How to Make a Metal Duct Transition in the Field

In some cases, you may have a plenum or evaporator coil that needs to be connected to a furnace. To start off, measure the plenum and use the measurements to mark the lengths of the S-locks. Generally, you’ll take 1/4″ off of each side to prevent overlap. Once you’ve lined up the S-locks, you can cut the excess metal off with a bandsaw.

Michael makes the S-shaped part of the S-locks facing the outside when he attaches them to the plenum. You’ll also want to prep the S-locks that attach to the furnace using similar practices (S-shaped ends facing the outside, etc.).

Then, you will want to measure the height of the space between the furnace and the plenum and add an inch to each side to account for the farthest point of the plenum. After that, you can mark up your sheet metal based on your readings and cut it accordingly with metal shears. Then, you slide the sheet into one of the S-locks (Michael uses the bottom, in this case). Ensure that you have an inch of excess on each side so that you can fold the sheet metal later. Then, mark where the corners contact the S-locks.

Take a spare S-lock and align its inside edge with the marks. Then, draw a line along the opposite side of the S-lock to ensure that you have enough clearance. After you’ve drawn your lines, cut along the sheet metal along the lines.

Notch the corners by cutting a slightly angled sliver where your corners have been marked. With the corners notched, use a folding board to create one-inch folds on the vertical sides of the sheet. Then, slide the sheet into place between the furnace and the plenum. You should be able to slide subsequent sheets into the folds on the first piece.

Repeat the first few steps three more times to prepare the other sides of the transition (the two middle sheets should be without folds, and the last piece should have folds). Slide them into place as you did with the first piece, though you may need to remove the bottom S-lock that connects to the final piece. Once the last piece is in place, reinstall the S-lock. Be sure to fasten everything with screws as necessary to finish the metal duct transition.

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