A repository for documents related to Corsi-Rosenthal Box air filters in the SCA Kingdom of AEthelmearc

View the Project on GitHub jswelling/AE-Air-Filters


The goal here is to minimize the risk of COVID transmission at indoor events in AEthelmearc, while hopefully allowing as many activities as possible without masks. Inexpensive air filter boxes are believed to reduce transmission risk. We can never make indoor events as covid-safe as outdoor events where the air is replaced with fresh every few seconds, but using air filters can greatly reduce the air replacement time. As usual, the safest option is belt and suspenders- use filters in your indoor event site and encourage attendees to wear masks.

Making Filters

Full instructions for building these air filter boxes can be found at DIY box fan filters - Corsi-Rosenthal box. We are doing our planning based on single-fan, single-filter versions of the Corsi-Rosenthal box like this one. We chose this design because it seems to be most efficient in terms of clean air per dollar, and because it doesn’t have to be broken down for transportation between sites. Try to use a 2-inch-thick MERV-13 or MERV-14 filter with this design for best results. One filter box of this type can provide approximately 5 air changes per hour for a 20’x20’ room with 8’ ceilings.

If you have already made filter boxes with a different design, you don’t have to change them. A filter box in the ‘classic’ Corsi-Rosenthal design made with one fan and 4 filters apparently puts out only about 25% more air than the single filter version. That means you can use fewer of them to get the needed air filtration for your site, but it probably won’t save you money.

Transporting Filters

The filters should be good for several months of operation, long enough to serve at many events and practices. Handling the used filters is safe as long as you don’t lick them. We’ve learned that COVID is not generally spread by contact with surfaces, and that applies to the filter boxes. The easiest way to transport the single-filter units is just to put them in a 30 gallon trash bag and put it in your car. For planning purposes, one of those units is about 20”x21”x7”.

Filter Placement

Obviously if you place a single-filter unit so that either the filter or the fan is against a wall, you’ll restrict the airflow and defeat the purpose of the filter. The same thing can happen to a lesser degree if the filter or the fan is too close to a wall. Please place filter boxes so that the face of the filter and the fan are both at least 10” from a wall. It’s fine to put the side of a single-filter unit against the wall as long as both the input and the output sides have good clearance.

Likewise, if you use one of the cube-shaped 4-filter units, please do not put it under a table. We believe that you’ll end up creating a pocket of lovely highly filtered air underneath the table while the rest of the room air is not filtered.

Our calculations assume that the fans are set to ‘low’, because at higher settings the noise level can become a problem.