Installing a High Static Pressure Duct

Test for leakages. Turn on your HVAC unit and test for leakages with a duct blaster fan attached to the furnace. This unit blows a visible, nontoxic smoke through the system to reveal leaks. You can also use a blower door that pressurizes the whole house with the air outlets sealed and opened. The difference in pressure between the two is then measured to determine if there are any leaks.

Remove the old duct. Make sure your HVAC system is off and locate the duct attached to the plenum. If the duct is attached with a collar, remove the collar from both ends and separate. Old ducts can also be removed by hitting the drives parallel to the duct. Once the edges go up, the duct should come off easily. Another option is to use a hammer to gradually separate each end if your ductwork slides in to each other. Rust may need to be scraped off before you begin.

Install new plenum. High static pressure ducts require a plenum with larger duct openings. Remove the old plenum. If the new plenum has a standing flange on the opening, affix "S" clips, attach the plenum and then use metal screws to secure it. If there are flanges on all four sides, metal screws can be used on all four sides to secure the plenum.

Attach duct to duct openings. You will need to attach square take-off collars to your duct opening or screw on your duct to the duct opening. If you're using collars, attach the collar to the duct and insert in to the plenum. Stretch the duct to the first opening of the supply air trunk and attach a second collar to connect. Make sure you don't have a sag or that the duct isn't stretched too tight. It's safer to start with a piece of duct that's too long, and then cut it down to size.