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How do Electrostatic Filters work?

How do Electrostatic Filters work?

It seems like manufacturers are coming out with new filters all the time.  Filters for homes with pets, filters for allergies, filters against viruses…and on and on.   I recently ordered and replaced my HVAC filter with a brand name that was a pleated filter advertised as an “Electrostatic Air Cleaning Filter”.  I had to find out what that meant!

First of all, I thought that electrostatic filters had to be hooked up to electricity.   Actually, only some of them do.  Electrostatic filters encompass a broad range of devices, and the ones that do require electricity are usually called electrostatic air precipitators.  The principle behind these units uses elements to impart a charge to incoming air particles, and then attract them (make them stick to) an oppositely-charged plate.  These systems could be portable, or installed in your HVAC system, or be a part of a huge commercial operation (like a smokestack).   Typically these systems are powered by a high-voltage, low wattage system.  The “filter” is the entire unit, and instead of replacing any parts, the charged plates are simply washed clean. 

However, I bought a regular pleated-type replaceable filter with a large wire grid over it, presumably just to keep the filter in shape as you try to manipulate it to fit in your HVAC.  I don’t have any electricity supplied to the filter box of my HVAC.  How could this filter be electrostatic?

The answer lies in triboelectricity, also called static electricity.  As air whizzes through the synthetic fibers of the filter, the fibers become charged with static electricity.  Then they start to attract the particles of dust in the air.  Making the filter “pleated” increases the surface area of the filter over a flat filter, so that it can attract more particles.  Eventually, however, the fibers become coated with dust, and it no longer acts as an electrostatic filter, but continues to trap dust by the other methods of normal filters (we discuss those four methods here).  And, hopefully before the filter gets so full that dust starts to bypass it, it gets disposed and replaced or cleaned.  

Reusable, washable electrostatic filters are also available.  Washable filters typically have aluminum or galvanized frames and polyester filters so they can withstand repeated cleaning.  However, these typically have a lower MERV rating (only up to MERV 9), so they are not capturing the smaller particles of bacteria, viruses and mold spores that disposable filters can handle.  For more explanation on MERV ratings, check out this article

You can also make your “electrostatic” filter perform even better by using an ionizer in the same room as your filter.  Because ions are charged molecules in the air, as they collide with dust, they impart the same charge to the dust.  This “pre-charges” the dust and causes a greater attraction to the fibers of the filter.  In this 2015 study, researchers showed that unipolar ionization (as opposed to bipolar) enhanced the filtration by 40%, with a lower pressure drop than filters that remove finer particles.  

Whether your HVAC filter is electrostatic or not, the most important maintenance task is to clean or replace it regularly!  This is the only way you can ensure that the filter is capturing the most particles possible, making your air as clean and allergen-free as possible.  

Photo by Lorena on Unsplash