The “Box Fan with a MERV 13 Filter” air cleaner is an easy-to-assemble, inexpensive and effective way to improve indoor air quality and to reduce exposure to Covid-19. The concept is really simple – attach an effective air filter to a box fan and clean the air. This article gives more details about the construction and the efficiency of the device. https://www.texairfilters.com/how-a-merv-13-air-filter-and-a-box-fan-can-help-fight-covid-19/
The impetus for writing my article came from a call from a reporter from Wired. They had heard about this “air cleaner” and wanted to do a story on it. The idea of using a box fan as the air mover of an air cleaner has been around for many years. It works. I volunteered to buy a fan and do some testing. The result was this article: https://www.wired.com/story/could-a-janky-jury-rigged-air-purifier-help-fight-covid-19/
In the article, Dr. Richard Corsi of Portland State University talked about the issues of the strain put on the fan when filters are attached. The air has to be forced through the filter by the fan. His suggestion was to build a “box” of wood to provide more openings for greater filter area. This box could provide openings for two or more filters. Thus the fan would have to do less “work” and it would prolong the life of the fan motor.
In our testing of the device, we tried to achieve the same of objective of less strain on the motor by increasing the depth of the pleated filter. Resistance of a filter is determined by media area. As the depth of the pleated filter increases, the media area and the airflow through the device increases. In the case of a MERV 13 – 20″X20″ pleated filter, the media area for a 1″ filter is 5.5 square feet; for a 2″ deep filter it is 12.2 square feet and for a 4″ deep filter it is 18.51 square feet. Thus when we tested the airflow through the “Box Fan with a MERV 13 Filter” Air Cleaner, we found the following:
| Filter | Flow Rate |
| No Filter | 780 feet per minute |
| 1″ MERV 13 Filter | 320 feet per minute |
| 2″ MERV 13 Filter | 400 feet per minute |
| 4″ MERV 13 Filter | 460 feet per minute |
But what would happen if we created a “box” using multiple pleated filters? Good question. Let’s find out.
To test this we used 5 filters – two 16X20X2 and three 20X20X2.
We taped them together to form a box. The sixth side being the box fan.
We taped the filter box to the fan on the intake side – using gasketing to form a tight seal.
I would like to dedicate this hybrid variation of the “Box Fan with MERV 13 Filter” air cleaner to my friend Dr. Richard Corsi. We have had the pleasure of supporting each other for many years. I was fortunate enough to support his initiatives and researchers at the University of Texas. He was gracious enough to serve on the Board of the Asthma and Allergy Foundation of Texas for over 10 years when I was President. No one has done more to promote the science indoor air quality than Rich Corsi as evidenced by his almost daily contributions to our understanding of how to stop the spread of Covid-19.
So how did our “Corsi Box Fan with 5 MERV 13 Filters” Air Cleaner perform? Great. Airflow from the fan was 580 feet per minute. As usual, Rich is right. More surface area of the filter media – better air flow with minimal strain on the fan motor. Plus the filters will last for a very long time (I would estimate 6 months).
Thanks, Rich!
Update: To ensure the most effective performance of the “Corsi Box” we recommend that a “fan shroud” be added to the face of the fan. Here is the explanation of why and how to do it.
Update #2: A very important question concerning all DIY Box Fan and MERV 13 filters air cleaners is: “Do they put a strain on the fan motors that could lead to overheating?” Thanks to some excellent research by UL (supported by the EPA) we know that the addition of filters to the fans will not create overheating. Even with a 1″ filter on the fan – which has a much higher resistance than a Corsi/Rosenthal Box – the temperatures on the fan did not exceed normal parameters. For more details of these tests see: