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• Air conditioning / heating filter defects & repairs - a photo library of air filter mistakes
• Air conditioning / heating filter suggestions for optimum indoor air quality
• Air conditioning / heating system filter product sources and recommendations
• Air conditioning / heating system air handlers - Fan Coil Unit Inspection, Diagnosis, Repair, Replacement

This website answers almost any question you might ask about air filters for heating or air conditioning systems. We explain how an air conditioning service technician will diagnose certain common air conditioning system failures or defects. We include photographs to assist readers in recognizing cooling system defects. We continue to add to and update this text as new details are provided. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

AIR CONDITIONING FILTERS - Air Conditioning Filter Location, Condition, Problems, Repairs

Locate and document the placement of the HVAC system air filters - for examination and regular changing during the cooling season. Then inspect the air conditioning filter type and condition. What about filters that are missing completely or are very dirty? What problems can a dirty or blocked air conditioner filter cause for the air conditioning system and how do we fix these snafus? That's what we'll cover in this article.

Air conditioner filter location: filters should be readily accessible

Filter accessibility: Air filters which are hard to access are rarely changed as often as necessary. I frequently see HVAC systems designed by someone who obviously has never had to service them. Placement of filters and air handler access doors in very hard-to-access locations such as at the far end of a minuscule attic behind a forest of trusses means that the system is very unlikely to receive the periodic inspection and maintenance it needs. I prefer to see A/C and heating filters placed at the building side of the air return register or grille, so as to protect the return duct from debris accumulation. The more common filter placement on many systems is right at or in the air handler.

After reading the text just below, if you still cannot find your heating or air conditioning system air filter read our detailed instructions on how to find air filters in our article: Air Filter Location

• At the central air return register, grille located in a wall or ceiling if your system uses centralized air returns instead of individual room-air return ducts. There may be several central return points, depending on the design of your system. If there are more than two, chances are the filter was placed at the air handler instead of at these grilles. Unfortunately that means that the return ducts themselves become more soiled with dust and debris from the building.
• At an attic air handler look for a slot which has a removable cover. The slot may be just an inch or so wide if 1" thick filters are used, or it could be several inches wide if a wide high-capacity pleated or similar filter was used. The return air plenum on an attic or basement air conditioner blower unit will usually be a large metal enclosure about the same dimensions in width and height as the air conditioner blower unit itself. Look for a filter slot right where the return plenum contacts the blower fan assembly.
• At a basement air handler we also look for a filter at the return air plenum which is often next to the bottom of the air handler if the system is an "up-flow" unit (or vice versa for the less common case of return air entering at the top of the air handler and exiting at its bottom).
• Next to an electrostatic air cleaner: if your air conditioning air handler has an electrostatic air cleaner installed, look for the filter, if there is one, next to the electrostatic air cleaner. In addition, the electrostatic air cleaner, which is a type of particle incinerating filter itself, needs to be removed and cleaned periodically. (Check with your unit's manufacturer for cleaning interval and procedures. Often the electrostatic unit can be cleaned inside a dishwasher). Often there is also a thin metal washable air filter installed along with the electrostatic air cleaner.

Change your air filters every month when the air conditioning system is in operation. Make sure you find all of the filters as some systems have multiple filters and even multiple types of filters installed, such as a fiberglass or pleated paper filter, a washable filter, and an electrostatic air cleaner. These last two are cleaned, not replaced, when they're dirty.

Dirty Air Conditioner Filters Cause Multiple Problems for an Air Conditioning System

Dirty Air Filters: are a source of increased operating costs and poor cooling system operation. Dirty air filters can: reduce air flow in the building cause dirt to accumulate on the fan blades, wasting your energy dollars cause excessive dirt build-up inside the duct system, leading to mold or allergen problems in a building and to the need for more costly duct cleaning or replacement block the cooling coil itself with dirt, reducing system effectiveness and possibly leading to costly repairs lead to frost build-up on the cooling coil and reduced or totally blocked air flow in the system eventually permit dirt to bypass the filter where it soils and blocks the blower fan itself, leading to more costly repairs. The filters on an air conditioning or hot air heating system should be changed monthly when the system is in use. Discuss with your heating/cooling service professional the possible need to clean the blower fan and duct work.

Improperly-Fit or Wrong Sized Air Filters Cause Bypass Leakage

Bending over the end of an air conditioner or heating air handler filter such as shown in the photo at left above is a bad idea. If the filter does not fit there will be bypass leakage past the filter, soiling the blower fan, slowing air flow, and leading to more costly cleaning and service later.

Furthermore when you bend the filter as this owner did, you interrupt the structural integrity of the filter's frame, risking filter collapse. A collapsed air filter can be drawn right into the blower fan, causing damage to the fan motor or even leading to a fire!

The photo at right shows how a college HVAC maintenance crew kept the A/C system running when the school did not have the proper filter size in stock. This filter installation also will have severe bypass leakage around the filter where the pleated section contacts the edges of the filter slot.

Installing a filter that is the wrong size for the heating or air conditioning air handler defeats the purpose of air filters because of leakage and it may be unsafe. Install a properly-sized filter in locations like this as soon as possible and watch out for unsafe filter collapsing.

How to Construct or Obtain Large or Special-dimension HVAC System Air Filters

The same college HVAC maintenance staff who was struggling with improperly-fit air filters we discussed earlier was also faced with the task of coming up with a much larger air filter for the air conditioner air handler over their computer center. The neatly-taped "built-up" air filter shown in this photo was nicely constructed but we don't recommend this practice: The filter may come apart and send fragments into the blower assembly, damaging the blower or leading to overheating and a fire. The filter is not delivering the total cross-section of filtering area that was anticipated by the HVAC design engineer who specified the dimensions of the return air plenum at which this filter was installed - we're seeing less total CFM of airflow.

Air filter suppliers and manufacturers have no trouble providing air filters of special dimensions. Furthermore if the filter is built by a manufacturer it's more likely that they'll understand the structural and strength requirements of the filter as well as the required airflow characteristics and filtering ability. We list some suppliers of air filters at SOURCES FOR AIR FILTERS

Incidentally, except unusual cases with special requirements, wouldn't it have made sense for the HVAC or duct system designer to have specified a filter that is one of the many standard sizes?

Missing Air Conditioner Filters

Look closely at this photograph. On the right we can see a tan "Air Filter Cover" plate which marks the intended location of the HVAC air filter. But there is an open slot to the left of the air filter cover, possibly where another filter was previously being installed. When the new air filter slot was constructed and nicely covered (so as not to leak) the old slot was simply left open. You can see my piece of adhesive tape bending into the opening, demonstrating (not too scientifically) that there was airflow into the unit from this location. This is a great way to draw attic insulation fiberglass into the air handler and to blow it into the living area. And of course any other unwanted attic dust and debris is also being invited into the air handling system and blown into the occupied space. Failure to properly filter dust from the return air supply will load the fan and cooling coil, dirty the duct system, and lead to the problems listed above. As the ductwork debris level increases you increase the risk of forming an allergen or mold reservoir, especially if there are water or condensate leaks into the duct system interior. If a filter is not present, have one installed. Installing a filter is normally a minor expense. Duct cleaning or duct replacement can be a significant expense. Cleaning up a moldy HVAC system, where mold may have been caused by coil icing which was caused by a dirty coil or filter is still more costly.

For more information about fiberglass as an indoor air quality concern see:

• Fiberglass in Indoor Air, HVAC Ducts, and Building Insulation
• Fiberglass Particle Identification in the Fiberglass Test Laboratory
• Goodman Gray Flex Duct Deterioration and Failures
• Mold in Fiberglass Insulation
• Optimum Indoor Air Filters
• Owens Corning Flex Duct Deterioration and Failures
• World Trade Center Dust Particle Identification

For more information about asbestos as an indoor air quality concern with focus on easy ways to identify asbestos materials in buildings, see:

• Asbestos Identification in Buildings: How to find & identify asbestos-containing materials
• Asbestos in Good Condition
• Asbestos HVAC Ducts - Asbestos Materials On or In Heating and Cooling Duct Work
• Asbestos paper duct wrap
• Asbestos Transite Chimneys, Flues, & Pipes in Buildings
• Asbestos under the microscope
• Asbestos Vibration Dampers & Asbestos in the Air Handler
• Ceiling tiles: How to recognize ceiling tiles that may contain asbestos
• Examples of unusual uses of asbestos in buildings

AIR CONDITIONING SYSTEMS Chapter Index

To continue reading this air conditioning inspection guide, use links to the document chapters at left or below. Links shown in green font indicate where you are in this document.

