Whether it's a plumbing pipe, an air handler or a support structure, obstacles are a fact of life when you're installing heating and cooling ducts, and constructing offsets is an inevitable part of the job. Each offset calls for angled cuts, and the most difficult part of the job is calculating the angle. The OWL method provides a useful tool for doing this, and although it's an approximation, it usually produces acceptable results. This method calls for three measurements: the length of the offset, the width of the duct and the offset distance. For the method to work, all the measurements must be in the same units -- usually inches.

Exposed ductwork in lounge space.
credit: Photo by Drew Coffman on Unsplash
HVAC pros have to create offsets when obstacles block the path of ventilation ducts

Overview of the Procedure

When a straight run of ductwork is interrupted by an obstacle, the installer terminates the run a foot or two before it, installs an angled offset that clears the obstacle and continues the run. If the offset distance is the O, the width of the duct W and its length L, the angle of the cut the installer needs to make to clear the obstacle can be determined by the following formula: O times W divided by L (O x W ÷ L). This calculation is usually referred to by the acronym OWL.

The width of the duct is usually a known quantity, but the other two numbers must be measured. Once they are determined, the OWL calculation produces the length of duct that must be cut from the offset side to produce the desired angle.

Step-by-Step

Step 1 Measure the Offset Distance

Place a straightedge on the obstacle and extend it in the direction of the straight run of ductwork, parallel to it. Make sure the straightedge is long enough to extend past the duct opening. Measure the distance from the side of the duct farthest from the obstacle to the straightedge. This is the offset distance, O.

Step 2 Measure the Offset Length

Keep the straightedge in place, and measure the diagonal distance from the far edge of the duct opening to the point at which you plan to continue the duct after installing the offset. This point is typically close to the obstacle, but allows enough clearance to make connections. This distance is the length of the offset duct, L.

Step 3 Calculate the Offset Angle

Plug O, W and L into the OWL equation. The number you get will be the number of units you need to cut from the side of the offset duct closest to the obstacle. You'll also have to cut the same distance from the opposite side of the duct on the other end to allow you to connect it to the continuing ductwork.

Example: Suppose you're installing a duct with a width of 8 inches, and the offset is 5 inches. You measure the length of the offset duct to be 20 inches. Plugging these numbers into OWL gives a distance of 1.8 inches, or approximately 1 3/4 inches.

Step 4 Prepare the Offset Duct

Cut a length of duct equal to L plus 2 inches, using tin snips. You need the extra length to create a 1-inch flange on either end for connecting the duct. Arrange the offset in the same orientation as the installed duct, and mark the offset side, which is the one closest to the obstacle.

Using a felt marker, draw a line to mark the distance from the end equal to the result you got from the OWL calculation. Draw lines on the top and bottom of the duct that angle from the ends of this line to the opposite corners. Draw mirror images of these lines on the opposite end of the offset. That is, the OWL line is on the side of the duct farthest from the obstacle, and the angle lines extend to the opposite corners.

Step 5 Cut the Angles

Cut along the lines you drew, using tin snips, to produce a length of duct with ends that angle in opposite directions. Snip all four corners of the duct on both ends for a distance of 1 inch, and bend back the metal slightly to form flanges.

Step 6 Connect the Offset

Slide the offset onto the straight run of duct that has already been installed, and push until the flange covers it. Drive sheet metal screws into the flange to hold the offset to the duct. Use the same procedure when connecting the duct to the opposite end of the offset to continue the run.