Ductwork installation doesn’t have to be difficult—these expert-backed tips make installing round HVAC ducts easy, efficient and DIY-friendly.
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Whether you’re adding new heat runs in a basement or changing the layout of an existing HVAC system, you’ll probably be working with a round metal ductwork pipe. Here, our expert Bob Schmahl—who’s been a tin bender for more than 40 years—shares a few round ductwork installation tips that should help make your next job run that much smoother.
Adding new heat runs in a basement can alter the airflow to other rooms in your HVAC ductwork. To maintain proper air balance, each register should have its own adjustable damper. If the dampers aren’t accessible from below, install them close enough to the register so they can be reached through the register opening. Schmahl recommends 4 x 10-in. boots (not shown)—they’re large enough to fit your hand for easy damper adjustments and offer more grate cover options than smaller sizes.
If you have to disassemble existing ductwork fittings, there’s no need to peel off the old foil tape first. Instead, just score the tape at the seam with a utility knife and remove the screws right through the tape. When it comes time to retape, just clean off the dust and apply new tape right over the old.
There’s no question that a flexible duct is easier to install than metal ductwork, but it comes with drawbacks. Over time, flexible ducts can degrade, collect dust and become nearly impossible to clean. It also needs to be larger than a metal pipe to maintain equivalent airflow. One of the most common issues Schmahl has seen during round duct installation is improper routing.“People get careless and turn corners too sharp, which creates kinks that severely restrict airflow,” he says.
When assembling a pipe, one of the most useful DIY duct installation tips is to start at one end and work the seam together like a zipper. Use one hand to align the edges and the other to press them down. Support the pipe with your leg, a workbench or the floor. If you need to take it apart, just slam it seam-side up on the ground—it should pop open easily.
Aviator snips work fine to cut holes in a trunk line, but only if there’s enough space. If you’re dealing with close quarters and you own a right-angle drill or attachment, you may want to invest in a sheet metal hole cutter. Otherwise you might have to take down the trunk line.
When you’re installing a pipe between two fixed parts, it’s tough to get the standard 1-1/2-in. overlaps at both ends using crimped connections. In this case, overlap one side as usual and create a butt joint on the other. Use a draw band connector to secure the joint. If your supplier doesn’t stock draw band connectors, make your own by cutting a short piece of larger pipe to bridge the ends. Screw it into place and finish by sealing the HVAC ducts with tape. For a 6-in. pipe, use a 7-in. piece for your custom band.
Caulk (don’t tape) the connection between the trunk line and a take-off (elbow) before you connect pipes to it. That way, you’ll be able to turn the take-off out of the way to caulk above it. Regular silicone is fine.
If the ducts are going to be concealed, all seams need to be taped or caulked. Here’s Bob’s trick for taping a seam on a pipe that’s installed close to the subfloor: Cut a piece of foil tape to length. Peel off part of the backing. Slide the backing up and over the pipe. Finally, pull down on the backing, which will pull the tape along with it. Inspectors will want to know you’ve used an approved tape, so buy the stuff with writing on it, or keep the roll on-site until inspection.
When cutting pipe, Schmahl recommends one of his go-to DIY duct installation tips: mark the length you need on both sides of the open seam with a marker. Flat metal is much easier to cut than curved, so he uses his knee to support and flatten the pipe before opening it up. Then simply sight the far mark as you cut—it’ll come out straight every time. Schmahl suggests snips made by Malco, which are available online for under $35. Be sure to wear gloves because sheet metal edges are sharp.
Each pipe needs support. You can use just about any support you want, but adjustable steel support brackets are quick and easy. Don’t forget to screw the pipe to the joist hanger so the pipes won’t rattle when someone stomps across the floor above. Every connection needs three screws. They don’t have to be evenly spaced. Use 1-in. galvanized zip screws designed for sheet metal.
Schmahl recommends moving one “gore” (elbow ring) at a time, starting with the connected side. Don’t make 90-degree turns if you don’t have to. A 90-degree elbow creates the same resistance as adding 5 ft. of pipe.
