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How to Repair HVAC Ductwork

Jul 8

A damaged duct system greatly decreases the energy efficiency of a heating and air conditioning system. Ideally, from the furnace to the living space, every bit of conditioned air goes directly. In extreme cases, the cost of energy lost from torn or damaged ducts exceeds the price of repair materials by far more in a single season.


Types of HVAC Duct

The same purpose is served by all duct systems. However, the type of material used, its installation method and its location are often dictated by local building codes. Wear the right personal protection equipment, such as a dust mask, eye protection and gloves, at all times.


  • Galvanized sheet metal systems: Such systems begin with a rectangular or plenum trunk line. The system then runs branch lines to the individual floor penetrations, produced with either round sheet metal or smaller rectangular ducts. The parts of sheet metal ducts are held together by S-shaped clips and cleats, often called s-locks and drives; screws hold round sheet metal ducts together. Installers use foil-backed duct tape or mastic compatible with sheet metal to seal ducts of sheet metal. To avoid condensation while using air conditioning, some applications, such as attic or enclosed systems, need insulation.
  • Fiberglass duct board: Compressed fiberglass boards with an outer layer of foil are used for this type of duct. Each joint is held jointly by a foil-backed duct tape, mesh, and duct sealing compound. 1-, 1.5- or 2-inch thick ridged fiberglass sheets are offered by manufacturers.
  • Flex duct: There are three layers in this easy to install and cheap type of duct: an inner layer that uses a spiral wire rib to hold its shape, a center layer of fiberglass that provides thermal protection and an outer layer that acts as a vapor barrier. A technician slides the inner liner over a sheet metal collar when you install a flex duct. He then seals the inner liner with a code-approved duct tape and locks it in place, called a duct strap, with a large zip tie or worm-gear clamp. A flex duct is often supported every four feet by installers. This restricts sags and prevents the collar from pulling connections away.
  • Spider systems: A centralized plenum that feeds multiple branch lines is used by a spider system. A boot that delivers air to an interior room is connected to most branch lines. However, a large branch line feeds each additional plenum when the system needs more than one plenum.


Sheet Metal Duct

Galvanized sheet metal ducts are used by many homes situated in northern climates. These ducts run through the joists on the floor. Heat radiating from these ducts helps the flooring to warm up. The sealant loses its grip occasionally and allows heated air to escape.


  • Expose the leaking seam: On both sides of the repair area, loosen the hanging straps or brackets. Lower the duct enough to examine the joint that is leaking. Usually, only extra sealant is needed for this type of leak; however, large gaps should be completely exposed and inspected. If the large gap has been caused by a missing or incorrectly installed S-lock, take the joint apart and reconnect correctly.
  • Repair the S-lock: When applicable, slice the old mastic with a utility knife from the perimeter of the joint. Remove any screws held in place by the S-lock. On the ends of both drives, straighten the tabs and pull them from the joint. Take the joint apart. Full separation is usually prevented by tension. Slip out of the joint the old S-lock. With a screwdriver, open your S-lock and slide it back into place. The flat metal should slip into the S-lock gap, the gap created by the screwdriver, on each duct end. With a hammer, install the drives.
  • Seal the duct joint: Using a paintbrush to smear the mastic, cover the joint with a 1/8 inch thick layer of code-approved duct mastic. Manufacturers produce mastics specifically designed for galvanized sheet metal ducts.


Fiberglass Duct Board

Beyond the occasional interior cleaning, heating and air-conditioning duct systems using fiberglass duct board rarely require maintenance or repair. High powered vacuums are used by professional duct-cleaning technicians to evacuate dust accumulation from the inner surface of the duct. While this sort of duct withstands moderate abuse, the fiberglass is contaminated and destroyed by prolonged exposure to excessive moisture. Damp sections often dry without any problems, but technicians often recommend replacing sections of saturated duct board due to mold concerns.


