Increase Filtration with a DIY Filtered Box Fan

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Air cleaners (also called air purifiers) can be an important tool for reducing your exposure to airborne particulate matter and thereby reducing the health risks associated with that exposure.

While air cleaners CANNOT eliminate COVID-19 or any virus, they can help minimize cross-contamination by enhancing the level of filtration which reduces the amount of virus-carrying particulates circulating in the air.  While the virus is roughly 0.1 micron, and too small to be captured by the vast majority of commercially available filtration systems, the virus can attach itself to larger particulates (0.3 or larger, which includes most things that make up common household dust like skin cells and pet dander) which can be captured by high-efficiency filtration.

An easy and inexpensive way to make your own air cleaner is to attach a very high-efficiency furnace filter to a basic box fan.


How to Assemble

  1. Unwrap the box fan and filter from any packaging.
  2. Look at the side of the filter, most will have arrows indicating the direction of the airflow. Make sure these arrows point to the BACK of the fan.
  3. Align the side of the filter indicated by the “airflow” arrows to the BACK of the fan (the air intake).  DO NOT attach the filter to the front of the fan where the air comes out.
  4. Securely attach the filter to the fan using the duct or painters tape. Make sure there are no gaps.

A 4” filter will allow you to operate the fan on a low or medium setting with good results.  Many people find that running fans on “high” speed is too loud for comfort, so it is important to use a thick filter so you can maximize capture efficiency and minimize noise.

Note: Most newer model fans have an automatic shut-off to prevent overheating.  If you are using an older fan, make sure to monitor for signs of overheating, such as odor. You can also run the fan on a lower speed to reduce risk.

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The fans need to be able to circulate air through the filter, so place them in open spaces not against the wall or in a corner. They can also be placed in front of an open window to bring in fresh, filtered air.

Make sure to place the fan on a flat, stable surface to decrease the risk of the fan tipping over.


How many do I need?
Plan on 1 fan per occupied bedroom. During the day, if the bedrooms are not occupied, you can move those fans into your living spaces like tv rooms, home offices, and study spaces.

How often do I replace the filters?
Every 3-4 months or when the filter is looks dirty.  If you live somewhere with a lot of outdoor pollution or if you have asthma/allergies you may need to change the filter more frequently.

To change the filter, we recommend being in the garage or outside and wear an N95 mask. Cut the tape where the filter is attached to the fan and place the filter in a garbage bag, seal the top and toss it in the trash. We recommend whipping the fan down with fragrance-free soap and water and let run outside for 10-15 min before bringing it inside and replacing the filter.

Can I use thinner (1-2” filters)?
Thinner filters are not recommended but may be the only ones available. They can reduce the airflow on the fan which may cause it to heat up, it can be louder and the filter will fill up faster.

Can I use something other than MERV 13 (or equivalent) filters?
Although filters lower than MERV 11 may capture large dust particles they are not able to capture smaller bacteria and virus-carriers particulates. MERV 11 is a minimum and MERV 13 is recommended.


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Joe Medosch
Joe Medosch
Hayward Score Healthy Building Scientist Joe Medosch has almost 30 years of experience as a contractor and 10 years as a master trainer with expertise in home performance, particularly in health and energy-efficiency. His proficiency includes building science & diagnostics and Indoor Air Quality (IAQ) assessments. He has served on numerous committees developing industry standards including: RESNET Standard 380, Equipment Sub-Committee, and BPI multifamily standards development.
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