Technical Reviewers

OPTIMUM INDOOR AIR FILTERS - Air Filter Choices & Placement for Optimum Indoor Air Quality

Selecting Air Conditioning System Filter Types & Air Conditioner Filter Cascading for Optimum Indoor Air Quality and maximum reduction in airborne particles involves selection of high efficiency filters and other filtration equipment and placing them in the correct order and in the most effective location within the air conditioning system air handler and duct work.

Air conditioning and heating system filter efficiency

Highly efficient air conditioning or heating air handler or duct air filters which trap smaller particles are available, right down to HEPA filters. We recommend improved filtration installed particularly in buildings where occupants have mold, allergy, or other respiratory concerns. It may also be appropriate to install and use a blower fan which can operate continuously for maximum reduction of indoor dust levels and improved indoor air quality.However before installing a high-efficiency HEPA filter on your system you need to review the air flow capacity of the blower and the duty-cycle rating of the blower fan motor (can it be run continuously). Cascades of filters may reduce airborne particulate debris but may also cause too much reduction in total air flow.

Optimum air conditioning and heating system air filter design and air filter location

At my forensic microscopy lab where it is important to keep the dust and airborne debris to a minimum, when I replaced my heating/cooling system we specified the following filters in the order shown, moving from the building air return to the air handler itself:

1. Front end filter: at the central air return we installed a typical inexpensive 1" disposable filter which protects the return ducts from high levels of debris. This filter is changed monthly.
2. Electrostatic air cleaner: at the air handler return plenum
3. HEPA air filter: at the air handler return plenum, after the electrostatic air cleaner. This arrangement permits us to use the more costly HEPA filter for many months before it needs to be changed out.
4. UV light treatment: is also available for HVAC systems and may be slightly helpful for areas where occupants are at extreme medical risk.I have not see enough supporting data to be convinced that these lights have much effect on air flowing past them as the contact time for particles moving in the duct system will be minimal. Some installers place these lights in the return or supply plenum on the hypothesis that they are disinfecting the surfaces there. This is an appealing argument except that nothing is disinfecting the other 99% of the surface area of the duct system. I did not install this equipment at my lab.

We also installed a two-speed blower fan which the technician selected as capable of providing adequate CFM of air flow in the system with the cascade of filters I describe above.

How to use a cascade or other filter system

For maximum IAQ improvement in buildings: for the new blower installation we selected a fan unit which had both the capability of delivering adequate CFM of air flow and a blower fan motor duty cycle which permits continuous operation if we wish to run the system that way. Running the blower continuously at low speed resulted in continual air scrubbing in the building. When the heating or cooling needs of the building require, the fan shifts automatically to high speed.

Measuring filtration effectiveness:

The filter cascade system installation in our lab provided an opportunity for careful testing of its effect on indoor air quality in the lab, since unlike most building owners, we have an extensive set of indoor air quality monitoring equipment at hand for our IAQ and environmental work. The airborne dust levels of all particle sizes dropped dramatically when the new system was installed (and the existing ducts cleaned as part of the project). Airborne particles in the 1-20 micron range were reduced by about 70%. Simple subjective visual inspection of lab surfaces showed that dust accumulated at a much slower rate.

Air Filter Design Basics: MERV, HEPA, How Air Filters Work

The Definition of Air Filter MERV and what MERV means when buying an air filter

MERV is an acronym for "Minimum Efficiency Reporting Value". The MERV rating on an air filter describes its efficiency as a means of reducing the level of 3 to 10 micron-sized particles in air which passes through the filter. Higher "MERV" means higher filter efficiency. The purpose of the MERV standard is to permit an "apples to apples" comparison of the filtering efficiency of various air filters.

