et al. 2002). Moreover, field studies have found that there are considerable unhealthy microbial components (fungi, bacteria) in duct dust (Pasanen et al. 1997; Lu et al. 2009; Li et al. 2010)”. In the NIOSH study, Building Related Risk Factors and Work Related Lower Respiratory Symptoms in 80 Office Buildings, a strong correlation was found between both dirty ductwork and poorly draining A/C condensate lines and occupant health complaints. Clean ducts matter.
So cleaning ducts is important but is it true that no particular method is better than another? In Is Ventilation Duct Cleaning Useful? A Review Of The Scientific Evidence. by M.S. Zuraimi, one is left with the impression that sometimes duct cleaning helps air quality and sometimes it worsens air quality with no one method particularly better than another. Similarly, in the study Effectiveness of HVAC Duct Cleaning Procedures in Improving Indoor Air Quality, they conclude with “Post-level bioaerosol concentrations, taken two days after cleaning, were found to be lower than the pre-level concentrations suggesting that the cleaning procedures were effective to some extent” with Compressed Air being the best. It’s a lukewarm endorsement for duct cleaning at best.
I found these statements to be a bit odd and decided to spend the money to read Zuraimi’s full review as it is most often held up as evidence for not needing to be particularly concerned about duct cleaning. In Zuraimi’s study, what you find is that there is very little definition given in terms of the actual equipment used and procedures followed. The other study duct cleaning study seems to follow along the same loosely defined terms.
Setting aside the methodology for determining the actual air quality before and after cleaning, I have a lot of questions about how the ducts were cleaned. Was the vacuum unit brought into the building? If so, did it have HEPA filtration and was it confirmed that it effectively captured particles below 0.3 microns? Many so called HEPA vacuums fall short especially when they aren’t properly maintained. Were rotary brushes used in Air Sweep methods or only the Mechanical Rotary Brush methods? Was the A/C coil and blower tented off and cleaned at the same time as the ductwork? Was the equipment used to clean the ductwork decontaminated from previous jobs before it was brought into the building? These are just a few of my questions. They matter.
And yet, here we have official sounding papers being written by people that clearly don’t have a clue about what it takes to clean ducts well and prevent cross-contamination. I say this because of the very crude groupings of duct cleaning methods they used. I can assure you that if a duct cleaning service follows the steps I’ve outlined, the air quality will be dramatically better – assuming the ducts and HVAC equipment haven’t been cleaned for a while.
On the other hand, if someone shows up with a high powered vacuum that hasn’t been well maintained and then brings the unit indoors, I can tell you how it’s going to turn out regardless of how expensive the unit is. The air quality will be worse. It’s pretty basic stuff but it seems to completely slipped by the attention of the folks that did these studies. Given the way the studies were formatted, my guess is that these authors have very little hands-on, practical experience.
Don’t be misled. A high powered outdoor vacuum in combination with rotary brush and compressed air tools will definitely make a difference in air quality. This is of course assuming proper procedures are followed along with the A/C coils and blower being cleaned. For the rest of this article, I’m going to get into the details about what to watch out for and how to ensure the job is done well. I’ll also cover cleaning Heat-Recovery Ventilators (HRV) for those in colder climates and Energy Recovery Ventilators (ERV) for those in warmer locations. Let’s get into the details.
Duct Cleaning Specifics
Here in the Green Bay area of Wisconsin, I called on Ken at Mr. Power Clean to help clean our ducts. Ken took over the business from his Dad and recounted to me how he first started going out on jobs at an early age. He’s got a positive, no nonsense attitude that I totally relate to. In addition, I was struck with how conscientious Ken was and willing to follow my lead on how I wanted cleaning to be done. It was a positive experience and I learned a lot.
Right on time and with booties in hand to cover his shoes, Ken and a helper showed to clean our ducts. Our home is different than most. We don’t have central air. We use heated water and small mini-split air conditioners to condition our air. However, we do run a Heat Recovery Ventilator (HRV) that connects to 5” diameter metal supply and return ductwork through out the house. Anyway, my point is that every home is different and this leads to the first item in the numbered list below on how to ensure a quality duct cleaning job.
- Know your HVAC system. When the Technician arrives, he/she is going to do their best to figure out how the ductwork runs through out the house. The reason for this is so when they are fishing cleaning tools into the ductwork, they can be sure that no section is left unclean. Tools often hit snags here and there. If the Technician knows they need to feed in about 20 feet of hose to get as far as they can on that run and the line stops at only 10 feet, they know to work the line a bit to get in deeper.
Before Ken showed up, I’d mapped out all the ductwork runs. I then went around with him and at each register would make comments like, “It’s 5 feet to the first 90-degree elbow, then another 15 feet to a transition into rectangular duct that we can get from another location”. Although you may not get all the details correct, you can be a help to the Technician. Before they show up, have someone make a loud noise at each register while listening at other registers and in the basement to get a sense for what runs go where. As the job is progressing, make notes about how much line was fed into each register as a way of making sure every inch of ductwork is cleaned.
