Quiet, relaxing air filtration for a hand tool shop. With backup capacity.

[Scott Winners]

I have been fooling with this all winter, seems to be working good.

I built a Corsi-Rosenthal box, September-ish, using 4 20x20 furnace filters and have been running it in my shop area with power tools or significant hand sawing. This one: https://sawmillcreek.org/showthread.php?293986-Shop-air-quality-on-a-budget-projects-one-and-two&highlight=

Most of the time I carry my lunchbox planer out to the driveway, but when it is really too cold, I have been known to operate my lunch box with no dust collector, inside my two car garage sized shop. Running the 4 furnace filter box with the fan on high, I can get the air quality (PM2.5 is usually the controlling variable) back down to moderate in about three minutes after I turn off the planer, and AQ back down to good within ten minutes. I have a couple different air quality monitors, and it sure is nice to know when I can take my respirator off.

I have a similar box I am running in the main living area of the house with the woodstove, and a third one in service at my wife's office where they have a mold problem in the central HVAC duct work. At this juncture I am going to put in a plug for MERV 13 level filtration as adequate for baseline healthy people without underlying respiratory issues. There are folks who need, or would need, higher level filtration to be around wood dust. At team orange home store, you are looking for team orange rating system "level 10," a filtration level that meets MERV 13. From 3M, both the 3M 1900 and 3M 2200 filters meet the HVAC industry spec for MERV 13.

20x20x1 inch nominal 3M 2200 filters from amazon a dozen at a time are not too terribly expensive. But it is the top end for a reasonably priced filter. You can go to MERV 14, or a 2" thick filter with more pleats, or go all the way up to HEPA level filtration, but all those are a lot more expensive than good old MERV 13. A MERV 13 filter will capture 90%of bacteria sized particles, first pass, and 75% of virus sized particles, first pass. I find them more than adequate, and my AQ monitors tell me they are getting the job done on mold particles, wood smoke and wood dust.

If you build something like this thing I am building, and it isn't enough improvement to air quality, you can buy more expensive filters in the 20x20 form factor and fit them in this thing I am building.

I am very much starting from this youtube video by user "3D handyman."

https://www.youtube.com/watch?v=fixd7LqnWow

The difference is I would personally never use a powered hand sander like that indoors, and just the 3000cfm fan would get my chakras all out of alignment. I don't want to be around anything that noisy when I am trying to relax. So I am sticking a bog standard Lasko 20" box fan on top of mine, good for 1000cfm on high speed, but soothing white noise at lower speeds.

First pic is the dirty filters on the C-R box I built in September. Second is my exterior pieces done, 4 uprights, glued up "L" shapes from 1x2 furring strip, and 2 pieces of 23x23 plywood for the top and bottom. Third pic is using a saw with a nib as a compass. I found the center of my top, nailed through the center into my bench, hooked the nib on the finish nail, used a tape to find the notch in the saw plate that matched the radius, get a pencil in there, spin the plywood on the nail under the pencil. Beautiful and done.

Each vertical will end up with two openings 20" tall. I cut my L's to 56" (arbitrary) to make sure there were no knots in the glue up area at each end. So I am going to need four pieces of plywood 16" tall and 20" wide to fill in the gap in each vertical. I used a bunch of glue (TB2) for making the Ls, I want airtight joints so I was looking for good squeeze out all around. Cleaning up the inside corners of excess glue was a bit of wear and tear on my shoulder plane, but I don't expect to spend eternity in a lake of fire for having done it.

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This build should go fairly quickly. I have built three of these now, this one won't be as nice as the one in my wife's office, and I have all the materials on hand. Right now I have to cook dinner, but after dinner I expect to get furring strip glued to the inside surface of the top and bottom so that can dry overnight. Tomorrow I am on call, but I should be able to get the outside frame glued up.

[Scott Winners]

I do agree with youtube user and inpire-er 3D handyman that 4 rotating and zero fixed casters under one of these is a good idea.

[Scott Winners]

I have used a fair number of clamps here while dinner was in the smoker. If you build one of these, keep the offcuts when you are trimming your Ls to length. I am currently working with 5 offcuts, ought to be plenty. One, I used the off cuts to calculate the length of my horizontals, used them for mock up, and then used them again for the actual glue up, pics 1-2-3. These joints do not need to be airtight, but use plenty of glue with decent squeeze out. Air tightness on the horizontals will be provided by duct tape between frame and filter here in a few days, but these bits are structural and need a good glue joint.