Air filter efficiency refers to the relative ability of a filter to remove particles of a given size or size range from air passing through the filter. If a filter were 100% efficient, none of the particles in a given size range would escape the filter and air which has passed through such a filter would contain zero particles.

The MREV Efficiency Rating Scale ranges from 1 to 16, with 1 being the lowest efficiency and 16 describing the highest efficiency. The particle size range addressed by the MERV scale is 3 to 10 microns. A logical inference is that if an air filter is removing particles down to 3-10 microns, it is certainly also at least that efficient at removing larger sized particles.

What are typical air filter MERV values?

A very low-cost disposable fiberglass furnace filter may have a MERV rating of 1 to 4 - pretty low.

A high efficiency high MERV pleated air filter in the MERV 13-16 range would be expected to remove smaller particles between 0.3u and 1.0u in size at about a 75% efficiency. But before getting too excited about 75% looking not as good at 99.97% (see HEPA filtration discussed below) remember that in a heating or air conditioning system air is being recirculated through the filter system. So if each pass is removing 75% of the particles we want to capture, over time the heating or air conditioning filter system will do a good job of removing a very large amount of airborne dust. This is exactly what we saw when we tested some filtering approaches in our forensic laboratory. The longer we ran the air handler fan the more dramatically we saw the airborne dust level drop in the test area.

A "HEPA" rated air filter (High Efficiency Particulate Air [Filter] or better put, High Efficiency Particulate Arresting [Filter]) has to meet more demanding US DOE standards than a high MERV air filter. A HEPA filter is required to remove at least 99.97% of airborne particles down to 0.3u in size (in diameter or in their longest dimension). Any filter, high MERV or HEPA, can be expected to filter out larger particles with an efficiency above its rating.

So do we want to use a high MERV air filter or do we need to install a HEPA filter on our air conditioner or heating system air handler? A high MERV air filter is much less costly than a HEPA filter and can be more effective over time in a residential or office air conditioning or heating system, but there are two warnings that have to be considered:

1. Air filter bypass leakage if the air filter does not fit properly and tightly, no matter how efficient it is, it may be leaking a significant of air around itself at the mounting, reducing the overall efficiency of the air filtration system. You can see this effect by noticing how much debris collects in the return air plenum behind the filter. If at each filter change you see that the ductwork or return plenum is pretty clean, the filter is not bypassing much air. Some air filter manufacturers specialize in providing air filters that include a gasket to seal against air bypass leaks.
2. Air filter density and reduction in airflow: some air filters may be so dense that the cubic feet per minute of air that can pass through them is reduced drastically. Check with your air conditioner or heating system installer or service technician to review the intended air flow rate (in cubic feet per minute) of your system and on the effects of installing a better, more efficient but slower-air-passing filter. An HVAC technician and some home inspectors have equipment that can easily measure the air flow rate at a given point in the system. In our lab experiments we made certain that the new air blower assembly we were installing and testing was able to move enough cfm of building air with the improved cascaded filter scheme we were testing.

Smaller airborne particles than those covered by the HEPA or MERV rating might be filtered out with still higher efficiency. Their smaller mass means they may "stick" to the filter surface well but some filters, such as the budget lowest-cost fiberglass furnace filters, will pass these small filters right through when they are new, but filter out more of them as the filter surface becomes clogged and dirty with debris (leaving smaller and smaller openings through which air must pass.

How do air filters actually remove particles from the airstream?

