Annotations on building a box fan + air filter (Corsi-Rosenthal) box

With the recent horrendous air quality in new york, I decided this was the time to build an air purifier. These were originally conceived to reduce the amount of covid-carrying aerosols in a room, but the same high-quality air filters that work against covid aerosols also work very well against harmful particulates.

For my box I used:

• A 20 inch square Utilitech box fan
• Four MERV13-rated Filtrete electrostatic filters with dimensions 20 inch times 20 inch times 1 inch
• A roll of duct tape (I used white so the box would look neat)

General architecture

The filter box is a cube. The fan is the top side of the fan and and sucks air from the inside of the box and blows it upwards into the room. Air enters into the box through the four filters that make up the sides of the box. On the bottom, the box is closed with a 20 inch square piece of cardboard I cut out of the box the fan came in. All the 6 sides are taped together with duct tape to make the box air tight in all corners– we want all the air to enter through the filters and exit through the fan.

The fan has four thin sides (edges), and one “air-out” side and one “air-in” side.

The filters also have four thin sides (20 inch times 1 inch), one “air-in” side (20 in square) and one “air-out” side (20 in square) However, it can be tricky to figure out which is which of these two sides. On the air-in and air-out sides, the filter does not extend all the way to the edge, but is inside a frame, approximately an inch wide for mine.

The problem of filter direction

Most (all?) air filters are optimised to work with the air flowing one way through them. Unfortunately, for the filters I bought, the only indication of the correct airflow direction was a printed arrow on the side of the filter (the thin side– the one that is 1 inch wide). No indication was evident to me in the packaging, nor was there any discernible difference in texture or feeling of either side of the filters. Shame on the producer for not doing the bare minimum of putting a piece of removable tape or something on one of the filter sides to help orientation. If you have problems seeing the arrows, but have a camera, I would be happy to help if you mail me photographs of the filters: Put a piece of tape on one of the 20x20 inch sides and place the filter standing on one of its thin sides in front of you, with another thin side facing you, and the taped side pointing to your right.

Take a photo, ideally while standing up pointing your camera pointing down towards the filter. this way, the photograph will show two short edges (the one pointing towards you and the one pointing at the ceiling) if the direction indication is not printed on all sides.

Building the box

Once you have decided which side of each filter is “air-in” and “air-out”, we can start putting together the box. When taping seams, take care to have the tape run all the way along the seam, we are aiming for air-tight seams for the best performance. To start with we’ll tape together the side-walls of the box, made up of the four air-filters, with the air-out side pointing into the box.

1: connecting the filters.

1A making the first corner:

Put the first filter standing on a thin edge in front of you, with the air-out side pointing away from you. The future inside of the future box is on that air-out side. Take the second filter, and orient it so that its air-out side is pointing to your left. Then place it so that it makes up the right side of the box, seen from your side, and put the thin side of the first filter against the frame on the air-out side of this second filter. In front of you, you now have a 20 inch high and 21 inch wide wall (since the last inch is the thin side of the second filter). Use the duct tape to tape the seam between the two– the duct tape will go on the thin side facing you on the second filter, and on the frame of the first filter.

1B second corner

Now you can rotate the box so the second filters air-in side is facing you, and repeat the last step with filter 3: orient the third filter so that air-out is facing left and tape between the thin side facing you and the frame on filter twos air-in side facing you

1C third corner

Rotate again, so that the third filters air-in side is facing you, and orient filter four to point the air-out side left and into the box. Place it similar to the previous– with the right thin side of filter three against the closest part of the air-out side of filter four, and with the seam facing you between filter threes air-in side and filter fours thin side. Tape that up!

1D: tape the last corner

rotate one last time, now with filter fours air-in side facing you. On your right, filter fours thin edge should now be against filter ones air-out frame, and you can place tape covering the right-side frame of filter fours air-in side and filter ones thin side facing you.

2: floor

Now you should have a square that already feels a little bit more sturdy. Next, lets make the floor:

2A: make a floor plate

make or take a 20-inch square piece of cardboard– I put the box on top of the cardboard box the fan came in and cut around it.

2B: tape the floor in

tape the cardboard in place as the floor of the box– take care to tape all seams carefully.

3: the fan.

3A: placing the fan

place the box with the cardboard floor down, and the construction should be sturdy enough for you to simply place the fan on top of the filter walls.

3B: tape the fan put

Now tape all around the filter-fan seam to make an airtight seal. You’ll need to pay extra attention to the corners– it is easier to cut some 20cm / 8in pieces of tape there so you can snuggle them into the corner better.

4: finished!

plug in the fan and enjoy some very clean air over your face.

5: optional fan shroud:

If you want your fan to move air efficiently, you might want to make a shroud around the fan– covering all parts of the fan outlet that does not contribute to the air flow. In particular on the outside corners of the fan outlet grill, the air might actually be moving into the box and not outwards!

5A: where to put a shroud

the easiest way I found to determine how big the shroud should be was to take a tiny piece of tissue, move it around over the fan outlet, and note the places (or the radius from the center) where the tissue was being sucked down towards the fan.

5B: ideas on making a shroud

you can cut out a shroud of cardboard with the correct diameter, but it was easier for me to just put tape over the fan outlet grill where the air was going into the box (mainly the corners/edges of the outlet)