I love mine but the replacement hepa filters alone cost almost as much as building this box. I have to take the pre-filter out a lot and wash it. Seventy-year-old houses accumulate a lot of dust out in the country over the years! I also have small portable hepa filter units in two of the bedrooms.
The guy that designed this box is as good with duct tape as any mountain hillbilly I ever met. 😁
The carbon “pre-filter” catches a lot of dust on mine, so I vacuum it a couple of times. Eventually, I replace it, but getting that dust out of there allows it to flow air. Not sure how good it is, but it really helped, with the other HEPA filter, to reduce the smoke smell in the house.