My mom's furnace has ductwork until right next to the furnace, where the return air instead goes into an obsolete electronic air cleaner that is basically as big as the furnace itself.

The issue is the furnace filter, which sits inside this big empty box. We're not looking to remove or rework everything. Since my dad recently passed away and I'm use to regular ducts and filters I don't know what to tell her with these. Do we buy new ones? Are these the kind that just get washed and reused?


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2 Answers 2


Replace. Some say that filter media is meant to be washed and reused, but my experience is it passes ginormous amounts of dirt.

If it were up to me, I would throw that away and install 2" pleated MERV 7 (or 8) disposable pleated filters. I usually purchase these from a local supply house that specializes in air filters because they will often sell any size needed for a reasonable price. The prepackaged media available in hardware stores can be okay in a pinch, but they are often a poor value and those stores usually do not sell anything thicker than 1" pleated.

A 2" pleated Merv 7 Filter looks something like this:

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Actual width and height are typically 1/2 inch under nominal. Actual thickness is typically 1/4" under nominal. (16 x 20 x 2 nominal is actually 15.5 x 19.5 x 1.75 to give installation clearance). Air filter supply houses specializing in this media can usually fabricate these to order at a reasonable price. These are called "pleated air filters, MERV 7 (or MERV 8)." The MERV rating refers to the particle size that the filter traps. Higher number gives tighter filtration, but most residential blowers cannot tolerate a filter more restrictive than MERV 8.

  • 1
    Prefilters are not made to get all the dirt in the air, just the larger particles that the electronic filter cannot handle. I hope the unit still has the cartridges that do the bulk of the cleaning. I see where they were, but they are not in the picture. If the filters are used that you prescribe, the cartridges will not fit unless that part of the unit can be altered to use media based filters instead of electronic filters.
    – Jack
    Oct 29, 2015 at 2:31
  • Good point concerning the prefilter media fitting next to electrostatic cartridges. My impression was the unit is an empty box at this point. If the OP has electrostatic cartridges for this that might be reinstalled and made operable, then I would agree on washing and saving the prefilters for use with the cartridges. If not, then these make lousy stand alone filters.
    – user39367
    Oct 29, 2015 at 5:14
  • Also, electrostatic precipitating filters tend to pass dirt if not religiously maintained, so that is another reason to put 2 inch pleated media filters in the empty box and just change those occasionally (unless the blower is too small to tolerate the pleated media). Details, details...the devil is always in the details.
    – user39367
    Oct 29, 2015 at 5:33
  • OP here, these are the only filters present. It's been this way for years and the house isn't dusty, but we're open to changes (short of removing/modifying the whole big empty box) So, for what you're recommending I replace the filters with, can I get a picture, or an exact term to google? Thanks!
    – user20127
    Oct 29, 2015 at 10:57
  • I added a photo to the post.
    – user39367
    Oct 29, 2015 at 15:12

The blue fibrous mats are prefilters, they can be washed and reused. What I see though, is the filters do not lay that well in the unit, but if there is something that helps them lay flat to filter the air the way it should them keep them. If they are to lay flat on their own, perhaps new ones should be gotten

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