Thread by @Poppendieck

Are you opening windows and using fans to move air into or out of a room? 1) Try to isolate the fan so it doesn't capture air it just blew. 2) Open a second door or window so the air has somewhere to go/come from other than cause condensation in walls (mold). 1/6

Typical $20 box fans claim to move 2,000 cfm (cubic feet per minute) on a high setting. A typical class room around 8,000 cubic feet. This equates to an air change rate of about 15 per hour, or taking 12 minutes for 95% of the original air in a classroom to leave. 2/6

The high setting is rather loud and flows are optimistic. @JohnSemmelhack showed yesterday numbers from 1200-1400 cfm with no filter. If we assume the flow (no filter) is about 1,000 cfm then the air change rate is 7 per hour, or about 25 min to change 95% of class air. 3/6

These numbers assume that you have a good seal around the fan and it is not drawing in air that it has expelled. This is a large amount of air going into/leaving a classroom. It is unlikely an HVAC system will be able to control moisture or temperature in the classroom 4/6

In summary, a box fan can reduce particle concentrations in classrooms on the order of 30 minutes by themselves (no portable air filters or HVAC removal). They move so much air that moisture and mold could be issues in certain climates. 5/6

Using box fans to increase ventilation can be a part of layered risk reduction approach (masks, portable air filters, HVAC filtration, fewer students and teachers) in classrooms and but should used with care as temperatures change. 6/6