Monthly Book Giveaway - March 2023

Corsi-Rosenthal Air Filter Box

I decided that I really need to take better care of my lungs so I am going to take a few steps to improve the air quality in my shop.  I have been more consistently wearing dust masks when doing dusty operations and I may eventually look into a PAPR for a more comfortable experience but for now, I am going to concentrate on cleaning the air.  If nothing else, I hope this reduces the fine dust that pretty much ends up covering every surface in the shop.   My CNC and my Lathe are the biggest offenders and I definitely need to improve my dust collection but with those 2 machines in particular, there will always be dust in the air.   Except maybe during sanding operations, dust collection at the lathe is nearly impossible.  

The Problem
I have a small Wen Air filter hanging from the ceiling that does seem to help a little but is just too wimpy to really improve the air quality in my shop while I am working and definitely is not enough to capture a significant amount of the dust before it settles out of the air all over my shop.  I recently added a Minisplit to my shop and have added some filters to it to prevent the dust from fouling the coils.  Initial results with the minisplit filter seem to be good so far.  Almost no dust has found its way onto the internal screens or the coils.  

The Corsi Rosenthal Air Filter Box
After watching a couple of videos about the Corsi-Rosenthal Filter boxes where they simply tape 4  1-inch filters and a box fan together, it appears that this inexpensive approach may actually be just as effective as some of the air filters that typically hang from the ceiling in hobbyist’s shops.   Here is a video by one of the inventors of the C-R filter box introducing the filter for those want to reduce the risk of airborne viruses indoors.

In this video YouTuber The 3D Handyman takes a somewhat scientific approach to measuring the effectiveness.  This is what ultimately prompted me to try making one to improve the air quality in my shop.   

A screengrab from that video shows his airflow test results along with Wood Magazines results for 3 common units and one unit for inside the home..  

These tests indicate that for a little over $100, you can cobble together a simple air filter with 1” filters and a box fan that can filter nearly as much air as the Jet AFS 10008 air cleaner which goes for about $500 according to a quick Google search.   Note that the Jet replacement filters are much more expensive, especially when you consider the area of the filter which probably also means that they have to be replaced more often than 5 20x20 filters would.  

I did a little testing with an old square box fan that I have had for years by taping a 4” filter of unknown MERV rating (I am guessing MERV 8) to the back so that it sucks air through the filter.  One thing that was noted in several of the articles about the C-R filter boxes is that square box fans will actually suck  unfiltered air from the front near the edges when configured with the fan pulling the air through the filters and blowing outward.  I verified this phenomenon with an anemometer.  The anemometer actually reverses direction as you move it to the edges of the fan near the case.  I put tape around the perimeter of the blade to ensure that only filtered air was blowing out the front.

Most of the online examples simply make a cardboard cowling to fix this but this was just some testing I did while I waited for the filters I ordered to arrive. I will use a box fan that I already have that has a round cowling that does not have this issue, though I will have to close the gaps between the round cowling and the square filter box.   

Let’s Make a Filter Box
I decided to order 5 4x20x20 inch MERV 13 filters from ($112+tax) to make a unit similar to the Air King+5 Filters in the graph from the  screengrab above except with 4” instead of 1” filters.  I decided to go with 4” filters to improve the volume of air it can handle and to reduce the frequency of filter changes.  They arrived today and I duct taped the 5 filters together and placed the box fan with a round cowling on top and closed the gaps around the fan with some pieces of cardboard.

I chose to orient the fan with it blowing into the unit rather than outward as is usually done on most C-R demos I have seen.  I think that this has a few advantages over an orientation that blows outwards and pulls the air through the filters.  For one thing, the way this particular fan is made, it creates an easier way to mount the fan.  The diameter of the grayish body you can see above is a little larger than the filters so would be awkward to mount that way.  Also, the power/speed selector switch would be inside the filter box making it difficult to operate.   I also think that it will be better to have the dust trapped inside the filter box rather than collecting on the outside of the filters where bumping it or even moving air could dislodge some of the collected dust.  The other considerations are about air flow.  On one hand, having the fan blowing outward may help keep dust suspended longer so that the filter can capture it before it settles out of the air.  On the other hand, blowing outward might tend to stir any dust that has otherwise settled around the shop or saw dust that initially settled on the floor, benches or tools into the air where you might breathe it before it gets filtered.  In the winter time at least, having a fan blowing outward on high might make it feel cooler so might make it less comfortable to be in the shop when it is running.  I typically run a fan in the summer even when cooling with the AC so I will likely do that and get the potential benefit of keeping the more of the dust suspended until it can be filtered.  