If we look at a high efficiency, high MERV or HEPA air filter under a microscope we'll see a mat of randomly criss-crossed fibers of filter material (fiberglass, polypropylene, paper, or other materials). The space between the filter fibers will be larger than the smallest particle size which the filter is asserted to remove. So how do these filters stop the small particles? Let's look at three air filtration mechanisms in order of decreasing particle size:

1. First, some airborne particles smack directly into a filter fiber, either because they happen to be on that unlucky (for them) trajectory or more likely because the particle is so large that it can't pass through the opening, or part of the particle smacks into the filter fiber. Air filter scientists call this method "impaction". Impaction captures larger particles which in a HEPA system are those larger than about 0.4u.
2. Second, some airborne particles moving in the airstream past the filter happen to be close enough to a filter fiber that they stick to it. How close? the particle has to be closer to the filter fiber surface than one radius or diameter of the particle itself. In other words, a 2u particle which is only 1u away from a fiber will probably stick to the fiber. Air filter scientists call this method "interception". Interception captures particles mostly in the 0.3u size to 0.1u size.
3. Third, very small particles, say below 1u in size, collide with larger gas molecules swirling around in the air turbulence caused by air moving by the filter fiber. This collision, called "diffusion" in filter science, causes the particle to zig zag out of the airstream passing through the filter opening and to stick to the filter surface.

What airborne particle sizes are an IAQ concern?

What particle sizes are a concern? What's not addressed by much of the science in the air filtration and IAQ field is just what particle sizes are a worry. In general, larger particles, say 30u or 50u or long fibers, say 200u, are so big that they tend to be filtered in the nose of a human breathing that air. (1u here means 1 micron in size). Very small particles, say in the range of 5u and below are so tiny that they tend to be breathed more deeply into the lungs and might be more of a health IAQ concern for building occupants.

Some allergists have told us that they can tell by the nature of a patient's complaint what they're probably allergic to. If a patient has chronic rhinitus, for example, they may be responding to large mold spores like Alternaria sp.. If a patient has lower respiratory complaints (in the lungs) they might be responding to very small mold spores like some of those in the Aspergillus sp. or Penicillium sp. size range, which we often see in our lab can be down in to the 1u range in size.

Gasketed air filters

A gasketed heating or air conditioning system air filter improves the filter effectiveness. The benefits of a high efficiency air filter material when used in an air handler and duct system are not fully realized if air bypasses the filter around its installation frame due to poor fit or poor installation. That's why some manufacturers emphasize the value of air-sealing gaskets on their products.

Simple observations made in the field will make for agreement with the manufacturers that filter bypass is an issue at some heating or air conditioning installations. When changing an air filter at a return air inlet or at an air handler, when the old filter has been removed, take a look inside the return duct at the return register, or inside the return plenum at the air handler, to see just how much debris is passing the filter.

Implications of dust and debris in the return air plenum:

On an old heating or air conditioning system where filters have been ineffective, not properly installed, leaky, or not maintained and changed on schedule, we often find quite a bit of debris entering the air handler and duct system. This is in fact so common that most HVAC installers and technicians consider dirty ducts to be "normal". And they're right that typical house dust that has collected inside of a duct system is not itself a highly toxic substance.

But
1) a high level of dust and debris inside of an air handler or duct system indicates that system filtration has not been effective,
2) the dust level indicates that the system has been delivering dust and debris to the occupants providing poorly-filtered air, and
3) there is an increased risk of mold in the duct or air handler system if moisture enters the system from a leak or from air conditioning condensate ms-handling.

Washable air filters for heating and air conditioning systems

Washable re-usable air filters are very appealing from a cost viewpoint.I would look for data to indicate whether or not the washable filter runs at a lower MERV or filtering efficiency. In our cascaded filter proposal (discussed at this website) we included and continued to use a washable filter that came with the equipment since it was not reducing the airflow CFM, but we preceded it with other filters for different purposes.

Air Filters and Fiberglass Contamination in Indoor Air

Air filters and their contribution to fiberglass fragments found in building air and dust

In our experience, concern for filter shedding, say of fiberglass fragments, is not a significant issue. The contribution of an air conditioning or heating furnace air filter fiberglass to the overall level of airborne or dust-borne fiberglass particles in a building is likely to be insignificant, probably below the limits of detection by other than the most rigorous means, and certainly in most buildings will be insignificant compared with the contribution of other fiberglass sources such as building insulation in unprotected ceilings or walls.

