How to Make a High-MERV DIY Portable Air Cleaner

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A high-MERV portable air cleaner can reduce your risk of getting COVID-19

Twitter is an utter hellscape in some ways, but it does have a few redeeming qualities.  I've seen a lot of good information about the COVID-19 pandemic there, including ways to filter the indoor air for coronavirus and the emerging science about the spread of SARS-COV-2 via aerosols in addition to droplets.  On the filtration front, one thing I started seeing there over the past few months was designs for portable air cleaners you could make with a box fan.  Of course, the easiest way to do it is just tape one filter to the back of the fan.  To reduce the pressure drop, though, you can attach two or more filters to the fan, and Shawna Henderson of Blue House Energy in Nova Scotia discussed the ones I've seen in this nice article:  3 Principles for Good IAQ:  Eliminate, Ventilate, & Filtrate.

The one with four filters is the one I like best, so I made a couple of them, one for my home and one for the Energy Vanguard office.  John Semmelhack sent out the tweet below last month of this four-filter design by Neil Comparetto, a top-notch home performance contractor in Virginia.

Portable air cleaner with MERV-13 filters - the Comparetto Cube

It's a modification of a design that I think Dr. Richard Corsi (of Corsi Code fame) promoted.  Corsi's version uses five filters, which means you have to have a base or some other way to support it (or else have one filter that doesn't get any air).  Comparetto used four filters, with the bottom side covered by cardboard.  Since it has six equal sides, we can call it the Comparetto Cube.

The cardboard base allows it to sit on the floor without any other support.  It also blows the air straight upward, and that makes it ideal for when you have company in your home.  Just set the Comparetto Cube in the middle of the room, and if anyone does happen to be sick, a lot of the droplets and aerosols should get pulled into the fan and filtered out.  The photo at the top of this article shows it in use at my house on a recent visit by my mother-in-law.

How to make a Comparetto Cube

I probably don't need to give any instructions here because it should be obvious what to do, but here you go.  First, get the materials you need.  I ordered the 20" box fan and 20" x 20" MERV-13 filters from Amazon for a total cost of $90.82.  The box fan (Comfort Zone CZ200A) was $25.20, the filters (FilterBuy) $65.62.  Once you have those, the only other things you need are tape and scissors or a box cutter.  I used the box the fan came in for the piece of cardboard that goes on the bottom.

All you need for this high-MERV portable air cleaner are a box fan, filters, tape, and cardboard

The first step is to tape the filters together.  I used little pieces of tape at the corners to hold them together first and then covered the seams with long strips.  Then I went back and double-taped the corners for a little extra reinforcement.

Two notes:  First, make sure you tape the filters together with the arrows pointing into the center of the square.  The fan will sit on top, pulling air in through the filters and sending it up and out through the top.  Second, arrange the filters in a square, as you see below, not a rectangle.

Arrange the filters in a square, not a rectangle

Next, cut a piece of cardboard the size of the filter square and tape it to one end.  Energy Vanguard's Luke Bertram is taping the cardboard bottom onto the filters in the photo below.

Luke assembling a high-MERV portable air cleaner for the Energy Vanguard office

Finally, turn the filters over and tape the box fan to the other end.  I used blue painters' tape, but you can use duct tape if you'd like.  (Just don't use duct tape on ducts!)

The assembled DIY high-MERV portable air cleaner

How to stay safe from COVID-19 in your home

The number one thing you can do to prevent the spread of COVID-19 in your own home is not to have other people in it.  My wife and I have done that since the middle of March now, with only a few exceptions.  We've had a few people come into our house, though, and we've worn masks, run the ventilation more, and opened windows when the weather is nice enough.  And now that we have a Comparetto Cube, we run it whenever others are in the house (with masks on) and for a while afterward.  As mentioned earlier, keeping the portable air cleaner in the middle of the room should greatly reduce the risk of transmission, if someone happens to be infected.

Filtration, whether with a Comparetto Cube, masks, or HVAC filters, isn't the whole answer, of course.  Social distancing and ventilation are also really important.  Now that we have nice fall weather in Atlanta, we occasionally have people over in the backyard, spaced about 10 feet apart.

So yeah, Twitter is a hellscape.  But in addition to the good building science community there, I've found other gems.  Paul Bronks (@SlenderSherbet) puts the best captions on animal videos.  The Auschwitz Memorial account (@AuschwitzMuseum) provides sobering reminders of how bad things can get when we lose sight of the humanity of others.  Stewart Brand (@stewartbrand) of Whole Earth Catalog fame puts out some of the most intelligent social media posts anywhere.  From him I discovered the novel Stoner by John Williams.  That tip alone was worth all the pain.

Who knows?  Maybe you found this article on Twitter and have built your own Comparetto Cube.  Maybe you even use it when you're reading Stoner.


Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.


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Now I have to wonder, since you are highly qualified to answer these questions:

Did you measure pressure drop across the filters? Do you have a CFM estimate for the fan on high speed?

The main reason I am wondering is because I come across standalone HEPA towers for less than $100 and I want to compare cost vs. air filtration rate.


Bobby, that's a great question.  No, I haven't measured those things yet, but John Semmelhack just did and will be releasing his results soon.  And he did for different configurations.  Stay tuned.

Allison, Why not take these MERV 13's to near HEPA. All you need is four negative ion generators, placing each one in front of the filter inlets. The agglomeration produced will dramatically improve your filter efficiency. AND according to some folks, it might even improve your marriage. Just don't set them near a wall.....

I actually built one of these recently! All total, it cost me around $70 to make. One thing that I ran into was most of the air being pulled in through the open corners of the box fan as well as some through the filters. I blocked off the corners of the box fan with strips of duct tape and it really helped mitigate that.
Other than reducing the area for air to bypass the filters, do you have any suggestions for dealing with this issue?

Question: Would it not be better to suck the air into the fan or place the fan pointing down and put this up on legs? My thought being laminar air flow. If droplets were headed down anyway and if people are above the fan if the fan is blowing up and mixing the droplets and atomized cov 2 and shooting it up to the ceiling to be distributed through out the room. Would it not be better to suck it in?

How often do you need to replace the filters? I'm thinking this would be great for a classroom.....

Years ago, saw a write up a box fan with a single one inch filter - same concept, just (1) not horizontal, and (2) using 1/4 the filters. Use it in the garage or basement when doing project and it works well (anecdotally).

Would vertical improve the operation?
Is there an optimal # of filters for an average box fan?


What is the area /volume this would cover?

Stoner Magazine? For Geology Majors? ;)

For the filters to work effectively, you don't want too high an airspeed moving through them, more filters means lower airspeed but also, not too powerful a fan.

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