DIY Air Filtration - The Corsi-Rosenthal Box

[majorhavoc]

A cheap 20" box fan, four HEPA (MERV 13 equivalent) filter panels and some cardboard and duct tape.  Seems a little too simple to be true, but I've done a little research and there is real scientific providence to this design.  Gathering the materials and construction details would be a worthwhile prep for the next pandemic, heavy air particulate disaster (wildfires, building collapse, etc).

https://www.npr.org/sections/back-to-school-live-updates/2021/08/26/1031018250/does-your-kids-classroom-need-an-air-purifier-heres-how-you-can-make-one-yoursel
https://www.wired.com/story/could-a-janky-jury-rigged-air-purifier-help-fight-covid-19/
https://www.wgbh.org/news/local-news/2021/08/17/diy-air-filters-for-classrooms-experts-are-enthusiastic-and-a-citizen-scientist-makes-it-easy
https://edgecollective.io/airbox/

[SCBrian]

I like the thought process, especially for environmental contaminants.  As for virus particles ... It's better than nothing?  wiring in a UV light inside the box might help, as well as moving to a higher merv filter. In this design using a HEPA etc, might be doable  even with premature death on the fan since they are so cheap compared to a home AC unit.  The caffeine hasn't kicked in yet, and I haven't googled back far enough back, but something like this may have been the grandfather of the commercial air purifiers we see at market today. 
*Might* be able to do a small desk top one for a pretty cheap price with a 4" computer fan & a Hepa filter cut into pieces.  small UV zone inside...

If nothing else, the placebo effect will help...

[Raptor]

That is a brilliant McGyver type device. There is no reason it should not be effective.

I have also seen a version of this with an air filter duct taped front and back to the fan.

The key is to use a high enough merv air filter that can address the particles you need filtered.

[SCBrian]

Read through the linked sources and it made me giggle.  Depending on the source, cost goes from $50 - $200.  and a number of the articles really are reporting a fact I wish they wouldn't:

"People are now reporting 600 cubic feet per minute in clean air delivery rates. That's phenomenal. That's actually better than a lot of the more expensive HEPA-based portable air cleaners," he said.

The fact is not a good one to report because it lacks context and the professor should have made it clear.  I can move 1800cfm of air by removing the filters.  But the air Isn't the same quality as the lower output HEPA filtered air.   (Hope that makes sense)

[RonnyRonin]

When we've been having bad wildfire smoke this summer and last my wife taped up two home furnace filters into one window and pointed a fan out the window at the other end of our apartment to try to move some cool air in the mornings; worked far better than I would have guessed so this makes a lot of sense to me. 

[flybynight]

I am not a HVAC technician. But would it not be more cost effective to just duct tape a single filter to the back of the fan?

[sheddi]

I am not a HVAC technician. But would it not be more cost effective to just duct tape a single filter to the back of the fan?

I think the problem there is that the pressure drop across a filter is a function of the flow through it, and a simple box fan can't develop much "suck". Using four filters in a box arrangement means a lower pressure drop and a better flow, plus less strain on the fan so a longer life.

Obviously European filters are rated differently but looking at these two tables I think MERV-13 is roughly on a par with EN779 grade M6. I think ...
https://www.iso-aire.com/blog/what-are-the-differences-between-a-merv-13-and-a-hepa-filter
https://www.spcb.co.uk/guides/air-filter-classification-and-grades

[Raptor]

Two things to keep in mind.
1.)This is not an all or nothing proposition. While a true HEPPA will likely perform better sometimes good enough is better than nothing.

2.) The air filter used has a huge impact on its performance. If you uses a simple fiber glass filter your results will not be as effective as a 3m 1900 air filter.


BTW this is a great video that tests various "furnace filters" a.k.a. HVAC air filters.

http://www.youtube.com/watch?v=RkjRKIRva58&ab_channel=ProjectFarm

[Brekar]

Two things to keep in mind.
1.)This is not an all or nothing proposition. While a true HEPPA will likely perform better sometimes good enough is better than nothing.

2.) The air filter used has a huge impact on its performance. If you uses a simple fiber glass filter your results will not be as effective as a 3m 1900 air filter.

What Raptor says is exactly right. The better the filter used, the better it'll work, up to a point. If you get something to restrictive, you'll burn up the fan instead, not good enough, it won't capture much. My wife has done something similar for years now and it always helps.
Take a box fan and zip tie a furnace filter with a 1900 rating or higher and it seems to help with my kids asthma, pet dander and dust. When we were pulling all the 20 year old carpet up in the entire house, you would be surprised how nasty and dirty the filters got after we put a filtered fan in each room.