If you have a room that's too cold in the winter and too hot in the summer, a booster fan can make the room more comfortable. There are three types of boosters. The simplest is a fan unit that sits on top of the register. Another option is an in-line fan that you can install in the duct line. The third type, shown here, is a recessed fan that fits into the duct and is flush with the floor. Recessed duct fans are sized to fit either a 4 x 10-in. or 4 x 12-in. floor duct. If you have either of these sizes, order the fan and install it in place of the register. Then plug it in and you’re done (Photos 1 and 2). However, if your floor duct is smaller (like 2 x 10 in. or 2 x 12 in.) you’ll have to cut out a section of the floor and install a larger boot. If the ceiling below the duct is open, you could do most of the work from below. If you have a hardwood, tile or other finished floor surface with a finished ceiling below, it may be easier to open the ceiling below the duct and patch it when you're done.
Replace Undersize Boots
Buy a larger 4 x 10-in. or 4 x 12-in. right-angle boot and a matching recessed booster fan (one choice is the Tjernlund RB12 , available through our affiliation with Amazon.com). Pull up the carpet and padding around the duct to find and mark the joists on each side of the duct. Set the cutting depth to cut through the subflooring and any underlayment. Then cut and remove the flooring and the old duct boot. You may have to cut the old boot apart with a tin snips to get access to the old sheet metal screws.
Next, swap in the larger boot. Enlarge the duct opening in the cut flooring pieces to fit the new boot. Reinstall the flooring with screws and staple the padding back in place. Lay the carpet over the duct and cut the opening to match. Then tuck the carpeting back onto the tack strip. Insert the fan into the duct. Then install the register and cord and connect to a receptacle or an optional fan speed controller.