Initial Testing
After quickly assembling a prototype filter box, it was time to see how well it works.  My shop is really dirty right now with dust covering nearly every horizontal surface.  I ran the filter for about 15 minutes and used an air quality meter to check the particulate levels.  The initial reading showed a PM2.5 level of 2 and the PM1 and PM10 levels were both  zero.  I put on a dust mask and used my leaf blower to stir up a bunch of the dust around the shop and took a reading.

Not entirely trusting the cheap meter, I actually bought 2 different types.  This one may ultimately go inside the house but after stirring up the dust, it went from a PM2.5 of 2 to 65.  It was place on opposite side of the filter from the other meter and only about 3 feet away from it.  

Looks like my new meters work pretty well.  I turned the fan on the filter box on high and left the shop.  After about 15 minutes I took another reading:


To prove that the filter box was actually responsible for these results, I turned the filter box off and found some new dust to disturb with my leaf blower and took another reading.  Looks like I hit the dust mother load. 

And about 15 minutes later  with the filter off the reading still shows an unhealthy air quality.  

I turned on the filter and took another reading about 15 minutes later.  I’d call this a success!  

Estimated Volume
I noticed that you can feel the air coming through the filter so I took some rough measurements of the outflow from the filters with the fan on maximum.  Holding the anemometer about 3 inches from the filter medium, it shows an outflow between about 120 and 200 feet per minute depending upon where you hold the meter.  The highest flow rates seemed to be areas farther from the fan.  Being conservative and assuming that each of the 5 filters has about an 18x18 square inch opening of exposed filter medium (1280 total) and an average  flow rate of 100 FPM, that yields 880 CFM.   With my 21x21x9 shop (not accounting for space taken up by the contents which are considerable), that means that the filter box can filter the entire volume of the air in my shop in under 5 minutes or 3+ times in 15.  Obviously, not every bit of air passes through the filter during that time but it obviously does effectively remove the worst of the suspended particles from the air near where the filter is.  Note that the meter was about 6 feet away from the filter box for all measurements.  Also note that if you use the lowest FPM rate that I measured of 120 FPM, that yields over 1000 CFM moving through the filter box.  

I also measured the outflow from my Wen Air filter and calculated an airflow of 200 CFM, a little under the airflow noted in the screengrab above.  Its outflow vent is only 6x4 inches so it is pretty easy to figure out how much air is coming out of it.   I changed its external (5 micron) filter a couple of days ago and the internal 1 micron filter is still pretty clean.     I suppose that I really should repeat my test for my Wen air filter to see how effective it is and to see if the seem to indicate that it is about 1/4 as effective, based upon my rough volume estimates.  If the DIY filter seems to work as I hope it will, I may eventually sell the Wen filter.   

 My goal is to make one  or two of these units that I can hang from the ceiling.  I may later try one that I can put on wheels to move next to whatever machine I am using to capture as much of the dust as possible near the source. I threw together a quick and dirty model in Sketchup to try out a few construction ideas.  

Thanks for following along.  I will add some pictures once I add some structure to the box so that it can be hung from the ceiling.  Feel free to share how you solved your shop air quality issues. 

Update: 01/08/23
I decided to test my Wen Air filter today using the same testing methodology to see if it was worth keeping.  Again I stirred up a bunch of dust with the leaf blower and turned on the Wen Filter. 


15 minutes later

Not quite as dramatic as the C-R filter box but still pretty good results.  I guess the Wen is not quite as wimpy as I thought it was.  It does a pretty good job with the PM2.5 levels but took longer to eliminate the PM1 and PM10 levels for some reason.  I didn't notice this until later so I may have to repeat the test and monitor it a little longer to see if this happens again and how long it actually takes to get them closer to zero.  


--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.

32 Replies

i love the smell of napalm  (sawdust)  in the morning !!!!

working with my hands is a joy,it gives me a sense of fulfillment,somthing so many seek and so few find.-SAM MALOOF.

Nice write up. I’d think it could be well implemented into a rolling table, maybe with three filters instead of four, but then you could put a tool on top of it to multitask the amount of room it takes up. 👍🏼 

Ryan/// ~sigh~ I blew up another bowl. Moke told me "I made the inside bigger than the outside".

