How Do I Add a Duct To My Air Conditioning System?

How Do I Add a Duct To My Air Conditioning System?


There are several ways to get air conditioning ductwork installed in your home. The first way is to hire a contractor, which is going to be pricey at best. The second way is that you can do it yourself and save a lot of money. But before you do anything identify whether or not your system has sufficient airflow and BTU capacity to handle the additional load.


To do this you’ll need to go online and do a Hey Alexa or Hey Siri search to access some type of program or software that will allow you to determine the heat load for each living area, and then determine whether or not the system has the correct capacity. We are going to assume you’ve already done that, and that you have sufficient BTU and airflow capacity to add additional outlets. The next step is to determine the raw materials you will need. There are two ways to do this. The first way to use a program that will tell you the various duct sizing that you need to carry the main trunk line, and what size reductions you’re going to need along the way. There are online programs or software available to do this. However it’s basically a waste of time. The easiest and best procedure to follow is to determine the size of your existing ductwork, and just use that exact same size to run your main trunk for the full run. That way frictional losses along the way will be kept to a bare minimum. You will simply use the registers at each outlet to balance air flow. At the end of the day this is by far the easiest process.


Now that you’ve identified the materials that you need and have purchased them, it’s time to take a look at the tools you will need to do this particular project. They’re fairly simple. What you will need are as follows: aviation snips, diagonal cutting pliers, a utility knife, a power screw gun, 5/8 inch drywall screws, a power screwdriver, plumbers banding, which is either steel or plastic that’s about ¾” wide, comes on a spool about 50 feet long, and is perforated about every two inches (This is typically used for pipe hanging). You will also need flexible duct tape, the type that’s metal foil not the type that’s cloth, as the cloth type will wear out. Then you’re going to need stainless steel type gear clamps. These clamps must be a sufficient size to fit around the flexible duct, and also around the insulation on the outside of the flex tubing.


Now that we have the basics out of the way, you have all your parts, all your tools, and you’ve already made a diagram to show you exactly where everything is going to be, it’s time to start the process.


Determine where the registers are going to be in each room. Typically these will be on the ceiling and they should be located between ceiling beams, but approximately two feet away from any walls. A pretty good register to use is the MV 4 type, which is made from plastic therefore they will not accumulate condensation. These have a built in collar and include a template so that you can draw the outlines of the hole to cut right on your ceiling. Now that you’ve cut the hole in your ceiling, you will note that you need some type of bridge or wood support to anchor the register to. This means you will have to do some type of a build out on the ceiling rafters, and we strongly recommend using 2” x 4” or 2” x 6” as these will not be conductive, and condensation will not form. It’s important to note at this point that generally speaking anything you do in this project that has a metal surface that can conduct low temperature, and therefore form condensation, should be avoided and/or covered with sufficient insulation so that you avoid condensation problems.


One further note on this project, this is as, an addition to a current system, your return register, plenums, duct work, etc. are all already in place, and that you already have a plenum on the discharge side of the unit which is feeding into your current system. That being said, the process we’re going to follow from this point is to disconnect the last register on our existing run, and assume that the run is the same 8 inch diameter throughout. Step 1 is to install an 8×4” T at the discharge point for the last register in the existing system. We’ll use the 4” side to connect that register, and that will leave us with an 8” T to which we must connect our new flex tubing. To connect the existing flex tube and the new flex tube to the T is a fairly simple process. Flex tubing can be stretched a little bit and pulled over the sides of the two openings on the T. Once that’s done, use a stainless steel band style clamp to secure the flex tube to the T’s.  There are several manufacturers of this type of clamp they are Ideal, Lokman, Hydrofarm, Hitichi, and Tech Team but we like the Tech Team clamp the best because it is a full 9/16” wide and therefore much stronger than ½” wide clamp of the other manufacturers. The Tech Team item which is an eight inch clamp, easily makes this connection for us. What you need to do now is to cut back the insulation covering on both of the flex pipes so that you can pull it over the T. And then this time you’re going to secure it in place with a 12” stainless steel gear type clamp. And once again we chose Tech Team’s for exactly the same reason


Now that everything has been mechanically locked into place it’s time to hook up the register that you just disconnected. Since this has a 4” flex tube on it, you use the same process, but this time you’re going to attach it with a 4” clamp. In this case we have used Tech Team’s 4” clamp which has a very clever thumb a screw type toggle arrangement for tightening it and makes the job extremely easy. And once again here we’re going to wrap it with the foil duct sealing tape so that there are no exposed metal surfaces in the fitting, and everything else is properly insulated.


  1. We have that out of the way. Next step is to move on to the other registers that we’ve installed in the ceilings. And each one of these locations except for the last one leave an 8×4 T. At the last one we will use an 8×4 elbow. At each one of these locations put the elbow in place and use the same technique as on the first one to secure it with banding and properly insulate it. At each one of these locations we’re going to leave the 4” end hanging down into the opening. For this you should leave about 12” to 18” of extra flex just in case. At this point go into the rooms and connect the registers to the 4” flex, and since this is 4” for this application you will use the Tech Team 4” stainless steel gear style clamp again. Make sure the insulation is covering everything, tape it up with the foil air conditioning tape, set the register into the ceiling, and secure it in place. Once this has been done, turn on the system make sure everything is operating properly. The final step is to go up into the attic again, and this time, use the banding tape to secure the duct work in place. The recommended interval is 1 securing strap about every 3 feet. For this you will need a screw gun and the screws. Secure everything so it doesn’t move and there’s no additional play, and your job is finished.