How air filters work to remove particles from the air stream
Operation of heating and air conditioning filters
Definition of impaction filtering
Definition of interception filtering of airborne particles
Definition of diffusion filtering of airborne dust and debris
Particle size vs. air filter characteristics vs. indoor air quality
Air conditioning / heating system filter product sources and recommendations
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This article explains and explain just how air filters for HVAC systems actually work to trap and remove particles from indoor air.
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. In these articles we are referring to filters installed on central air conditioning or central heating systems that move air through air handlers and duct systems. Standalone "air cleaners" are generally ineffective in buildings.
We include photographs to assist readers in
recognizing cooling system defects. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
The page top photograph is of a low-MERV HVAC filter in an air handler.
Readers should also see our INDOOR AIR QUALITY IMPROVEMENT GUIDE article series.
How do air filters actually remove particles from the airstream?
This photograph shows a soiled surface of a conventional air filter up close. The fibers are clearly visible but not the openings through which air has to pass.
If we look at a high efficiency, high MERV or HEPA air filter under a microscope we'll see a mat of randomly crisscrossed 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:
Impaction filtering: 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.
Interception filtering: 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.
Diffusion filtering: 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 mold, house dust dust, allergen fragment, mite fecal, cat dander, or other airborne particle sizes are a concern for indoor air quality? In the photograph shown here the large black Stachybotrys chartarum mold spores can be seen against our eyepiece micrometer which, after
calibration, shows that these particular spores were about 7u x 15u in size.
The brownish tubular structures are fungal hyphae. Another, smaller fungal spore
is in the background. What's not addressed by some 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).
A more complete discussion about the size and behavior of problematic indoor air particles which form
an indoor air quality concern can be read at Particle Sizes & IAQ.
Critique, contributions wanted: Contact Us to suggest corrections or additions to articles at this website, and if you wish, to receive online listing and credit as a contributor. Particular thanks are due to the many experts and also consumers who read and critique technical articles at InspectAPedia.com.
Additional technical contributors & reference sources for this article are listed below.
Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects"
section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and
home inspection educator.
Thanks to Jon Bolton, an ASHI, FABI, and otherwise certified Florida home inspector who provided photos of failing Goodman gray flex duct in a hot attic.
Carson Dunlop, Associates, Toronto, have provided us with (and we recommend)
Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
US EPA article on air filter efficiency: epa.gov/iaq/pubs/airclean.html 06/18/2010
Wikipedia provided background information about the definition of HEPA and airborne particle interception.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Carson Dunlop, Associates, Toronto, have provided us with (and we recommend)
Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
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