When the basement isn't quite as warm or as cool as you'd like it to be, the solution is usually a few well-placed vents in the ceiling. You can't just tap into any piece of duct work, however. Some ducts are sized to the room they serve. Others return air to the furnace and central-air units to be reheated or cooled and recirculated. Whether you're installing a ceiling vent in an unfinished or finished basement, the steps are similar. Often the difference is only in the trim pieces.
Things You'll Need
Left- or right-cutting sheet metal shears
Hex-head sheet metal screws
Drill with bits and hex driver bits
Duct starter collar
Large flat-head screwdriver
Flexible metal duct (for drop or hanging ceilings)
Rigid metal duct (for no ceiling or drywall ceilings)
Register box (for rigid duct)
1-by-2-inch lumber for mounting the vent
Metal duct tape
Determine the correct main duct into which to cut. At the furnace, find the air filter. The filter is always on the return duct, right before the air is fed into the furnace. It is the other main duct that you will want to tap into. Follow this duct and mark it as the delivery duct.
Choose a location on the main delivery duct that is closest to where the new vent will be. On the side or top of the main delivery duct, use the starter collar to trace a cutout for the new duct. The path to the new vent should have as few bends as possible, which makes the side of the main duct ideal. Use the top if necessary.
Place the head of the screwdriver just inside the traced circle at a 45-degree angle. Strike the butt of the handle with the hammer to penetrate the duct and start the cut. Use the sheet metal cutting shears to cut out the circle for the starter collar.
Place the starter collar in the new hole and bend the ears back around the inside of the main duct to hold the collar in place. Secure the starter collar with three equally spaced hex-head sheet metal screws.
Run duct to the location where the vent is to be installed, starting at the starter collar. Use duct strapping every 3 to 5 feet as necessary to secure the duct work to the joists with sheet metal screws. Use as few bends and angle pieces as possible. Each bend slows the air flow.
Mount the register box, if required, to the ceiling joists with 1-by-2-inch lumber cut to size and nailed to the joists. This step is not required for suspended or drop ceilings.
Make any adjustments to the installation and secure all the joints. With flexible duct, metal duct tape will permanently join the sections together. Secure rigid duct with three evenly spaced sheet metal screws at each joint. Seal all the joints with metal duct tape regardless of the type of duct.
Install a return air register for each room into which a ceiling vent was added. Follow Steps 1 to 7, except tap into the main return duct rather than the delivery duct. If the basement is unfinished, you probably don't need additional return ducts. Return air ducts can be at ceiling height, or the space between studs in a wall will serve just as well.
A 6-inch duct is standard for all but the smallest rooms. For rooms 150 square feet or smaller, 4-inch duct may be used. For drop or suspended ceiling installations, run the flexible duct before you install the grid. Then install a vent cover in a ceiling tile. If you always start at the main duct with the starter collar, you'll never go wrong deciding which end of a duct piece should point at the vent or at the main duct. The starter collar will always be your starting point. You can install a vent right into the main duct. Buy the vent at a home center, cut out the opening and secure the vent with hex-head sheet metal screws. Sheet metal shears come in left, right and straight versions. Use straight-cutting shears for making straight cuts and a left or right model for making round or curved cuts. Return air ducts facilitate the flow of air through a room. If a room does not have a return air duct, chances are that room has a reduced flow of heated or cooled air.
Wear gloves and safety glasses when handling and cutting sheet metal duct pieces. Check with your local building department to ensure you follow all codes. If you plan to install more than three or four new vents, you should talk to a licensed HVAC professional before proceeding to ensure that your system can handle the new vents efficiently without changing the heating/cooling capacity of your home. Never cut into a single, round duct that serves one vent in a room. Some main and auxiliary ducts are round and those can be tapped into, but not duct pipes that go directly to a vent in a room. You'll identify these round auxiliary vents by their large size and they will have more than one vent being served from them. If you have doubts about what can be tapped into and what can't be, consult a professional.