I started out by buying three 20" box fans and some lumber. I had a sheet of 1/2" OSB on hand, so I ripped it lengthwise to get two 2x8 sheets. I took the fan covers off, and used the inner perimeter of the fan frame to trace three opening outlines on an OSB sheet, which I cut out with a sabre saw. I took the fan front screens and mounted them over the openings on the exterior side of the OSB, and mounted the fans themselves on the interior side. I decided to augment the attachment of the front screens and fans with the use of some silicon sealant along the perimeter of each... but this proved to be a mistake. As it turns out, the sealant tube, although never opened, was old enough to expire, and as a result it never set up after application... it remains gooey even now. This issue was not critical enough to halt progress, however. I wiped off as much excess as I could from the fixtures, and continued.
I decided to use 2x6s for most of the unit's framework; although heavy, it provided the rigidity and substantial mounting surfaces I sought. The top piece is 1x6. The inital design was sketched, but actual construction details and dimensions were determined as the work progressed. At first I had chosen 20"x20" filters for the design, but later changed those to 20"x25" for better air flow. Once the frame was built onto the exterior OSB, I cut out the six pieces of OSB that would comprise the interior surface, and assembled them as required. Then I fabricated the filter framework out of 1x2 boards and attached them to the interior OSB.
I attached grab handles to the interior side, to make this big, ungainly, heavy unit a bit easier to handle. I ran the fan plug wires along the interior framework and out of a central exit slot. I mounted a power strip to the interior trim, and tacked down the cord to keep it secure and out of the way. I had removed the top handles from the fans, but left the four-position rotary switches intact in their stock location. They are easily accessible thru the filter openings. I experimented with different types of filters, and initially chose the pleated type. These fit the openings best. The second photo below shows the unit in place under the garage door, with a filler board inserted on the right side.
And here is an exterior view of the installed unit; the first photo with the fans off, the second photo with the fans on.
It occurs to me that the main source of incoming air in this configuration will be the gap along the top of the garage door opening that is created when the door is partially raised. I'll have to insert some sort of screen or filter along that gap, so I won't be sucking in dust, pollen and bugs from the outside. That should be easy to configure using the cheap green filters, or screens. I also thought about a gap seal along the bottom of the fan unit... but an easier solution would be a long strip of carpet that would be laid down where the fan unit sits, compensating for the uneven apron surface. Other sealing requirements around the edges may have to be addressed... and since it's so big and heavy, I'll have to put some thought into how I intend to store it when not in use. But at least the unit is basically complete, and it moves a lot of air, so I'm happy with the result. Another prep step complete!
