Better Box Fan Air Purifier

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A better more efficient and odor eliminating homemade air purifier than just taping a 20x20x1 filter to a box fan. Sometimes you need to help clear the air in a dirty environment but don't want to use an expensive air purifier such as in a room after renovating and painting. Here's a cheap and easy way to build one.

Air purifiers can be expensive and you've probably seen articles recommending to just put a 20" x 20" x 1" furnace filter on a cheap 20" box fan and POOF! instant cleaner air for not a lot of money. It really does clean the air pretty cheap.

There's a problem with this though. These fans weren't designed to be run with a filter. The filter will restrict air flow which will put a higher strain on the motor causing it to use more electricity and in worse cases could be a fire hazard. The higher the MERV rating (cleaning efficiency) of the filter the more stress it will put on the fan.

Don't worry! You can still have your cheap air purifier as long as the filter area is increased to decrease the effect of air resistance. Instead of using one 20x20x1 filter we'll use two 20x25x1 filters which increases the filter surface area over 250%. It's a little more expensive because you're using two filters instead of one but the increased filter surface area also helps the filter last longer before it gets clogged up and we're saving on energy use compared to a single filter.

I can't take credit for the design, I found it via Marshall Hansen Design but I'm using different filters.

I recently remodeled my basement and there's a lot of dust (including drywall dust and sawdust) and fumes from paint, cleaners and other building materials. It's an underground basement and mold has to always be a conern too. Because part of the space will be my home theater in the future I plan to get a more expensive air purifier that's smaller and quieter such as this Fellowes Quiet Air Purifier AP-300PH with True HEPA Filter because it has great reviews, is reasonably priced and the replacement filters aren't that expensive or a Sharp Plasmacluster True HEPA Air Purifier or Blueair HepaSilent Air-Purification System if I find some extra Benjamins in the couch cushions. But I don't want to put that in the dusty environment right away because I'll have to replace the more expensive filters fairly quickly because the air is so bad. I also don't want to spend the money right now but I do want cleaner air now.

So I decided to get things going with a homemade air purifier that has filters that not only clean the air of particulates but also reduce VOCs and odors. Even after I purchase an air purifier I'll still have an air purifier I can use for other purposes like when I'm painting or cutting wood.

Also, don't forget to add more plants to your home. They're natures air purifiers. Not only do they convert carbon dioxide to oxygen but houseplants filter toxins and emit negative ions too!

What You'll Need



  • A pair of scissors or utility knife

Quick Info About Filters

The nice thing about this box fan filter setup is you can use any filters you want. If you are only concerned about large particles generated from something like cutting wood, you can use cheaper, lower MERV rated filters. If you're worried about finer particles from allergens such as mold, you can use a higher MERV rated filter. To understand more about what the MERV ratings mean have a look at Table 2 on this EPA article on residential air.

I chose to go with the Filtrete Home Odor Reduction Filters which are a new type of filter from Filtrete. They contain a lot of activated carbon to help filter out odors and VOCs.  They are rated MERV 11 and are equivalent to their 1000 MPR furnace filters. They should do a good job with even the fine drywall dust and the activated carbon will help a lot with odors. I bought a 4 pack for around $57 which gives me 2 sets of filters.

Filtrete Odor Reducing Air Filters
The filtering power of these Filtrete Odor Reducing filters isn't listed on Filtrete's website or on Amazon but I contacted them and found out that these filters have a MERV 11 rating and filter 90% of particles between 1-10 microns which makes them more efficient than the Filtrete Micro Allergen Reduction Filters I've been using in my air conditioning for the past few years. Plus Filtrete packs over 180 grams of activated carbon to help reduce odors.

After I go through both sets of carbon filters I'm probably going to switch to the Filtrete Elite Allergen Reduction Filter which does a much better job of filtering small particles. If odors are a problem I'll get some Cut-to-fit Carbon Prefilters and tape them over top of the Filtrete filters.

Okay, enough of the why, now for the how!

Step 1: Unbox The Filters

You're thinking "OMG! He's actually describing how to open a cardboard box?!?!?!" The best deal I found for these filters were on Amazon and with my Prime membership (or supersaver shipping) the shipping is free. Plus the packing box is going to be part of the filter so we need to keep in intact.

The box is sealed very well. Not only is it taped but the seams are also glued together. To get the filters out of the box I cut the top where it's perforated with a utility knife as shown.

Be careful not to cut too deep and damage the filters. Keep the box someplace safe to the side while we continue. 

