How to Cut Through Duct Work for Plumbing

Slide a piece of pipe insulation over the end of a plumbing pipe. Measure the diameter of the insulation with a tape measure.

Mark the hole location of both sides or the top and bottom of the duct with a dark-colored permanent marker. Ensure the holes are perfectly aligned. Secure a hole saw in a drill -- 1/8- to 1/4-inch larger than the diameter of the insulation.

Drill a hole at each marked location. Make sure the circular cutouts do not fall out of the hole saw and land inside of the duct.

Push a piece of thin-walled conduit through the drilled holes. Allow one end of the conduit to protrude 1-inch past one side of the duct. Set the body of the permanent marker against the duct with its tip sitting on the conduit. Run the marker around the circumference of the conduit to create a thin line. Repeat the process on the other side of the duct. Make a second line 1 inch off the last drawn line.

Cut the conduit on the last line to make the conduit 2 inches longer than the duct width. Make a series of cuts -- 1/4 inch apart -- with aviation snips that start at the ends of the conduit and stop at each line representing the sides of the duct. You will have multiple slats measuring 1/4-by-1-inch.

Slide a pair of pliers over one of the cut slats. Align the tip of the pliers' jaws with the marked line. Push the pliers toward the outside of the conduit until the slat sits 90 degrees to the conduit wall. Repeat the process to bend all the slats on one end of the conduit. Push the conduit through the drilled holes until the bent slats sit tight to the duct. Tap the slats on the other side of the conduit with a hammer until all remaining slats sit tight to the duct.

Load duct mastic on a disposable paintbrush. Apply the mastic to the connecting point between the slats and the duct. Ensure both sides of the duct have a thick layer of mastic that has no holes. Allow the mastic to dry for 10 to 12 hours before installing the plumbing.