- Before I get into more details, I suppose I should talk a bit about selecting a service. As noted, you’re ideally looking for a 10,000 cfm or more truck mounted vacuum and compressed air cleaning tool service. Make sure to get multiple bids and check references. There are a lot of shady duct cleaning services. Ken referred to them as “Blow and Go” services wherein they just screw a plate over a main trunk without every cutting a hole to connect their vacuum! The guy in the basement turns on the unconnected vacuum and then smokes a cigarette while a second Technician vacuums a bit at each register to make it look like the ductwork was cleaned.
I laughed nervously at hearing this because we all know there are services like this. It’s so outrageous what some people will do. Even after you think you’ve picked a good service, make sure to be there when they are working and don’t be afraid to ask them to get out if the job starts to go “south”. See Ken’s 10 Questions to Ask Before You Hire an Air Duct Cleaner and Dateline: Chris Hansen – Air Duct Cleaning Scams.
For pricing, check out the chart at the bottom of Duct Cleaning: A Solution in Search of a Problem.
- Take a minute to let the Technician know about your health status. Keep it short and simple. You want to let them know that you’re struggling to recover your health and its super important that the job is done well. In addition, my general approach is to let the details of how I want the job to be done to unfold as the work progresses. If you call a service and start saying you want this and that done, it may scare them away. If you get everything ready for them and support their work, the job will go smoothly and everyone will feel pleased at the end. If the job does run long, then pay them for the extra effort.
- Before they even walk into the house, make sure the truck gets parked as far away from the building as possiblemake sure to stand upwind of the truck and we. These trucks have been used in some really nasty mold and fire remediation projects. Although they run the exhausting air through filters, as discussed, this massive amount of air will be laden with all sorts of nasty microbes. The trucks come with a lot of hose. Depending on your layout, they may be able to move the truck a good 20 feet or more away from the house.
Close all the doors on the house and seal the door or window opening where the large vacuum line enters the hose. Close all the windows too except one or two on the opposite of the house well away from the truck. That vacuum is going to pull massive amounts of air out of your house and if you close the house up tight, it’s going to draw in air through bath fan, stove, hot water heater, and whatever other piping that leads to outdoors.
Note: You can buy inexpensive 1×2 (pronounced one-by-two) sticks of wood and sheets of plastic at any big-box building center. The sticks can be cut so that they press into place without using any nails – although I always screw mine together. A stapler does a nice job for attaching plastic to this makeshift wood frame. Prepare a plastic covering over the door or window that hoses will pass through ahead of time. The key is you don’t want to be pulling air into the hose that is coming off the truck. To make the job easier, I like the idea of using a Heavy-Duty Zipper for Dust Barriers.
- The large vacuum line and hoses suffer the same cross-contamination issues. I didn’t ask Ken to blast them off the vacuum hose with his 175 psi compressed air because in my situation the large vacuum hose is hooked up to the outside of my house. Remember, we only have HRV ductwork piping. If I had a conventional system, I’d definitely ask that the large hose is blasted off with compressed air before being brought into my home. By the way, if you suffer from CIRS, make sure to stand upwind of the truck and well away of items being blown off outside.
- Along the same lines, use your own tarps. Ken showed up at the door with runner cloths in hand to protect the flooring below the large vacuum line that travels from the truck down to the ductwork in the basement. Just like the truck, these tarps have been everywhere. We have hard flooring that I could mop up after so I wasn’t concerned about the small compressed air lines brought into the house. Others should spend the money and buy inexpensive tarps that they lay out ahead of time.
- Get your dampers in order. It’s common practice to install dampers in ductwork. Dampers are flat metal plates installed inside ductwork that can be rotated with a small metal lever on the outside to block off air flow. They’re often used to “balance” the supply and return air flows. This prevents the house from being in a constant state of over or under pressure. This is important. For example, you don’t want to be sucking in cold air through nooks and crannies in the winter because your HVAC system isn’t balanced. If you do, you run the risk of condensation issues and mold developing inside your walls. In warmer climates, cool inside air driven into the walls creates condensation issues.
So find any dampers and mark their proper orientation. You’ll want to go back after the cleaning and reset them all. After marking them with a big, black magic marker, open them up. To do this, turn the lever so is runs parallel with the ducting. To prevent the damper from moving, make sure to tighten the wingnut on the damper handle. Note: More sophisticated ducting systems use dampers that are controlled with small electric motors called “zone dampers”. Normally these dampers default to open when the furnace and A/C is turned off. All furnaces have a switch on them that you should turn to the off position to ensure any zone dampers are open. Besides, you definitely don’t want the blower to kick on while the ductwork is being serviced.
- Remove your furnace filter. If you’re using an HRV or ERV, you’ll need to remove the heat-transfer core and ideally pull the blower motors inside. This is easy enough to do. Just make sure to shut off the power and take pictures before you start the job – just in case you can’t remember how parts went together.
- Confirm that there is suction at all the registers. This is super critical. On our HRV, I’d removed the blower motors and heat-transfer core to ensure good air flow. I’d also set the motorized dampers in the wide open position. However, 12,000 cfm of vacuum slammed one the those dampers closed. When we went to clean the first register, there wasn’t any noticeable suction. One thing is certain, you don’t want to start kicking up a lot of dust inside the ducting until you’ve confirmed that there is a strong vacuum to pull debris and the cross-contaminated compressor air out of the building. Check for suction at each register before cleaning that runduty plastic garbage bag to make sure a damper hasn’t inadvertently been drawn closed.