You could lay out all these joints to the nearest Angstrom and make careful knife marks, but there is no point. The furring strips at nominal one inch are much closer to 3/4 actual. The filters (I have been dealing mostly with 3M 1900 and 2200 this winter) are going to be 19 3/4 or 19 15/16 (not square) by a bit less than 1"nominal. I will pull a stack of six 3M filters after dinner and post a pic.

I do have the L's pointed all in the same direction around the clock. You could have a wide side and a narrow side if you like.

[Scott Winners]

FWIW I came up with 20 3/4 inch furring strips for my horizontals.

[Scott Winners]

Tonight I went ahead and glued the case together. The nominal length, 56", is longer than my 48" bench. There was nothing for it, it needed to be done. I ended up hanging the top and bottom panels off the end of my bench, but starting from dry I had 2 of the L's on my benchtop, with the top and bottom panels resting on them so it didn't all go tumbling in the heat of battle.

One pic is dry clamp up with hand spring clamps, glue bottle still closed. Other (first) is after glue up. Once I had it tacked pretty darn close -with glue and hand spring clamps- then I used the ratchet straps to pull everything together. It was maybe a little fussy, pulling each handspring clamp put a little slack in two of the ratchet straps every time, but not too much cussing and not that much patience really. Now all I have to do is glue stuff to the case once this glue is cured.


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I cannot probably express how important I think it is to do a dry clamp up before the glue bottle is open. If it is a common thing you have done a few dozen times before not so much, but with new problems I get a lot of confidence from knowing my clamping plan will work because I just did a dry clamp up assembly and it worked out fine.

For the money, I think my ratchet straps are the best value clamp in my shop. I haven't had to buy new one's lately, but a set of four in the 800-1000# rating range each for a set of four, not that long ago, was $20-25 at any sporting goods store.

[Scott Winners]

Here is a stack of 6 3M 2200s (20x20x1 nominal) still in the bulk wrapper, I come up with 5 1/16 (5.0625") for the stack, 0.84375 each, a few thou under 7/8 actual.

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[Scott Winners]

And I had a brilliant idea to change my design mid assembly. This happens to me a lot, I am thinking about mentioning 'changing design mid project' as the most hated tool in my shop. I am going to mix some 1x3 in with my 1x2 furring strip. I bought a bunch of them today, and have them stickered stacked and weighted for seven days.

While I am waiting on the 1x3 to show me how it is going to move, I did get a pair of diagonals in tonight. They start at the same lower corner, but spread to opposite upper corners to give me some stiffness. I think this will be enough diagonal, because each of the four (56") faces will have two filters at 20" tall, and 16" of plywood that can be glued on four sides to add even more stiffness.

My goals here are 1)effective air filtration 2) quiet 3) easy to build and 4) inexpensive. I am sure someone with a powered mortiser and a tablesaw could make a much more complex version of the same thing.

I have a metric fun ton of charting to do tonight, and I am looking at least 14 hours on the clock tomorrow, not whining, but updates will be somewhat time constrained for the next few days.

For the bevel angle, just cut the piece a couple inches too long, and then position it as best you can. Eyeball the angle with a pencil. Cut it. Put the diagonal stock back in the box and check again. When you are in the ball park, set your bevel gauge to what you got and trim the piece a bit at a time, quarter inch is probably safe if you started two inches over finished length, and possibly dial in the angle on your bevel gauge as the stock gets shorter and closer to correct length.

[Phil Cyr]

From time to time I think about something nicer, but as it stands my 20 inch box fan with a 20 inch filter does a good job. I love when the light shines in the shop and the dust particles sparkle on their way straight from the board to the filter.
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[Scott Winners]

Nice work Phil. The FPR7 stamp on your filter is from the team orange filter rating system, that scores MERV 11 on the HVAC industry (ASHRAE) standard. If it is working for you, that is great; and I personally am happy to see you doing something proactive about your respiratory health.

There is an article here: https://www.hvac.com/air-quality/merv-13-vs-merv-11-vs-merv-8/ about the relative qualities of MERV 8, 11 and 13.

I personally don't worry too much about the dust I can see, the insidious trouble is the particles so small I can't even see them.

Can I ask how long you have been running that same fan with however many single filters taped to it? I am very curious to know. The thing I am building with multiple filters should present less air flow resistance to the fan, so my fan should last "longer"by running "a little" cooler, and while I won't have to change my filters as often as you do, when I do have to change them there will be more of them to change. Besides our relative filter cost (a wash), and fan life (probably not significant), the other thing my system should be able to do is capture dust particles near the level of my nose without waiting for those particles to settle to the level of your single filter.

