Making Your Own
You’ll need two or three 8-foot lengths of flexible 6-foot aluminum clothes dryer tube with couplers, a 4-by-8-foot sheet of 1-inch-thick urethane foam wall insulation with two foil sides, and two two-speed, 6-inch-diameter cage fans; look for the latter at discount stores during summer. Cut the insulation board into two 1-1/2-by-4-foot sections, two 1-by-4-foot sections and three 12-by-16-inch end pieces that fit snugly inside the open ends. Cut a centered round hole in one of the end pieces and in one of the 18-inch sides 9 inches from one end to fit fans. In the other two ends, cut round holes to fit the dryer tube.
Use construction glue to assemble, and duct tape to hold together. If possible, let the cement set in a garage or outside for a couple of days. Assemble the 4-foot-high column so it measures 18 by 14 inches. Cement one end of a duct tube into one of the ends with a duct hole. Extend the duct fully, and push the assembly duct-first into the column 8 inches so it lies next to the fan hole in the side. Join one more duct tube section to the end of the first, and fully expand both. Carefully coil the duct inside the column. Insert the other end into the two-hole end piece, and push the latter into the column to form the far end. Push one fan exhaust end into the end of the column. Push the other fan exhaust end out into the large hole in the side. Use the third duct piece to connect the unit to a two-hole panel cut out to fit under a partially opened window, which should lock at the height of the panel.
Outside Air Exchanger Operation
Cut the third 4-inch duct section as needed to connect the unit to the panel in the window. Plug in the fans, and verify that the side fan exhausts to the room. The end fan should force inside air into the interior duct and push it out the window. As the stale inside air moves through the aluminum tube, it loses its heat to the space outside of it, where the cold outside air comes in. The heat raises the temperature of the incoming air to a point where it is not chilly and the thermal exchange efficiency should lie close to commercially available units at about 80 percent. If the exchanger works well, finish the outside with veneer, paneling or paint.