How Air Filters Work on Air Conditioners or Warm Air Heat Systems
     

• How air filters work to remove particles from the air stream: impaction, interception, diffusion filtering methods
• What airborne particle sizes are an IAQ concern? - particle size vs. air filter characteristics vs. indoor air quality
• Questions & answers aboutHVAC filters used to clean indoor air: how filters work to remove airborne particulates and contaminants
• References

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Here we explain the three methods by which HVAC air filters work to capture and remove airborne particle contaminants from building air: impaction filtering, interception filtering, and diffusion filtering. 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.

In articles at this website 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. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. Readers should also see our INDOOR AIR QUALITY IMPROVEMENT GUIDE article series.

This photograph (above-left) 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:

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).