To explain the components of a forced air system: a fan draws air from the house into the system through return air vents (Image 1). During cold weather, gas burners create heat in the heat exchanger (Image 2), which in turn heats the air. The hot air goes up into the plenum and from there flows to the supply trunk line (Image 3) where it is distributed through vents throughout the rest of the home.
The conditioned air then re-circulates back into the system as needed. During hot weather, you turn on the air conditioner and cooling coils inside the plenum (Image 4) then cool the air, which is then distributed throughout the house.
You need a load calculation to make sure the furnace is large enough to supply the new room. You can get it online from companies like DuctWork or go to local HVAC pros to get a load calculation worked out. Generally speaking, if the room existed when the house was completed, the furnace is probably powerful enough. If it is an addition that was added after the house was built, you may need a new furnace.
A couple other components to a gas forced air system is a source of fresh air from outside the house to aid in combustion, and you'll notice chimney pipes that carry flue gases from the hot water heater and the furnace to the outside of the house.
Mark the center point for the takeoff and drill a pilot hole using the cutting bit in the hole cutter. Preset the cutting tool to the correct diameter. Place the center guide in the pilot hole and start the cutting bit along the circumference of the hole. Use a right-angle drill because you have to work in a tight space, but you can attach the cutting bit to any drill. The cutting bit then swivels around the center guide (Image 1), forming a perfect circle.
Note: Sheet metal hole cutters and right-angle drills are available at most rental centers or specialty stores.
After the takeoff is in place, reach in and bend the tabs over (Image 2) to secure it to the duct.
When doing any heating or ventilating project, you may need to pull a permit, so check with your local municipality. In some cases, you may even need to hire a pro to help with this kind of project, so check on that as well.
Install the ceiling register boot (Image 1) so you will know how long to cut the supply duct. Center it between the joist and screw into place with self-tapping screws.
Note: Use a power nut driver to drive self-tapping sheet metal screws.
Measure the length you need for the supply duct. Take the measurement from about 1" inside the takeoff to about 1" into the boot collar.
Cut the duct to length with a round duct cutter. You need to use a round duct cutter because the cutter actually removes a thin strip of metal along the cut line (Image 2), which makes the cut a lot smoother. It is almost impossible to cut the metal with any other tool.
Snap the duct together. It has a special snap lock built in at the factory.
Install the damper into the duct by drilling a hole into the side of the duct and then screw the damper into place. The damper control indicator will show you whether the damper is open or closed.
Put the crimped end in first, then put the other end of the duct in. The takeoff rotates to make it easier to fit the duct into place.
Secure both ends with a few 1/2" sheet metal screws through the collar.
Dampers are critical for the efficiency of a forced air system. They allow you to adjust the amount of air flowing into each room balancing the system.
Make sure the damper control is on the bottom of the duct.
For the system to work, you have to re-circulate the air back to the furnace. That is why you need a cold air return (Image 1).
Expert Note: If you had no return air in your room, you would create a positive pressure. That would then reduce the flow of conditioned air to that space.
Nail a piece of sheet metal onto the back of a stud cavity.
Cut two holes -- one in the stud cavity and the other one into the existing return (Image 2).
Secure start collars (Image 3) into both holes.
Use non-insulated, flexible ducting to connect the two returns (Image 4).
Use plastic cable ties to secure the ducting.