If necessary we can usually identify the source of the dominant fiberglass fragments in building air and dust. Samples of settled dust and building air are compared with samples of fiberglass from common building sources such as fiberglass building insulation and fiberglass HVAC duct insulation. Fiberglass fiber metric consistency and the color of fiberglass binders are often sufficiently distinct to permit positive identification of the source of fiberglass fragments in the building environment. (See our article on laboratory identification of fiberglass found at More Information.

In sum, I am doubtful that even a fiberglass-based filter is a meaningful contributor to the total load of fiberglass particles in a building. There are simply too many enormously larger sources that overwhelm the measurement. Any air filter, properly selected and installed and maintained, will reduce the overall level of airborne particles, including fiberglass fragments which are contributed to the building air and dust from other building sources.

Do test labs see or even look for very small fiberglass insulation fragments?

What about fiberglass particles? As we discuss in more detail in our fiberglass and asbestos IAQ articles some of the research on the possible hazards of airborne fiberglass is confusing because it asserts that the probable hazard of "large" fiberglass particles is low. What's tricky is that hygienists or others who check indoor air or dust for the level of fiberglass contamination, and even the labs which process these samples may examine and report particles only in the larger size range. That makes sense insofar as it's apparent that large particles dominate fiberglass dust. But let's consider just a tiny bit further. Fiberglass, particularly mechanically-damaged fiberglass insulation, say in a fiberglass-lined air duct which was mechanically cleaned, can break to release very small glass fragments, even in the 1u range and below. These particles are very hard to detect in the laboratory unless the lab is specifically looking for them. In fact, unless the laboratory uses a slide preparation media with a refractive index nowhere close to that of glass, they won't see the particles at all in the microscope, no matter how many of them are present! In other words, you don't see what you're not looking for.

A reasonable conclusion from these observations about the presence and difficulty in seeing small airborne particles below 10u and especially below 3u is that the better we filter the air the less we need to worry about them.

Variations in fiber size in air conditioning and heating air filters

One of our clients wrote that her fiberglass-based air conditioning and heating air filters were described by their manufacturer as using fibers of 20-27u (in fiber diameter).

The maker promises the filters do not shed fiberglass fragments and that the fibers are too big in micron size to become repairable even if they do shed.

Laboratory examination of a filter sample sent by the client to an independent microscopy laboratory found that the filter fibers were 31 microns in diameter, and had a blue coating on them [probably the binder]. The binder coating increases the effective fiber diameter.

These measurements of fiberglass fiber diameters were within a normal range of variation in product manufacture that we've seen in own forensic laboratory. Furthermore, the diameter of a fiberglass air filter fiber is as an indicator of the health hazard should the filter shed fibers is unlikely to be as useful measure as the number of particles released and their overall size. For example, a fiberglass fiber can break so as to release several sub-micron glass fragments.

The actual filtering efficiency of a filter should not be estimated simply on fiber diameter since more than diameter goes into the filter design, including fiber placement, randomness, average remaining opening diameter, total filter thickness, electrostatic properties of the filter, and other variables such as whether or not the filter has a gasket or is bypassing lots of air around itself.

Sources of Air Conditioning and Heating Air Filters, High Efficiency Filters, Gasketed Air Filters, and Air Filter Manufacturers

• Filtration Manufacturing Inc. in Alabama, are a manufacturer/distributor of air filters.They have a wide selection of air filters (air filtration products) and air purification products. This company can also manufacture and deliver quickly air filters made to custom dimensions. Filtration Manufacturing Inc. company has paid particular attention to air filters that don't leak or bypass air at their mounting location. Their website http://www.filtrationmanufacturing.com/ offers less discussion about filter composition materials than those who are concerned with filter fiber shedding or filter composition.
• Global Filter Source, is a supplier of for non-fiberglass filters such as their polypropylene filter with a supporting grid of wire and cardboard. Contact Global Filter 877-844-3293 offers "healthy home" filters and a MERV 11 pleated furnace filter. Website: http://www.healthyhomefilters.com/
• List your product here: manufacturers or suppliers of air filters and air filtration equipment are welcome to contact us to add product listings here - there is no listing cost (we reserve the right to add or remove product listings from our web pages) Technical critique of our website articles is very welcome and we give full credit and web links to technical reviewers or to contributors of technical content. We have no financial relationship with companies or products discussed at our website.

