Things You'll Need
2-by-2 sheet metal angle
Sheet metal screws
Plastic banding straps
Diffusers are the vents where heated and cooled air from your furnace enters a room. Installing a ceiling diffuser in a drop ceiling can be a little more tricky than when installed in a drywall ceiling. The weight of the diffuser and diffuser box must be supported by the ceiling grid and not the tile. If supported by just the tile, the tile will likely bend and break. You'll commonly find diffusers installed in drop ceilings in basements. Since the duct work will be above the ceiling in a basement, it is most logical to install the diffuser right into the drop ceiling.
Remove the ceiling tile that you want to install the diffuser in. Lift the tile up, and tilt it to remove it from the grid.
Measure the size of the diffuser and lay out the corresponding size on your ceiling tile. For example, if you are installing a 12-by-6 diffuser, draw a 12-inch-by-6-inch rectangle on the tile with a pencil. The rectangle can be located anywhere on the tile, depending on your needs.
Cut out the rectangular shape you laid out in the above step with your utility knife. Make slow passes along the scribed line, with each pass going successively deeper. Once through the tile, the rectangular cutout should easily pop out.
Lift the ceiling tile back up into the ceiling. Remove the adjacent tile and set the diffuser box on top of the ceiling tile so that it is centered over the cutout.
Cut two pieces of 2-by-2 angle at a length of 23 3/4 inches. This is the distance across the grid space. Place one length along the side of the diffuser box. Secure to the box with sheet metal screws. Place the other length on the other side of the diffuser box and fasten it to the diffuser box. The angles will bear the weight of the diffuser box, diffuser and flexible duct to the grid and not on the ceiling tile itself.
Connect the diffuser box to the appropriate supply air duct with flexible ducting. Cut the flexible ducting to the needed length; pull one end over the supply air duct and pull the other end over the collar on the diffuser box. Secure with plastic banding straps.
Lift the ceiling diffuser into the diffuser box from below the ceiling tile. The ceiling tile is between the diffuser box (on top of the ceiling tile) and the ceiling diffuser (below the ceiling tile). Screw it to the diffuser box above the tile with the provided screws. Tighten until no gap exists between the diffuser and the ceiling tile.