Note - I'm also looking at filtering air pre-minisplit and Jay also has a video on that (in which he discusses this cart again with time spent using it)
The furnace blowers are great for moving lots of air (squirrel cage) and their shape makes for an easy application. You can turn it into a small cart and possibly make the top perforated (peg board) for a sanding work surface to catch any fine dust that a DC misses. A cover for the peg board to seal it up when using as a room air filter.
I seem to remember that Mosquito built one similar to Jay Bates'. CL810 showed a furnace blower filter in my topic about the Corsi-Rosenthal filter box that he had sitting up on a shelf. When I look up the the airflow of the furnace blowers, my measurements and calculations seem to indicate that my box fan actually has a greater CFM than the theoretical airflow of a 2 ton blower motor. The volume I found online for HVAC blowers is 400 CFM per ton. Perhaps my methodology for computing the airflow is not accurate but the CFM of my filter box appears to move at least 800 CFM.
The nice thing about the cart approach is that not only can you use the top surface as a bench top but you can move it near the source of dust as you are working. I considered that for my C-R filter box but floor space is at a premium in my shop so I decided to hang it hear my minisplit to ensure that its intake air was mostly recently filtered. That seems to be working because the filter on my minisplit was barely dirty the last time I checked it.
--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.
Good points Nathan, a propeller type (box) fan will move more air (CFM) in a free-flow design whereas a squirrel cage will push air better given a ducted (restricted) output path. Not sure if that applies to the intake side as well which could make a difference as the filters load up.
I blow the air into my C-R filter box so that the dust collects on the inside. That way I am less likely to get a shower of dust when I bump it with a board or something. Before I replaced the 4- 4x20x20 filters about a year ago after 2 years of use, I measured the airflow on both the intake and outflow. There was plenty of dust on the filters but very little drop in air flow. Of course with 4 filters, I estimate that there is over 4300 square inches (30 sqft) of filter media.
--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.
My minisplit will be installed the end of the week. Going with a single head with some "wall fans" going into and out of the side room. I want to have a filter box, or like lazyman place a filter above, for the minisplit intake. Appreciate everyone sharing their experience. Sounds like a MERV 8 would be ok (higher may impact airflow based on this video), and ideally 4 inches thick.
I used Merv 8 on top of my minisplit. That seems to work pretty well. I did measure a slight drop of airflow with an anemometer but it does not seem to impact cooling or heating performance or change the amperage drawn when running with and without the filters. I had to lay the filters on top because I do not have the room that Dave and Jay Bates have over their minisplits.
BTW, I bought my filters from filterbuy.com. They have just about any size you might need so if your design calls for a specific size filter other than the ones you typically find locally, they probably have it. I think that they will even make custom sizes for you. The more filters you buy, the better the price and if you buy enough of them, they have free shipping too. I also bought a year supply of filters for the house while I was at it so it was not hard to reach the free shipping threshold. I use MERV 11 filters for my house units.
--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.
Thanks Lazyman - I plan to put filters over the wall fans too. One big culprit with dust is my miter saw. Plan to build an enclosure and get some connection to a vacuum as right now I just have a bag on the unit.
If the goal of the filters on the wall fans is to remove the visible dust (3+ micron) that settles around the shop, then a MERV 8 would probably do the job without restricting air flow too much, at least until the filters get clogged. Merv 11 is the minimum I would use if the goal is to reduce the amount of finer dust you breath. MERV 13 is sort of the sweet spot where you get above 90% of 1-3 micron particles at a relatively affordable cost. Remember what I said on one of the other threads about how fan blades without cowlings actually suck in air from the front of the fan so if you are pulling the air through the filter rather than blowing air into it, it may be that not all of the air coming out of the fan will be filtered. A cheap (Amazon) handheld anemometer is pretty handy for evaluating airflow for projects like this. You can actually see the air flow reverse direction around the edges of the fan, indicating that it is pulling air in around the edges.
--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.
Old furnace blowers are great but you have to have adequate air restriction or they can pull more amps than designed. I have a 1/2hp one on the wall of my shed and had to make a cardboard throttle for the air intake. It's also important to only restrict the inlet opposite the motor as the motor side is also 100% of the cooling for the motor.
