Corsi-Rosenthal Cube

The Corsi-Rosenthal Cube (sometimes called a Comparetto Cube) is an inexpensive, do-it-yourself air cleaner that can be easily constructed out of a box fan and MERV-13 furnace filters. The Corsi-Rosenthal Cube can give whole-room air cleaning performance comparable to commercial HEPA air cleaners that are 10x or more the cost. Total cost is around $100USD ($130CAD).

Construction guide

Figure 2. A visual guide to constructing a Corsi-Rosenthal cube with a box fan and MERV filters with a top fan configuration. Air is pulled into filters (red arrows) and blows out of fan (green arrows). Photo by CC-BY al.hu.

What's needed:

4 or 5 MERV-13 furnace filters. You will want to purchase 20" sized MERV-13 filters. For the 3M Filtrete brand of furnace filters, this is "FPR 1900." If MERV-13 is not available, MERV-11 filters can be used but performance will not be as great. Important note: see the notes on MERV-13 filter tests below for brands to potentially avoid.
A 20" box fan
Duct tape
Some cardboard

How to construct (Figure 2):

Figure 3. A visual guide to constructing a Corsi-Rosenthal cube with a box fan and MERV filters with a side fan configuration. Photo by CC-BY al.hu.

Figure 4. Cut the internal diameter of the fan shroud based on the brand. Photo by CC-BY al.hu.

Duct tape the filters together, forming an incomplete cube. Try to avoid taping over the filter media part of each filter.
When taping the filters together, make sure to arrange each filter so that the air intake direction of the filter goes inward. The filters should indicate which direction the air is supposed to flow. So you want each filter's airflow direction to point into the cube, not out.
There should be two empty sides of your incomplete cube. The box fan will go on one of these empty sides and the cardboard will go on the other.
Cut your piece of cardboard to fit over the bottom area of the cube, where the cube will sit against the ground. Duct tape it to the bottom.
Duct tape the box fan to the top of the cube. You want the fan to blow air out (not into) the cube. You can also place the fan on the side, rather than the top, of the cube (Figure 3). Having the fan point directly up into the air is a bit better because it's less obtrusive.
Optional, but strongly encouraged (47% boost!): for increased performance you can create a 'fan shroud'. Cut a piece of cardboard so that the circle created by the fan blades is exposed by the edges of the fan are covered (see diagram or these instructions). You can also use duct tape on the fan to create this 'shroud' as well. The optimal shroud opening has been determined based on the fan brand: For Lasko fans, cut the shroud with an internal diameter (Figure 4) of 15"; for Utilitech fans, use 13.5". For increased durability, you can use Coroplast (corrrugated plastic) or 1/4 inch thick plywood to make the shroud.

Filter brands to potentially avoid

Multiple independent testers of the "FilterBuy" brand "MERV-13" filters report that they do not perform as expected. As of 2021, FilterBuy appears to be a popular MERV-13 filter vendor on sites like Amazon. If purchasing FilterBuy MERV-13, you may want to test the filters with a particle counter or find some way of ensuring their quality.

Variations

Figure 5. A 5-filter version of the Corsi-Rosenthal Cube. The cube is raised slightly off of the ground to allow air through the bottom filter.

5-filter version (Figure 5)

Rather than placing a piece of cardboard on the bottom side of the cube, another filter can be added in place of the cardboard. In this case, you would want to raise the cube off the ground (without blocking the entire bottom filter) so that the bottom filter could be utilized. This would in theory lead to less resistance on the fan motor, potentially prolonging the life of the fan and filters, and increasing the air output.

2-filter "wedge" design (Figure 6)

Figure 6. A 2-filter "wedge" variation of the DIY box fan air cleaner.

A variation of the air cleaner can be made using two MERV-13 filters (add details). This version is slightly less expensive (2 fewer filters are needed) and has a slightly smaller footprint.

One filter design, the "classic" DIY box fan air cleaner design.

You can duct tape a single filter to the back of a box fan. In this case there is more strain on the fan motor, more air resistance and the air cleaning efficacy would not be as great. You would also have to replace the filter on the back of the fan more often. But this design has the advantage of having a low footprint and being the least expensive option.

Figure 7. A Corsi-Rosenthal cube featuring a Lasko 20" high velocity floor fan rather than a box fan. The fan's base was removed and a considerable amount of duct tape was used to seal the fan to the top of the cube.

Using a non-box fan (Figure 7)

With some effort (and potentially a lot of duct tape or custom woodworking), you can make a cube using a higher-CFM floor fan. The advantage of this design is that these floor fans put out a lot more CFM than box fans, so the unit will clean the air faster or be better in larger spaces. The downside is it's harder to construct, considerably louder and the cube is less physically stable. There is a guide to constructing a version of one of these using wood.

History

As discussed in his original blog post, the design for the Corsi-Rosenthal Cube was detailed by Jim Rosenthal, CEO of Tex-Air Filters, and was dedicated to Dr. Richard Corsi. Neil Comparetto also detailed a similar design in a popular YouTube video, which he called a "Comparetto Cube."

Room sizing & placement

Due to the DIY nature of the Corsi-Rosenthal box, exact figures on the proper room size for the box can't be predicted. However, engineers have taken measurements of various configurations of the box and reasonable approximations can be given.

Corsi-Rosenthal without fan shroud (typical box fan):

Likely delivers at least 5 equivalent air changes per hour in a 500 sqft room (8' ceilings). Likely at least 331 CFM CADR. Figures assume fan is on "high."

Corsi-Rosenthal with optimal fan shroud (typical box fan):

If an optimally-sized fan shroud is used (see instructions above), the cube may deliver around 5 equivalent air changes per hour in a 680 sqft room (8' ceilings). Likely at least 455 CFM CADR. Figures assume fan is on "high."

These figures are conservative. For sources for these approximate figures, see the scratch page here. Updates will be made to this page as more figures are obtained. Keep in mind these are approximations.

If using the Corsi-Rosenthal cube for COVID airborne risk mitigation, experts at Harvard and CU Boulder have created a downloadable calculator that can be used to help size air purifiers to a room. The "CADR" figures from above can be used as an approximation in this calculator.

The optimal place to put a Corsi-Rosenthal cube may be the center of a room, but the unit will work effectively as long as they are at least 3 feet away from a wall.

Power usage, noise & safety

As a DIY project, exact figures for power usage and noise can't be known. Based on readings with two popular fan box fan brands, the Corsi-Rosenthal Cube appears to use around the same amount of power as the underlying fans with no filters attached (e.g. between 58W and 88W on high in these readings).

The Corsi-Rosenthal box is around 51 dBA at 6', or slightly quieter than a typical refrigerator.

The EPA funded a study by Underwriters Laboratories (UL) which tested box fans with MERV-13 filters attached, finding them to not present a fire hazard even in extreme, modified conditions.

Governments that have suggested DIY box fan air filters include the San Francisco Department of Public Health and the state of Oregon (for wildfires).

News / media stories

Could a Janky, Jury-Rigged Air Purifier Help Fight Covid-19? - Wired article on a simple, earlier version of the DIY air cleaner which uses a single filter taped directly to the back of the fan.