Workshop Air Cleaner


Basement, Blog, Home, Woodworking / Wednesday, September 19th, 2018

I make a lot of things, but the thing I’m best at making is a mess. The mess can usually be justified, by me, because I have a fairly good memory of where things are in the shop. However, I don’t live alone so I need to keep a cleaner shop in order to not inconvenience my wife. 

One of my biggest issues is sawdust. I put up some temporary shop walls to help deal with the problem, but really it was only the first piece of the puzzle. The drop-cloth curtains do a great job keeping dust off the items on my shelving, but they don’t keep the dust from settling elsewhere in the shop. What’s worse, that sawdust stays in the air for quite a while and if you’re not wearing a mask then you’re likely to breathe it in and that is no good for anyone.

The plan then, was to clean the air.

I’ve watched a fair number of YouTube videos of homemade air cleaners of varying build quality. Some of them use multiple horsepower motors with homemade impellers and boxes, both optimized for the space and the motor. Others just find an old box fan and tape a furnace filter to it. Me, I wanted something in between the two. Something that isn’t the bare minimum but is still achievable without spending much money nor being so complicated that I set the project aside while I work on it. My requirements for the build include:

  • Cheap, less than $100
  • Powerful, enough to clean the air
  • Look decent
  • Out of the way
  • Easy to build
  • Accepts two filters
  • Use existing materials on hand, if possible

That’s when I found this page, simply detailing a ceiling mounted box fan filter. It looked fairly easy to build and would be low profile enough that I won’t be avoiding the part of the shop where it’s mounted to the ceiling. Also, since it only had a few pieces needed for it, I could build it using the old MDF I saved when I rebuilt my shop shelves.

Ceiling mounted box fan filter

The next step was to buy a fan and filters. You can really go a lot of different routes on which fan you use. Some people use old box and oscillating fans for their builds. Other people get old furnace blower motors. I’ve even seen a good number of videos where someone has a motor and they turn it into a fan just because. Since I was going the low profile, box fan route, I had to buy a box fan. Ellie and I didn’t have one on hand and I think if I were to get an old one from my parents it would have been too tired and old to work for what I had in mind. Also, there are surprisingly few box fans being thrown out in the trash.

I didn’t want the bargain bin box fan, I wanted something I knew was going to move a lot of air. For fans, this is measured in cubic feet per minute. Given the size of the space where my shop is and the ceiling height, I needed a fan to accommodate 3600 cubic feet. I found a list of the best box fans based on CFM, and decided on the Lasko 3723 (I think I paid about $33 shipped) because it has a rating of 2500 CFM. In theory, it would take about 90 seconds to circulate all the air in my shop area, which isn’t bad. However, once you put a filter on the intake side of the fan, the CFM drops dramatically. I can only guess what my fan is running at now but I know I’m glad I went with the best fan I could.

For the filters, I was building the brackets with the idea it would hold two filters. One would be a prefilter to get the bigger stuff, and the second would be a high quality filter to remove the fine dust out of the air. The idea behind the two filters is that you can replace the cheaper prefilter more often and only have to replace the second filter occassionally. Considering the prefilter was $10 and the second filter was $20, I’ll hope this holds true.

Box fan air cleaner bracket

Building the brackets to hold the fan was very straight forward. I cut down the MDF from my old basement shelving to the length of the fan. I then cut strips for the ceiling attachment and to hold the filters in place. Everything was held together with counter-sunk 1 1/4 in. screws. 

To hold the fan to the brackets, I had to remove the screens on either side of the fan to give me access to the inside. I needed to drill holes in the sides, big enough to fit bolts through. I drilled matching holes on the mounting brackets. The bolts are held in place with washers and wing nuts. With the brackets attached to the fan, I could do a test fit of the filters. I found that there was too much play in the slots for the filters, so I added some spacers on either side with some more MDF. This gave the filters a very snug fit but I was still able to slide them in and out with ease.

Box fan air cleaner bracket detail

Next came the hardest part of the project, mounting it to the ceiling. I decided to remove the filters and mount the brackets with the fan attached all as one piece. I wanted to make sure the mounting brackets were in the right place on the joists. Since Ellie was working on homework, I needed to accomplish this on my own. I ended up propping the fan up on top of my six foot ladder and then raising it higher by balancing it on a 4×4 I had left over from my ShopSmith bench build. It wasn’t safe. I held everything as best as I could while I frantically secured it to the joists. 

I slid the filters into place and plugged it in. The power cord barely reached the nearest outlet. I still need to wire up a better solution for powering the air cleaner. I have an idea that involves using the conduit that was previously powering one of the basement lights in last week’s blog. It’d be nice to have it all on a single switch too. 

I left the fan running for a while and I set about doing some sanding for another project. I figured it’d be a good test of the air cleaner. Even though I had the air cleaner, I didn’t throw caution to the wind. I still wore my respirator. When I was done sanding for the evening, I turned off the fan and inspected the air filters. I had a problem.

The way my two filter bracket is designed, I incorporated a flaw. The space between the two filters is too great that the prefilter doesn’t really accomplish anything because that space is also open on the ends where you can slide the filter in and out. I’m sure the prefilter did catch some stuff, but the second filter was a lot dirtier than it should have been. I think what I need to do is put a cap on both ends, and make it so one cap is on a hinge that can be locked in place. The hinge will allow me to open it up and clean/replace the filter as necessary.

All in all, I’m quite happy with the result, even if the prefilter is being ignored at the moment. For $63, I checked all boxes off my list. Since after maybe an hour of use the filters are visibly dirty, it’s clear the air cleaner is doing its job. Hopefully, it keeps things cleaner in the basement and Ellie and I can breathe a little easier.

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