Filter Fans

A furnace filter duct taped to a box fan makes a remarkably good and inexpensive particulate filter. Linda Wigington has been advocating for them and using them in ROCIS cohorts. 

This thread is meant to discuss what works and what doesn't.

I've got a Dylos again, and so I'm measuring stuff that I can't with the other IAQ monitors I have: Foobot, uHoo, Awair, and NetAtMo. Namely I'm watching small particles.

Linda Wigington uses a 4" MERV 11 or MERV 13 filter on a standard 20" square box fan. She's seen REMARKABLE results from using them.

Meanwhile, I'm a bit lazy. I went to Dollar General down the street from me and found a box fan and a 1" filter without a MERV rating. It was cheap and easy, so I bought a few.

When I was part of a cohort in summer of 2016, I found it worked remarkably well with the filter taped to the front of the fan (the exhaust). It knocked the Dylos small under 500, and often under 100.

What doesn't work

Taping the filter to the back of the fan with a 1" filter.


Something was stirred up this morning and my readings were 4000-5000 Dylos small, the highest readings I've ever seen in my finished basement, where my office is. I turned the fan on, no change whatsoever that I could see. Doh!


The 4 corners of the fan on the exhaust side are sucking pretty hard. My hypothesis is that rather than pull air in through the filter, it's just pulling from the four corners.

Linda found that with the 4" filter, both sides of the fan are effective because the media filters don't create a lot of static pressure.

Propeller fans and static pressure

Propeller fans are LOUSY at creating static pressure. The late, great Brian Baker took my head off in an email when he thought I was advocating for filter fans as whole house filtration. When met with static pressure they just pull from somewhere else and get noisy. I think he's still annoyed with me about this. =)

What have you learned?

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  • Recently Fan/filters have been referenced in print and social media, as well as technical webinars addressing filtration in reference to improving indoor air quality, whether in response to indoor particle sources, reducing aerosol transmission of COVID, or reducing exposure to poor ambient air quality.

    Here is the experience summarized by a ROCIS team member:

    an interesting home experiment that we saw with some DIY fan filters:

    "This is a home office in an unfinished dry basement with laundry and lots of things stored around. The experiment was performed over the weekend, so there was minimal activity in the space.

    The space is about 350 sqft and for maximum effect the homeowner moved two box fan filters with MERV-11 20X20X5" filters with 1" charcoal pre-filters. When on, the filters both ran at medium speed.

    The filters were shut on and off several times during the course of the weekend.

    The air conditioner was also turned on for one of the periods that the filters were off, to see the effect of the AC on particulate concentration.

    The peaks correspond EXACTLY to the three periods that the filters were off. With two filters running at medium speed it takes about 1 hour to bring the peaks down to the 1-2 µg/m3 range.

    The higher peak was during the period that the AC was on, but filters were off.

    It looks like it takes about 3-4 hours for levels to increase to peak levels after filters are turned off."

    Imgur: The magic of the Internet
  • ROCIS will be doing a survey soon of those participants who have been using either portable air cleaners or these box fan/filter devices. We would like to see how effective they have been in real houses and offices, and if there are strategies which optimize their use. For instance, there have been several ROCIS participants who habitually use air cleaners overnight in their bedrooms, knocking recorded particle counts down to near zero when they are sleeping. On the other hand, it may turn out that some people try them and then never turn them on again.

    That is why we need to survey users of portable air cleaning equipment.

  • Hi Nate,

    Thanks for your commentary and observational info.  This is my favorite intervention method.  I find it to be extremely cost-effective and most of all, simple.  

    I've been exploring some various fan filter practices my self.  I'm yet to try a 1" filter, but have heard of a colleague of ours using two 1" filters strapped together.

    Using a 2" MERV 13 filter has seen less of a drastic drop in Dylos numbers, however generally gets the numbers down quicker, whilst also providing more "breeze" (i.e. the air moves more easily through the media).

    Using a 4" MERV 13 filter by far does the best at dropping numbers to the lowest point possible, and keeping them there.  I have run a 4" MERV 13 for a few months now (24/7), and my average Dylos numbers went from the 2000 neighborhood down to the 200 range or lower.

    A caveat for the 4" MERV 13 that I've found, which speaks back on what you mentioned Linda saying- due to the thicker media, it doesn't do as well for CFMs, on comparable settings as the 2" MERV 13. There is more back-flow with the 4" from what I've seen thus far.  Both seem to be better functioning on the back of the fan "pulling" than on the front of the fan "pushing".

    This is what I've gathered so far, not necessarily true for all cases.  I'd be interested to hear other participants' experiences and anecdotes as well.


    • Awesome feedback! I learned a lot from your commentary. 4" MERV 12s are on the way, found them for $10 each on Amazon. 

    • Let us know how your new filters perform.  Our MERV 13 4" deep filters are Honeywell, and are considerably more expensive than $10 each,  We have seen a variety of high MERV filters, and these Honeywell filters have a media that is thick and soft - something you could imagine wearing or sleeping with.  Others have media that is more cardboard-like, and in the tests we have done in central air handlers, definitely have higher static too.

      I wonder if a inexpensive kill-a-watt meter is the easiest way to get a handle on filter resistance.   Please provide that feedback -- open fan at each of the speeds, and then new with the filter attached. 

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