Step 2: Hinge Filters

Start by taking 2 of the filters out of their plastic wrappers and lay them one on top of the other with the air flow arrows pointing towards each other. With the Odor Reducing Fitlers this means the black, carbon sides will be facing each other.

With the filters lined up, tape one of the short (20") edges of the filters to create a hinge between the two filters as shown.

I decided to go with white duck tape so it looks a little nicer but it makes it hard to see in the photos against the white cardboard filter frame.

Step 3: Tape Filters To Fan

Lay the box fan face down on a flat surface to mount the filters to the back making sure the bottom of the filters are about flush with the bottom of the fan and do not extend past the bottom.

Use a strip of duck tape to secure each side to the side of the fan.

Step 4: Cut and Attach the Cardboard

Cut out one of the large sides of from the cardboard shipping box, place it over the top of the of the filters and secure it with a strip of tape to the top of the fan. Position it so that it's pretty even over both filters but it doesn't have to be perfect.

Using a pencil, trace around the filters (while pressing down the cardboard so nothing moves) to mark where the cardboard needs to be cut.

Lift the cardboard up (the front piece of tape acts like a hinge) and cut along the lines to trim the sides. By the way, I really like these Fiskars Cuts+More 5-in-1 Multi-Purpose Scissors.

Repeat the process for the bottom cardboard. The other large piece from the box is seamed but it's taped and glued and very stiff so it shouldn't be an issue but you can run a strip of duck tape over the seem if it makes you feel better.

Also keep in mind that the bottom of the fan has feet. I positioned the cardboard underneath the feet and then pulled the feet out to tape.

After taping up the sides of the bottom cover to the filters, I added an extra strip of duck tape all the way around where the filters/cardboard meets the fan for added support. The rounded corners of the fan means there might still be an air gap there so check them and add extra duck tape if necessary. On the bottom there are also some holes, duck tape over those as well. Finally, replace the feet.

Step 5: Seal The Back

Where the hinged filters meet the cardboard in the back, add another strip of duck tape for support and to seal any gaps.

Give another look around the seams to make sure there aren't any gaps. If you find any, tape them up.

When using the fan, I always make sure that the cord isn't underneath the cardboard and instead is under the metal fan chassis, just in case something goes wrong and the cord overheats. Can never be too safe.

Replacing Filters

When it's time to replace filters, use a utility knife to cut the filters free but leave the cardboard in place. Then just hinge and attach the new filters. 


  1. Currently I bungee a 20x20 to the back of box fans, but I might try this instead!

  2. Very nice idea. I got to make one of these, the wife will love it.

  3. Thanks Mr.Tom!I just built this today.... I hope my allergies improve now!

  4. hi Tom, did you build this in the UK? I couldn't find box in amazon uk. thanks.

  5. this is a great plan, but the information about causing the fan to pull more current when the inlet is restricted is exactly backwards. Restricting the inlet reduces the amount of air the fan can push, (less work= less current)

    1. Less air flow at much higher resistance = more work/current.
      Doubling the filter area halves the resistance caused by the filters.

    2. Less air moved = less power consumption, but also less cooling. If the fan could force the air throuh the filter then the power consumption would increase, but that can't happen with axial flow fans.

    3. will 2 filters reduce the filtration rate? other diy sites just attach a furnace filter (though not w/ high MERV rating) directly to the box fan. i want hepa merv 12 or above if possible for a small room.

    4. Current draw isn't about air movement, its about fan motor load. More filter surface area for air to travel through=less air resistance so less load (thus better for the fan and more safe). Imagine if you use your hand to stop the fan. Zero air moving but there's still load, and a high current draw.

    5. The high current draw from the fan when you are holding it is obvious.

    6. The high current draw from the extra resistance of the filter should be obvious too. That's why you keep the outside unit of your hvac system clean and clear of debris, and why you change out dirty air filters. 2 filters won't hurt the filtration because the air still goes through the filters. If you had a cheapo filter on one side and a high filtering hepa on the other side, it would only be as good as the cheap filter though because there would be less air pulled through the higher resistance hepa than the cheapo.

    7. Blocking the air flow decreases the load of the motor and decreases the current (amps). Period. The fan is not using any form of closed loop control trying to keep a constant air flow. The article is wrong!!! The only damage that can occur is heat built up as the air the fan is circulating is the same air being used to keep the motor cool. Please do not give false information and confuse others. What I have stated is fact! This is not an opinion, this is factual information. This confuses people all the time, even at my work. We monitor load (amps) on pumps and fans while performing rational checks against flow meters. This is done via PLC but is checking to make sure the fan / pump is performing at its best in the event a flow meter is providing inadequate information.

