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DIY Box Fan Filter – MERV 13 versus HEPA Air Purifier

by / Sunday, 18 November 2018 / Published in Environmental Consulting, News
Air flow of fan filter on intake side. Wildfire smoke and air quality

DIY Box Fan Filter & Air Quality Experiment

Here in San Francisco and surrounding areas kids are stuck indoors during this smoke wave caused by our wild fires with little or no air filtration. In San Francisco the school buildings are older and are not setup to have building wide air filtration. Nor can cash-strapped school districts afford high-end air purifiers. Necessity is the mother of invention, and thus this blog about a DIY Box Fan Filter I built and tested.

 Since the last fires, I have been curious about the DIY box fan air purifier made from items that can be found in most hardware stores- box fan, 20-inch x 20-inch x 1-inch MERV 13 filter and some duct tape.

There is a serious need for air filtration in schools and homes. How well will this DIY set-up clean the air?

Some parents of children with breathing issues are deciding to keep their kids home.
The SFUSD current response to the unhealthy air quality is to close windows and stay indoors, no current plans for air filtration due to cost.

Following the instructions from the Mariposa County Health Department, “How to Build an Inexpensive Room Filter”, we made our box fan room filter.

Using our Lighthouse 3016-IAQ Laser particle counter, air samples were collected for 2 minutes, with a 2 min delay. This set-up was done in 2 rooms: one with the box fan and one with the HEPA filter.

The DIY Box Fan Filter was able to drop PM2.5 particulates down 77%, the HEPA filter dropped the PM10 down 94.46%

DIY Box Fan Filter with MERV 13 Filter

DIY Box Fan Filter with MERV 13 Filter

 See results complete results in the chart below:

After testing, I was impressed by how well the DIY Box Fan was able to remove particulates from the air. Yes, HEPA will provide more filtration, but if cost is an issue than the DIY MERV 13 Box Fan is a sensible choice. Further testing needs to be done with higher MERV ratings and over longer periods of time.

MERV 13 Box Fan set up at 2 feet from particle counter

MERV 13 Box Fan set up at 2 feet from particle counter.

HEPA air purifier (AeraMax 300) set at 2 feet from particle counter.

HEPA air purifier (AeraMax 300) set at 2 feet from particle counter.

Lighthouse monitor taking air samples.

Lighthouse monitor taking air samples.

HEPA air purifier (AeraMax 300)

HEPA air purifier (AeraMax 300) set at 13 feet from particle counter. This unit was currently in the classroom.


10 Responses to “DIY Box Fan Filter – MERV 13 versus HEPA Air Purifier”

  1. Alex says :

    Thanks for the suggestion. We do hope to run more air quality tests with the box fan. When time and budget allow we’ll retest more filters and update this post.

  2. Alex says :

    Thanks! You’re so right. Testing these additional variables would be super interesting. Time and money… we keep looking for more of both!

  3. Alex says :

    If there’s a silver lining to Covid, it’s that many more people are now aware of how particles and germs travel, and many people now have a better understanding of face masks and respirators and air filters. In the early months of Covid there was a fast push for “ionization” and “electro-static” filters to “kill Covid.” I’m glad to see the larger market shift away from these forms of questionable air purification strategies, and focus more on traditional media filters which we all agree work.

    In our home remodeling and contracting division, Home Stewards, we’ve helped a lot of people in recent years install more advanced ventilation and filtration systems. We’ve found customers much more educated since Covid struck.

  4. […] Use a high quality air purifier indoors, especially in your sleeping area. The air purifiers we recommend are made by Blue Air: and IQ Air: If you don’t have an air purifier available, an acceptable hack is to tape a MERV13 filter  to a box fan. […]

  5. It’s slightly disgusting to think about, but many of the viruses that make us sick are found in the air we breathe. When someone who is sick sneezes or coughs, they expel infected droplets into the air. Those droplets remain in the air and may be inhaled by others later, causing illness. Diseases ranging from the common cold to COVID are spread via airborne transmission.

    An air purifier for your home can help keep you and your loved ones safer from a number of airborne illnesses. According to the EPA, air purifiers can help cut down on the risk of airborne transmission of many common viruses.

  6. Kelly says :

    Nice research. I would love to see a test of common alternative configurations of the diy box fan to optimize performance. Such as: is the filter better in front or behind the fan? How much difference would it make to add a shroud to the corners of the front of the fan so it prevents back flow? There are studies that show the 4-filter corsi-rosenthal configuration is more effective, but the downside is it uses more filters and is harder to replace. How about a middle ground of using 2 filters, adding filters to both the back and the front?

  7. Dave Keeton says :

    Can you try a 4″ pleated filter and retest?

  8. Michael Coomes. says :

    Would like information and costs to conduct an air quality test on a box fan / merv13 filter system I have patented.

  9. Mort says :

    Fan flow is wrong. Fan does best in compression, not vacuum. So rotate for better performance.

  10. greg davis says :

    I have been using this configuration here in Lodi, Ca to take advantage of the cool delta air since I bought my house in 1996. Several of my neighbors asked how it worked, and they did the same thing after showing them the filters, ao if you drive around our nieghborhod you will see them in the windows.

    Sorry that it took you this long to figure that box fans are what they say – fans, no different than any other except they have a limited static pressure (<0.4" for house type) compared to centrifugal types.
    Greg Davis, PE M29759

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