Thanks for the info Lazy... I have a couple of air filters, so building my own is a no requirement, however, the air quality meters did get my attention.  Maybe with a meter, I might get that kick up my rrrs to actually turn my filters on.

What are they actually called and what obviously differentiates a good one from bad... other than exorbitant cost... Any hints a a good reliable one. 

If your first cut is too short... Take the second cut from the longer end... LBD

Thanks for the write up Nathan, some good information and the meters show the results. Sounds like that may be my ticket for a air cleaner. I have been thinking about using a box fan or two with the filters attached. When I built the shop I installed a few switched outlets in the ceiling just for that purpose.

I was thinking more in the lines of a stack of filters, so one could replace one and rotate the others out. At least that was the thought.

Main Street to the Mountains

Nice write-up, Nathan! I’ve been getting cranky with my ceiling-mounted Rikon filter. They strongly dis-recommend modifying it to be always on, because (according to the tech support person I talked to) if the filters get clogged, the unit will shut down rather than overheating the motor. But I really want a ceiling-mounted unit I can hardwire to the same circuit as the shop lights so it’s always on when I’m working.

Also, the cost of the replacement filters for the Rikon is pretty high. I think I’ve got five filters left to “use up” before I retire it for something like what you’re playing with.
Great write up Nathan.  I’ll have to watch the video later.
Eric, Testing results in the video above indicate that you need at least a 4" filter if you are going to attach a single filter to the fan; otherwise, you just do not get enough airflow for it to do much good except maybe right next to where you are sanding for example.  Stacking the filters does not really work unless maybe you stack something like a MERV 8 in front of a MERV 11 but it mostly just acts to restrict the air flow and does not increase how much of the finest dust it can collect.  I think it would just serve to prolong the life of the MERV 11 filter.  You are better off  using a 4" filter with a higher MERV rating because it increases the surface area so you do not choke off the airflow.   Reducing the outflow by adding a round cowling on the front so that it doesn't suck in unfiltered air from the front should also increase the amount of air that it pulls through the filter without reducing the net air being moved out the front.  Another thing mentioned is that you need at least a MERV 11 filter for it to capture the most dangerous particles but moving up to a 13 or 14  is sort of the sweet spot for cost and effectiveness.  MERV 15 is even more effective but the price jumps significantly for those.  One 4" MERV 15 filter can cost $60-100 compared to a little over $100  I paid for 5 MERV 13 Filters.

BTW, one thing that was shown in a couple articles I read indicates that in a 4 or 5 filter setup, even using 1 inch filters, because the multi filter setup doesn't restrict the air flow by much, it will cycle the air through the filters more often so even  MERV 11 filters will eventually remove  much of the below  5 micron dust from the air.  If you can filter the equivalent of the entire volume of air every 5 minutes for example, even a filter that removes lower percentages of particles below say 5 microns will eventually get most of them by just sending the the air through the filter so many times.  So with a MERV 11 filters for example, they should remove between 65 and 79% of the 1 to 3 micron particles in each pass.  If you can process the entire volume of your shop air 12 times in an hour for example, you can theoretically capture up to 99% (65% ^ 12) of those particles in an hour. 


--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.

What are they actually called and what obviously differentiates a good one from bad... other than exorbitant cost... Any hints a a good reliable one.  

Other than it working as it is supposed to I cannot really say what differentiates a good one from bad.   Dylos is one brand that I have seen in the past that was always held up as low end professional quality meter but they are pretty expensive and the reviews seemed kind of mixed to me.  I searched for "air quality monitor" to find others.   The meter above with the color display is the same model that the guy in the second video above used.  He seemed to think it worked well so I ordered that one. It has pretty good reviews.   

The other one I found by reading reviews and is made by Temtop who has several other versions including handheld ones.  I chose it because it seems to have decent reviews and selected the version that is WiFi enabled so I can look at the history in an app (on iPhone at least).  I think that you can also export the data for further analysis but have not tried that yet.  Below is a screen shot of the app during the above tests yesterday.  The first hump in the graph is the first test where I stirred up some dust and the filter brought the counts down by the time I checked on it 15 minutes later.  The second hump is where I stirred up the dust mother load and the graph shows that it gradually dropped for the first 15 minutes.  You can see where the PM2.5 count drops dramatically when I switched the air filter back on.    This one does not report PM1 or PM10 levels as the other one does. 