FWIW I am running a similar fan with three filters (and two sheets of cardboard) upstairs in my house with our wood stove and geriatric (long haired) cat. I am going to have to replace all three filters some time this summer, but i am already in the shoulder season of decreasing wood stove use and increasing pollen counts without having had to make time to change the filters on the upstairs unit. In my time limited existence it makes sense to change three filters all at once one time per year, rather than one filter three times per year.

I am going to PM Alan and David, and ask them to keep an eye on this thread. Phil's system is likely to keep him healthier longer and older compared to someone doing the same work with no air filtration at all. Also Phil's system is way more economical than the ceiling mount air filters from the usual suspects that use small (relatively) expensive filters. And it does work. Phil's system does work, and does improve overall air quality, but (my opinion) the MERV 11 filter is leaving small particles in the air that could be filtered out economically.

At the end of our various careers, the price of filters is a LOT more than the price of the fan and housing. 20x20 nominal are the cheap filters, and they come in lots of specifications. Sort of like the price of LPs compared to the price of a good turntable if you are as old as me.

[Phil Cyr]

I have had this setup about a year, though I don't get a lot of shop time. The fan probably has less than 50 hours on it. I would have made a multi filter setup like you build, but my shop right now is very small. It is only about 50 square feet. I wanted something small, fast, and quick. When I buy more filters I'll likely buy the better ones. I remember buying this while there were some wild fires burning so the selection was slim.

Regarding the filter height - ours are equivalent there. My bench is only about 8 inches tall. I am working on designing bench number two. I plan to increase the height maybe to about 16 inches.

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Edit: Thanks for pointing out that I should improve the filters I use. This thread is also a reminder that that filter is pretty dirty, time for a new one!

[chris carter]

I also have the 20” box fan with a filter. I use the 3M 2200 filters (MERV13). They cost me nothing as I take them from the house’s central air because I don’t care if I burn out the motor on a $20 fan (as opposed to the blower in the house AC!). That said, this fan has about a billion hours on it and is still going strong. That’s a bicycle tube holding it on. I run it on low 99% of the time, but if I have to use power tools or if I’m doing a lot of sawing I’ll crank it up temporarily. It’s very efficient: I once cut a bunch of wood on the table saw and forgot to turn it on. I could barely see. I turned it on and it cleared the basement in about 2 minutes flat. I have compromised lungs from the previous pandemic (H1N1) and asthma so I have to be really careful about air quality.

As for a box configuration with multiple filters, I don’t think it will work any better than a single filter if you are using a box fan. The reason for a box with four filters is so you can run a more powerful fan than a 20” box fan. If you run that many filters with a 20” box fan all you are really doing is making it so you don’t have to change the filters as frequently.

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[Richard Coers]

Why do you need dust filtration in a hand tool shop? Pretty tough to get scraper curls in your lungs.

[Scott Winners]

this fan has about a billion hours on it and is still going strong. That’s a bicycle tube holding it on.

Thank you for that data point. In theory more filters, that is to say more filter area, will provide less airflow resistance to the fan motor. I have had a hunch for a while the fan is not going to run enough cooler to make a meaningful difference, and here you are proving it.


Why do you need dust filtration in a hand tool shop? Pretty tough to get scraper curls in your lungs.

I have been making a little Susie Homemaker this winter as my garage/shop area is attached to the living area of the house. Among hand tools, at my place, it is sawing that makes the most airborne tiny dust particles that float around a while and settle later. Sawing is an extremely violent process at the cellular or similar microscopic level. Certainly chainsaws and radial arm saws make the big visible plumes. A 14 point carcass saw does the same work, makes the same dust particles, just not as many as fast.

I am not concerned with chips I can see. They fall to the ground pretty fast, and if I do breathe them in I got nose hairs for that. It is the little tiny floating ones that evade the nose hair filter and lodge in the lungs.

My filter housing could hold 8 furnace filters at 20x20. I am only installing six filters to start, with two openings on the upper level covered with plywood blanks, but all four possible filters installed at the lower level. When I need to, I will leave the unit running over night, but most of the intake air will be from near the floor, so as the little tiny floaters are settling out, the filter can still snag them. I am going to park the item at the end of my bench just past the trash can. So when I am whisk brooming my benchtop I will start at the tail vise end of the bench, sweep past the leg vise, off the end of the bench - where the heavies can drop into the trashcan and the light floaties can waft right over the trashcan to a receiving filter.

The plan anyway, I won't know if it works for about another week.

[Richard Coers]

Thank you for that data point. In theory more filters, that is to say more filter area, will provide less airflow resistance to the fan motor. I have had a hunch for a while the fan is not going to run enough cooler to make a meaningful difference, and here you are proving it.