Before calling or ordering an air filter online, remember to measure and write down the dimensions of the filters you are replacing in order to order the correct air filter size.

Other air filtration methods besides air filters

Beyond mechanical filtration methods using high MERV or HEPA rated air filters we've discussed above are other methods that can reduce airborne particle levels:

• Electrostatic air cleaners have been in use on HVAC systems for decades as highly efficient "air filters". These devices use an electrical charge on metal blades or plates and an electrical charge of opposite polarity on wires between the plates to cause particles in air moving past the wires to become "charged" and to stick to the surface of the metal plates or blades in the air cleaner. These electrostatic precipitators may also actually oxidize particles as well.
• Ultraviolet light (photo catalytic oxidation) is used in some air handling or duct systems, claiming to kill 99.95 of bacteria and viruses. While Wikipedia claims that independent research has confirmed this claim, we are very skeptical. For UV light to work as a sterilizer we need sufficient contact time. We are doubtful that little particles zooming along in the air path get enough time in the UV light. However the surfaces in the duct or air handler system which are exposed to this light are almost certainly being treated effectively. We'd like to see this approach tested in duct systems where condensate splash-by is at risk of wetting duct surfaces and at risk of a concomitant mold growth problem in that location.
• Charcoal or activated carbon air filters provide a porous material that can remove small particles along with some volatile organic compounds (VOCs). These are normally used together with a HEPA filtration system.
• "Air purifiers" or ionizers that charge their internal electrical surfaces and cause airborne particles to stick to a collector plate are a currently popular (2006) product which in our opinion are of limited actual effectiveness unless used in a small, clean, enclosed space. These devices do not move enough cubic feet per minute (CFM) of air to be effective in a large open space.
• "Air cleaners or purifiers" which use ozone, or "ozone generators" have been proposed as air cleaners and as "mold killers" by the manufacturers of these products. Ozone is a powerful oxidizer and it may also cause airborne particles to become electrically charged and to plate out on building surfaces. They are used effectively to reduce airborne smoke and they're used in some medical applications or facilities. Some other "air purifiers" may also generate (un-wanted) ozone levels in a building. But ozone generators can themselves be an IAQ problem source if they produce high levels (over 0.05 ppm) of ozone in indoor air. And as a "mold remediation" or mold cure ozone generators are junk science which fails to properly remove the problem mold and fails to correct its cause. We discuss the use of ozone generators in detail at The Use of Ozone Generators Indoors for Control of Odors and Mold Removal in Buildings: A Summary of Hazards and False Claims

See our Wikipedia citation below for more detail on these mechanisms.

For more information about fiberglass as an indoor air quality concern see:

• Fiberglass in Indoor Air, HVAC Ducts, and Building Insulation
• Fiberglass Particle Identification in the Fiberglass Test Laboratory
• Goodman Gray Flex Duct Deterioration and Failures
• Mold in Fiberglass Insulation
• Optimum Indoor Air Filters
• Owens Corning Flex Duct Deterioration and Failures
• World Trade Center Dust Particle Identification

For more information about asbestos as an indoor air quality concern with focus on easy ways to identify asbestos materials in buildings, see:

• Asbestos Identification in Buildings: How to find & identify asbestos-containing materials
• Asbestos in Good Condition
• Asbestos HVAC Ducts - Asbestos Materials On or In Heating and Cooling Duct Work
• Asbestos paper duct wrap
• Asbestos Transite Chimneys, Flues, & Pipes in Buildings
• Asbestos under the microscope
• Asbestos Vibration Dampers & Asbestos in the Air Handler
• Ceiling tiles: How to recognize ceiling tiles that may contain asbestos
• Examples of unusual uses of asbestos in buildings

AIR CONDITIONING SYSTEMS Chapter Index

To continue reading this air conditioning inspection guide, use links to the document chapters at left or below. Links shown in green font indicate where you are in this document.

Technical Reviewers

More Information on Building Diagnostic Inspections and Repairs

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