Along the same lines, Ken suggested that he likes to close all the registers and start working at the register farthest from the furnace. He’ll then leave the clean register open and then move on. I think this makes good sense. The only caveat I would add to this is that I would leave the registers slightly open and make sure to reinstall the register in a slightly open position after cleaning. My thinking is that you want a small amount of air to be moving through all the ducts to prevent dust from escaping while focusing most of the suction on the run being cleaned.
- If you have registers near hanging clothing, I would cover them with sheets. Typically, the Technician will blow compressed air into the opening after the grill is removed and before compress air tools are inserted. The reason for this is that the compressed air tools are aggressive and will send material flying back out of the piping if activated too close to the opening. I don’t recommend this procedure because invariably, some material blows back out of the opening.
Instead, I suggest you have a feather duster in hand and explain that you’d like them to gently sweep any dust in the first couple of feet into the pipe. Even then, some material at the very opening may inadvertently fall out, especially when the grill is removed. The sheet will protect this. Better yet, if you own a quality HEPA vacuum, have it ready to clean out the first few feet of piping. Don’t expect the Technician to move around your vacuum. Help him/her out. After they use it to clean out an opening, move it to the next location. Whatever you do, don’t let them bring one of their vacuums into your house for the reasons already discussed.
- As discussed, it’s important to tent off the area around the furnace using the same 1×2’s and plastic. Create an opening near the ceiling opposite to the door opening. The reason for this is that the vacuum hose will be brought in through the door and then the door will be closed. At over 10,000 cfm, you want to make an opening for air to get into your plastic tent to prevent it from imploding. Create the opening so you get good cross ventilation in the structure.
Make sure that any equipment like the blower or A/C coil that is removed to be cleaned outside is placed in a heavy duty plastic garbage bag before being transported through the house. Technicians should blow themselves off with compressed before leaving the tented air as they will invariably be covered with dust after doing the dirty work of pulling the equipment. They should stay on the tarps you’ve laid out and head directly out of the building. Obviously, blower and A/C servicing should be saved to last as the Technician will be contaminated after performing this work.
- Make sure you’re the first job of the day. You don’t want Technicians covered in debris from jobs they performed previously that day walking through your home.
I’m sure that some of these recommendations may sound somewhat extreme to those without CIRS. However, it takes very little to set off those that live with this CIRS. I cringe at the thought of a Technician showing up to clean my house after working on a place loaded with Stachybotrys earlier that day. For everyone else, why risk feeling less than optimal?
HRV/ERV & Filtered Fresh Air
I’m going to finish up with a brief discussion on HRV and ERV cleaning. HRV and ERV units exchange stale indoor air with fresh outside air. Their unique design allows the incoming outside air to be pre-conditioned thereby saving on energy costs. They do this by using a heat exchanger (special radiator for air) that brings the outgoing airstream in close contact with the incoming airstream – without ever mixing the two together. In so doing, the cold incoming air in winter is pre-heated by the warm outgoing air. Similarly in summer, the colder indoor air cools the warmer incoming air. The only difference between an HRV and ERV is that the ERV also allows for moisture in the airstreams to be transferred too.
HRV’s and ERV’s need to be cleaned regularly. The inner “core” consisting of many layers of fins that transfers the heat between the two streams of air becomes readily coated with dust. This dust in combination with condensation that occurs whenever warmer humid air is cooled is a recipe for mold. Expert mold inspector, Martine Davis, commented that these units are almost always moldy.
I pull out the core on my HRV and wash it several times during the year. In addition, I’ve installed a MERV 11 pleated pre-filter on the round incoming air duct. Even if you don’t have an HRV/ERV, many homes have a fresh air duct. I went to my local HVAC store and asked them to make up an adapter that would hold pleated filters and fit onto the round incoming air duct. I order the custom pleated filters online.
Even with the pre-filter, the fins on the two squirrel cage blowers in my HRV develop a layer of dirty buildup over time. I pull these motors on a yearly basis and clean them outside with compressed air and a toothbrush. While the motors are removed, I use a solution containing quaternary ammonia and wipe down the inside of the HRV and as far as I can reach into the round ductwork. Keep your HRV/ERV clean.
Footnote: In a 30’x50’ ranch, the main trunks are about 10”x18” and run down the length of the house for a total length of about 75 feet – return ducts are generally shorter. The area of the trunks is 94 square-feet. There is generally one 6” round supply duct for each of the three bedrooms, the bathroom, the kitchen, the dining room, and two for the living room that run from the center of the building to the outside walls. The area of these supply ducts is about (8x15x1.3)= 156 square-feet. There are generally two return ducts made up from wall stud and floor joist cavities that are more centrally located – nearer to the center of the building. Their area is about 2[(7×0.25)+(8×0.33)]= 9 square-feet. The total area is 94+156+9= 259 square-feet. This equates to (259/1.3)~ 200 feet of 6” round duct.