I have been making a little Susie Homemaker this winter as my garage/shop area is attached to the living area of the house. Among hand tools, at my place, it is sawing that makes the most airborne tiny dust particles that float around a while and settle later. Sawing is an extremely violent process at the cellular or similar microscopic level. Certainly chainsaws and radial arm saws make the big visible plumes. A 14 point carcass saw does the same work, makes the same dust particles, just not as many as fast.

I am not concerned with chips I can see. They fall to the ground pretty fast, and if I do breathe them in I got nose hairs for that. It is the little tiny floating ones that evade the nose hair filter and lodge in the lungs.

My filter housing could hold 8 furnace filters at 20x20. I am only installing six filters to start, with two openings on the upper level covered with plywood blanks, but all four possible filters installed at the lower level. When I need to, I will leave the unit running over night, but most of the intake air will be from near the floor, so as the little tiny floaters are settling out, the filter can still snag them. I am going to park the item at the end of my bench just past the trash can. So when I am whisk brooming my benchtop I will start at the tail vise end of the bench, sweep past the leg vise, off the end of the bench - where the heavies can drop into the trashcan and the light floaties can waft right over the trashcan to a receiving filter.

The plan anyway, I won't know if it works for about another week.
I'm well aware of what fine dust off of machinery and powered sanding can do, I thought you said you had a hand tool shop.

[Scott Winners]

I'm well aware of what fine dust off of machinery and powered sanding can do, I thought you said you had a hand tool shop.

And in the quoted text of mine, I pointed out a 14 point carcass saw does the same work and makes the same dust, just not as fast.

However, this is a fair question. I can get my saws sharper than new saws on the shelf pretty easy, but I would not be at all surprised if my saw sharpening skills aren't that great compared to average around here. I can say I have had to resharpen my Veritas 14 tpi crosscut carcass saw and Veritas 14tpi rip dovetail saw and more or less restored them to how well they worked when factory new. Ron Boontz I am not, but I am running neck and neck with Rob Lee's staff.

So it could be that Richard's saws are sharper than mine, and he makes less dust than I do for the same work done.

But, the little particles I am concerned about cause cumulative damage. Breathing in a teaspoon daily is better than breathing in a handful daily, but the ongoing damage is cumulative, so breathing in none is best. We can hang a name on these and call them particles at 2.5 microns diameter and smaller, aka PM2.5. These suckers are so small you can breathe them in and they can pass into your bloodstream.

Here is a reasonable introduction with footnotes: https://molekule.science/what-is-pm-2-5-and-how-can-you-reduce-your-exposure/ this website does want to sell you a portable airfilter, but the particle facts quoted are pretty well known in academic Air Quality circles.

And here is another look at how the various MERV ratings work on the little particles: https://www.greenbuildingadvisor.com/article/understanding-filter-ratings-merv-fpr-and-mpr

At the end of the day, the sooner you filter your shop's air, the less cumulative lung damage you will get, lifetime. I am running a unit similar to this upstairs, 3 MERV 13s with one 20" Lasko box fan. One day when my wife wasn't home I intentionally opened the loading door of my woodstove and let the Air Quality in the stove room get to hazardous from smoke rollout, I think I got to 365 micrograms per cubic meter, closed the door, turned the fan up to high and had the indoor AQ back down to good within 90 minutes in 1200sqft. It was a relief both to take off my respirator, and have the AQ restored before my wife got home.

If you live somewhere that doesn't get wild fires/ forest fires, but is seems like the wild fires are getting closer to your home every couple years - suddenly having the particle capacity of six filters on one fan in one housing might be more attractive. Also, 3M packages the Filtrete 1900 and 2200 in 6 packs, they ship in one box.

We do have severe air quality issues in Fairbanks and I will readily acceed this is one reason I have perhaps gone overboard. In the wintertime the city is prone to inversion layers, similar to Los Angeles smog. All the emissions from everything gets trapped under the inversion layer. Mostly smoke from woodstoves, but all the vehicles, all the power plants, all of everything sticking around. I think we hit about 185-190 mcg/m3 as a peak for the winter coming to an end, summer 2021 we did hit 350+ mcg/m3 on PM2.5 from forest fire smoke rolling in to town. Fairbanks is ranked in the top ten for worst air quality in the USA year after year, usually #1 or #2. A couple days each year we have worse air quality than Beijing, China does on the same day. I expect in the next ten years or so someone with academic credentials and data will be able to say living in Fairbanks is the same as smoking, mmm, possibly half a pack of cigarettes every day.