I believe that Temtop also makes a version that you can download the historical data from.   Both of these meters are powered by USB cables and have a battery so that they can operate for a little while without being plugged in.  I considered an Amazon branded monitor that is supposedly Alexa enabled but the reviews were sort of meh and it doesn't have a display so you have to rely on Amazon and Alexa to see what is going on.  A common complaint is that without an Echo Show you cannot see details.  


--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.

BTW, I decided to run the same test with my Wen filter.  I updated the OP with the results.  

--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.

 Dave Polaschek
Nice write-up, Nathan! I’ve been getting cranky with my ceiling-mounted Rikon filter. They strongly dis-recommend modifying it to be always on, because (according to the tech support person I talked to) if the filters get clogged, the unit will shut down rather than overheating the motor. But I really want a ceiling-mounted unit I can hardwire to the same circuit as the shop lights so it’s always on when I’m working. 

 In hindsight, I wish that I had ordered 16x25" filters instead of the the 20x20 ones.  That would give it a lower profile hanging from the ceiling with the same surface area but would require a smaller diameter fan.  Nice thing about using a box fan or one of the barrel fans the guy in the second video uses is that they would be easy to wire so that they come on with your shop lighting.  I may experiment at some point with some sort of manometer to help me see when it is time to replace the filters.     

One thing that I do not like about my Wen filter is how loud it is, probably at least partially because it is sending its outbound 200+ CFM through a 6x4" port.   This is one reason that I sometimes do not run it or turn it off.  It can get a little annoying, especially if I am listening to music while out in the shop.   My box fan seems to run quieter.  That may also be in part because I have it configured blowing into the filter box which may dampen the sound a few dB.  If the noise level of the DIY filters is not as annoying, I may just wire it to come on with the lights as well.    

--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.

A lot of good information here.

I started looking into going down this road, for the house, a couple years ago.  It started with building a new filter box so I could use three, instead of one.  Too, because my house HVAC uses 5" filters, I was going to stay with them.

The design grew from there. It uses allthread and jig knobs to allow me to use thinner filters, or to add screens or cheap glass filters to filter out kids, cats, bricks and what have you. As well, I could, if desired, add a charcoal filter.

Then the design schemes included a portable version, like the ones in this  post. It would, like my HVAC, rely on a three or four speed squirrel cage. It would use at least four 5" filters and would scrub a lot more of what we have to dust for out of the air. 

My HVAC was set up on a single speed, but I bought a stacked switch, jumpered the input terminals, installed it on the HVAC, then wired it in, to allow me to drop or raise the speed.

Though many may not know it, nearly every furnace or HVAC squirrel cage is capable of three or four speeds.

Another great add to these is, an manometer or a Magnahelic gauge by which the filter condition can be monitored.  This is a great help on my home system and my shop dust collectors.

The home HVAC uses a 1" Magnahelic gauge, which taps into the airflow between the filter and the fan. The more clogged the filter(s) become, the higher the gauge reads.  By noting the reading when a new filter is added, its condition can be tracked.

     NOTE: Merely that two companies sell filters with the same MERV rating does not mean the amount of air they allow to pass will be anywhere near the same.

My 3 horse dust collectors required a Magnahelic gauge with a much higher capability, since it would peg the needle installing it at the intake port.  

An alternative to overcoming pegging the needle is, use a valve that allows you do damp down how much air is drawn.  
This would be a MUST, if using a manometer on a dust collector, because if would empty the gauge before you could pull back from the install.

Kelly, Any recommendations on a cheap manometer.  I was thinking about just experimenting with a DIY version with some tubing and colored water.  

--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.

Thanks Lazy...

If your first cut is too short... Take the second cut from the longer end... LBD

Lazyman, I bought two from EBay. The first, for the house, was the 1" and perfect for the task. The one for the collector was 15".  The 15 ran only $20.00.

A hose and liquid would work great too. As I note, you'd have to all but shut off the line. For starters, hot glue and a fine drill bit?
Great report Nathan - Thanks!  I used an HVAC blower coupled with a filter I purchased from Grizzly .  In your research did you come across anything about using HVAC blowers?  I've purchased a particulate meter so I'm going to test the effectiveness of the Grizzly filter.

Again, thanks for taking the time to make this report.
 replied 2 days ago
Great report Nathan - Thanks!  I used an HVAC blower coupled with a filter I purchased from Grizzly .  In your research did you come across anything about using HVAC blowers?  I've purchased a particulate meter so I'm going to test the effectiveness of the Grizzly filter.