      Again, blocking the air flow decreases the load and the current on the motor. Period!

    8. I appreciate you linking this article, it was very informative. Specifically about how CENTRIFUGAL fans decrease load as airflow is restricted. Now I don't work in the hvac industry but I do know that centrifugal fans are the most commonly used fans in the industry. So the problem is that this article is talking about using an AXIAL fan to move the air. Axial fans move air parallel to the shaft of the fan. Perhaps the difference between the way the types of fans move air could cause the box fan to draw more power as resistance is increased. Only empirical data can show us, I might update with experiment results.

  6. Used 300 mpr furnace filters. Bought 20x30x1 as they were same price as 20x25x1 or 20x20x1.

  7. Nice.
    Indoor plants decrease air quality because of the soil. Bacteria in the soil will use up more oxygen than the plant produces. Outside, sunlight kills the bacteria, but most windows block UV light. The soil also supports mold and mold spore. Also, wet soil increases indoor humidity and increases other growth away from the plant. For indoor air quality, no plants, or only hydroponic plants.

    1. those are some very good points, thank you for the input.

    2. I will never forget the air of an apartment that had plants on every shelf all along the walls. There were hundreds of plants, if not thousands as it was a large room. It smelled like a vibrant forest. Unforgettable. Not sure what bearing the soil had but she was old and this was her hobby. It took hours to take care of the plants every day. It could be that the soil detracted from the air quality and if the same amount of plants were using aeroponics or hydroponics the air might have been much better though its hard to imagine anything smelling better than that. I need to visit many garden stores where there are many plants packed in smaller spaces. I heard there was an office building in India that was sealed from outside air because of the air pollution and all of its air was produced by plants inside. But what about nitrogen? That is a component of air. Plants don't produce nitrogen I thought. If you could produce all of your own air from growing food indoors, that would be sweet. And would most of that oxygen that they produce by gone in the morning because they use oxygen at night and expel CO2? Maybe the answer is to sleep in a different part of the house at night than where the plants are.

  8. using a 20x20x4 filter would decrease air flow resistance and is a lot less work

  9. Brilliant! Shared this on my page cause our area is saturated in wildfire smoke currently

  10. Been thinking about doing something like this. Thanks for already inventing the wheel. Built mine today, using the box the fan came in and MERV 12 filters.

  11. Is there anything on the market with permanent HEPA filter or does permanent HEPA exist? Something i can wash and reuse, instead of replacing these expensive furnace filters. MERV13 is ~$25-30?

  12. I don't know about a washable filter that works well. I have seen videos on Youtube and read that some people use HEPA filters on a box fan and clean them by blowing out with high pressure air. Not sure how well it works, but you could try that. Take fan outside before removing filter and letting dust from it return to house air.

  13. you can also do this with 4 20x20 filters, and one piece of square cardboard to make a cube. It will also allow you to use a high MERV filter without much drop in airflow.

  14. FYI, a box fan is an axial bkade design. Restricting the inlet air actually unloads the fan reducing the power it draws. Old hvac systems used to use inlet vanes to unload large fans. The only issue would be restricting airflow to the point that the motor is nit cooled sufficiently.

  15. Believe it or not, I just did a current draw test on my 20" box fan and to my great surprise, if I block the intake it draws MORE - not less current!!!!

    Here are thge actual figures:
    - normal operation @ 120VAC: 1.84A
    - intake partially blocked by a 3M Filtrete MERV 11 filter: 1.85A (no significant change)
    - intake completely blocked using a plastic garbage bag: 2.02A
    - exhaust completely blocked using a plastic garbage bag: 2.02A

    For fun, I did the same tests by simply laying the fan flat down on the floor, measuring current for both sides. Again, I got roughly the same results.

    This is totally unexpected to me. I did the same test with a vacuum cleaner a while ago and the current draw would plummet dramatically when I plugged the intake with the palm of my hand. Something like 7A down to 2A if I remember well. And the motor would start producing that caracteristic revving sound.

    The box fan does not behave the same way at all, probably due to that axial blade design thing.

    But still, the conclusion for this application is that it is totally negligible.

    Norm D.

  16. Norm wins. I love facts and stats.