On my 3 filter system upstairs, with the wood stove running all winter, combustion air for the stove taken from the conditioned living space and outdoor PM2.5 counts ranging from 0-200 mcg/m3, I routinely observe, indoors, 0-6 mcg/m3 PM2.5 with the wood stove running, and 10-15 mcg/m3 while cooking. Even when the outdoor AQ gets to unhealthy for sensitive groups and the woodstove is sucking outside air through the conditioned air space to the air intake on the wood stove, MERV 13 is getting the job done for me.

David and Alan will be much better able to answer these sorts of questions when they choose to chime in. I agree I am more sensitive to AQ issues than average, because I live in a polluted environment. I am personally certain a 14 tpi dovetail does produce some PM2.5, and I don't want it in my shop.

In other news, I got my caster pads glued to the bottom panel, and found my vertical rails are only 19.5 inches apart to embrace a 20 inch filter, so I am having to cut some notches as I work my way up the case. I suppose I should have measured a third time...

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I should add my 3 filter/1 fan system upstairs got me through the heating season with excellent AQ and is still going strong. I do have a geriatric long haired cat in the house. Whether or not I can make it through summer pollen season and just replace the filters next fall really depends on how much wildfire smoke I have to deal with this summer. I will definitely have to replace the filters upstairs in September to get through next winter, but I may have to replace them sooner if I see a lot of pollen or a lot of wildfire smoke over the summer.

[Scott Winners]

Also, the 3 filter/ 1 fan system I built for my wife came home today. She is the executive director of a small non profit. I am not sure if I mentioned in this thread, but she she is very sensitive to mold and their old building (she talked the board into buying a new building) had mold in the HVAC ductwork.

On this one the fan is one top, with BB panels on two sides. The other two sides each accept a filter, and I lifted the box off the floor with spacers and casters to get a third filter on the bottom with minimal air flow restriction. Finish was spray shellac x3 coats so accumulated dust can be wiped off fairly easily. I used a Utilitech (team orange) fan. I like that the Utilitech has the power knob on the front face, it is much easier to integrate than the Lasko with power knob on the top edge. Subjectively air flow is "similar," Several engineers have worked over a Lasko fan and they all come up with pretty close to 1000 fpm on high. I could not find robust data on the Utilitech, the Utilitech is probably similar but a little slower.

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On this one I did glue diagonal furring strip to the inside of the plywood filter blanks and probably will ongoing. I received zero noise complaints about reverb, buzz or thumping noises, substrate is 3/8" BB plywood.

[chris carter]

I'm well aware of what fine dust off of machinery and powered sanding can do, I thought you said you had a hand tool shop.

Dust from handtool woodworking, while not as bad as powertooling, is something to take very seriously. It’s not going to cause problems in the short term, but years down the road it will catch up to you. It’s not the particles you can see that are the really dangerous ones; it’s the ones you can’t see that cause the most damage to the lungs. And these pesky little things can take as much as a day to finally fall to the ground. Even just a hobby with 8 hours a week in the woodshop will add up over time if you don’t do anything about it. We used to not understand how bad this stuff was, but the more research that is done, the more we come to realize how bad these microscopic particles are.

Luckily, as many have pointed out, the solution is as cheap as s $20 box fan and a MERV13 filter.

[Richard Coers]

Dust from handtool woodworking, while not as bad as powertooling, is something to take very seriously. It’s not going to cause problems in the short term, but years down the road it will catch up to you. It’s not the particles you can see that are the really dangerous ones; it’s the ones you can’t see that cause the most damage to the lungs. And these pesky little things can take as much as a day to finally fall to the ground. Even just a hobby with 8 hours a week in the woodshop will add up over time if you don’t do anything about it. We used to not understand how bad this stuff was, but the more research that is done, the more we come to realize how bad these microscopic particles are.

Luckily, as many have pointed out, the solution is as cheap as s $20 box fan and a MERV13 filter.
I'll be 70 in September, and I started woodworking in 1972. Out of all those years, 24 of them were as a professional woodworker. When should I expect the boot to drop? Woodworkers Journal paid me for the shop tip of filters and a box fan around 1974 (can't remember that far back for the exact year. I do remember Fine Woodworking offering me more money for the shop tip after WWJ had already paid me). If you are a fan of Bill Pentz, he explains then real damaging dust is the super fines floating in the air that can get through the lungs and into the blood stream. I have yet to see dust floating throughout my shop using hand planes and scrapers. Even a hand sanding block with 320 grit is not going to put so much dust in the air that you need something more than a face mask instead of scrubbing the shop air. But if must insist there is a major issue that requires air filtration in your shop, then by all means have at it.