Lasko Power Plus Box Fan 20"x20" Filter Adapter/Air Purifier Conversion

This model converts a Lasko Power Plus model box fan to an air purifier. Lots of these types of fan filter conversions exist but they're all made for the basic Lasko/Cosco 20" box fan, which is quite a bit smaller all around than the Power Plus model. 

This adapter assembly replaces the plastic intake grill with a form fitting bracket that securely holds a 20"x20"x1" HVAC filter (more thicknesses to come) to the back of the Power Plus box fan. The original feet need to be temporarily removed in order to install the lower brackets but they'll fit back in the same way even with the brackets installed. There's also a slot in the bracket for the power cable and grommet.

The pieces are secured onto the box fan metal frame using 3M VHB adhesive tape (sold in stores as 3M Extremely Strong Adhesive). Stick the adhesive to the plastic frame pieces so you can line the pieces up before pressing down on the adhesive. 

IMPORTANT NOTE: These box fans are just bent sheet metal and aren't perfectly square, so the printed parts won't fit all around the fan perfectly. Test the fit of all the pieces with the filter in place before sticking the frame parts on. 

Small gaps shouldn't affect filtration efficacy even if they're located after the filter. I initially thought the gaps would behave like a vacuum leak and pull unfiltered air through but the suction force through the filter is strong enough such that air will favor the path through the filter rather than being sucked through the higher resistance path of the leak (smaller cross sectional area for air to pass through). The GIF below shows this in action. The gap shown is about ¼" but the vapor being gently blown toward the gap takes a detour down through the filter.

The filter is rated MERV 8 and the fan is on the lowest speed setting and it pulls an impressive amount of air through. This fan can probably handle a MERV 13 filter or higher without issue. For reference, MERV 17 would be roughly equivalent to a HEPA filter. With these higher rated filters the increased static pressure may result in more significant unfiltered air being pulled through the gaps, so taping over them might not be a bad idea with the higher filter ratings.

The filter slides in from the top and is held in place from falling out backward by a lip on the bottom and two side straight pieces. When I eventually add versions for this for thicker filters, only these three pieces will need to be updated for the thicker filters.

Since the box fan measures over 21"x22", this is a substantial print. I cut up the whole model into 8 pieces and if oriented as shown below, all 8 parts can be printed in two long (about 9 hours per session on my machine) print sessions across two build plates. I used 3 walls, minimal supports, and 15% adaptive cubic infill on a 0.4mm tip in PETG and in total this entire assembly ends up over 550 grams. I have an Ender 3 KE with a 220mm x 220mm x 240mm build volume, so this should fit in a lot of printers despite the large size.

I would test the fit before committing to the whole print. Print the two straight side pieces (scale them to be shorter to save time and filament but make sure uniform scaling is off so only the length changes) and test mount the two sides on the fan with your AC filter in between to make sure it'll all fit. The filter slot measures 19.835" wide. 

Industry standard HVAC filters are always about ¼" undersized from nominal, so a 20"x20"x1" filter will actually measure around 19.75"x19.75"x0.75", although this will vary from brand to brand.