Again, thanks for taking the time to make this report.

I saw some examples of filter boxes where they used HVAC (squirrel cage) blowers but none that did any sort of analysis about how quickly or well they filtered the air.  The closest that I have seen is from YouTuber Jay Bates where he made a cart using HVAC filters and and an HVAC blower.  He was happy with the performance.  I went back and forth about using an HVAC blower but when I saw the simplicity of the C-R filter approach where you can literally buy everything you need for under $100 at a Home Depot, I decided to finally give this a try.  One nice thing about it is that it will be much lighter so will be easier to hang from the ceiling than one that requires a plywood box.  

As far as the Grizzly filter is concerned,curious about how its surface area compares to a 4" MERV 13 or 14 HVAC filter.   The 4"  MERV 13 filters I bought from FilterBuy have about 2800 square inches per filter and I will have at least 4 of them one I make one that I can hang from the ceiling.   It would be nice if Grizzly or the other similar brands listed a MERV rating.  They just call the inner filter a 1 micron filter but do not say how efficient or effective it is at filtering 1 micron particles.  If you look at the MERV chart I posted above, a MERV 11 filter technically filters some 1 micron particles, just not very well.   It would be nice to find out what what the Grizzly's MERV rating is.  As you can see from my update above, my Wen actually work fairly well and uses a filter that is similar but smaller than the Grizzly one  so I would expect your results will be good as well.  Let us know what you find with your tests.  

--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.

I've been using this one for about a year . It works. I don't have facts and figures, but it works well and didn't cost much. It weighs just a few pounds, easy to move.

It's always on when I'm working.

Jump back, Jack! See ya later,alligator! James Brown

Very nice. I've got a small cart I built that holds 3x 12x24x2" filters with a small 4"x4" output squirrel cage fan. It doesn't move as much air as a box fan, but makes a fair wire wheel/power strop (buffer) stand. 

This summer I built one for a single 24"x24" which used a salvaged 3 speed blower from a dehumidifier. That one has a 6" duct outlet so I can use one of the portable AC vent mounts in the garage door as an exhaust (mostly for a spray painting or soldering fume extraction). 

One key I've found is to make sure you're running it before you start making dust and it keeps up much better. I also often run the fan on my furnace to help as well. I hate dust. I have a PSI dual fan ceiling mounted air cleaner too. I think those work more efficiently per CFM because of the air circulation, when properly installed

 I received the Temtop M10 air meter yesterday and was able to do some testing today.  First a few details of my air filter setup.

It is located on a shelf 8' high in my shop/garage.  The first filter you see in the pic above is a washable filter that I do not know the Merv rating for but I'm sure it is very low.  The second filter is a Grizzly 1 micron filter for their W1690 air filter system and rated for airflow of around 1,200 cfm.  (The reason I say around is on Grizzly's site it is hard to find the same rating twice.)

The surface area is approximately 7,560 square inches or 52.5 sq. ft  There are 6 surfaces measuring approximately 6" X14" X 15".  The blower is small.  I believe it came out of a house about 1,000 sq. ft. in size.  A 1.5 Ton HVAC unit (according to Mr. Google) will generate 600 cfm and a 2 Ton unit will generate 800 cfm.   Without disassembling the box it is hard to get an accurate measurement, but I estimate the blower is 8"X 8".  I have limited Google kung fu powers and could not find anything about how many cfm that size blower would generate.

On to test results.  

(Disclaimers: First. there has been virtually no shop activity since my last shop cleanup.  I must state that either I'm a better shop cleaner than I ever would have imagined or I'm terrible about stirring up dust.  I would go with the latter.)

First I measured the PM 2.5 levels in my undisturbed shop. It was between 1 & 2 as it kept going back and forth.  Next I turned on my air filter and used my leaf blower to stir things up. I set the Temtop meter on top of a ladder as you can see in the first pic.  The reading was 25.  I then moved it to the air outflow side of the blower and positioned it similarly to how it was positioned on the inflow side.  The reading was 3.  Dust must settle fairly quickly as about 10 minutes after the initial reading the PM 2.5 reading was 10 at the intake side.  I re-positioned the meter to the top of my workbench and got the same reading, 10.

So there are my initial results.  I hope to study this further when I have some more time.

Chris, when did your shop get so full?!!?  Last pic I remember seeing you had like 5 pieces of equipment in it.  You haven't become a collector have you??   ;-)