  • Drain the duct: Like a sponge, the fiberglass layer holds water, and an intact foil backing avoids drainage. Place a bucket under the duct and poke a small hole with a screwdriver in the duct's foil outer liner. Until it stops dripping, keep the bucket under the duct.
  • Cut out the section that was damaged: Through the duct, force a non-serrated knife blade and cut out the wet area. Use motions from in and out. The notches on a serrated knife grab the reinforcement threads of the foil backing, the visible triangle-shaped lines on the outside of the duct.
  • Create a patch for a replacement: Use the old section as a template, or use a tape measure to measure the opening and transfer the measurements to a new sheet of duct board. With a non-serrated knife, trim the duct board.
  • Install the: patch Insert the patch into the hole and press the patch until the foil support sits flush with the surface of the existing duct. Apply the seam with a piece of foil-backed duct tape, keeping the center of the tape centered above the seam. With a squeegee, press the tape. With a piece of mesh, cover the tape. Coat the mesh, using a paintbrush as an applicator, with a 1/8-inch thick layer of duct mastic.


Many duct board systems need the seam repaired or sealed around the air handler or furnace. This usually occurs when the installer failed to wipe all the production oil and debris from the surface of the air handler, or failed to seal around the full outside perimeter of the plenum.


Remove old sealant: Use a knife to cut off the old sealant, mesh, and tape. With a rag, wipe the exposed connection, removing any debris or oil.


Replace missing or damaged insulation: the furnace must touch the insulation of the duct-board. Otherwise, condensation forms and becomes a mold and mildew breeding ground.


Cover the joint with foil-backed duct tape: the plenum should be bonded to the furnace by the tape.


  • Apply mesh: Use mesh to cover the duct tape. Use a paint stick to push the mesh against the tape in tight spaces.
  • Seal the joint: Use a paintbrush to apply a 1/8-inch thick layer of mastic duct to the mesh. Attach a paint stick to the handle of the brush to reach areas that are hard to reach. Using the recommended dry time of the producer, let the mastic harden, then turn on the air handler and use a hand to feel for drafts.


A tight space prevents the installer from sealing portions of the outer surface of the duct in certain situations, such as when the home's design places the air-handler or furnace in a small closet. In these instances, an access hole must be opened by the installer and the joint must be sealed from within. When doing this, use caution to avoid dripping mastic onto the system's evaporator coil.


Flex Duct

A home inspection occasionally reveals an exposed air-conditioning duct collar. This happens when the outer liner and insulation are forced away from the collar by pressure. The exposed collar greatly lowers the energy efficiency of the system.


  • Solve the problem that caused the exposed collar: Next to the slipping connection, add a hanger strap, easing any tension. Throughout the duct run, place extra hanging straps.
  • Disconnect the loose liner: With wire cutters or pliers, cut the duct strap. Pull the connection away from the insulation. Adjust the outer liner, so it covers the insulation properly.
  • Attach the outer liner: apply putty to the butt edge of the outer layer. Slide the connection together and mash the material together until the connection is sealed by the putty, then lock it with a duct strap in place. Cover the putty on the outside of the joint.


Torn flex is another common type of air conditioning duct damage. When an installer drags a section of flex duct across roof trusses, tears in the outer liner often occur, and the outer liner catches on a connector plate of a nail or truss. Typically, the installer notices and repairs the damage. Occasionally, though, the damage slips through the inspection and the issue becomes yours after the warranty period.


Close the rip of the outer liner: Close the center of the rip with a 6-inch duct tape section. Use duct tape to seal each side of the rip, starting at the center and working out.


Seal the repair: Apply a piece of mesh over the repair's length. Use duct putty to cover the mesh. The mesh holds the mastic together, which, once dried, prevents cracks.


Replacing Flex Duct

It makes more sense to replace a section of flex duct in certain situations, such as a crushed duct or animal damage, than to repair it. The new section utilizes the duct collars and hangers of the original system. This determination is based on the cost of materials and the location of the repair.


  • Remove the damaged flex: peel away the old duct seal when disassembling a flex connection and cut the duct strap. Pull away the insulation from the connection, revealing the inner liner. Take off the duct strap and tape that holds the liner in place. Slide the damaged duct off the collar carefully. Skip to the third step. Technicians slice through the duct with a knife when removing a small section of flex duct. At once, the blade cuts all three layers and follows the wire rib of the inner liner around the perimeter. Snip the wire rib with wire cutters to remove the knife once it makes a full revolution.
  • Install a Splice Connector: Fold the outer liner of the existing duct over the insulation and slide it between the insulation and the inner liner, leaving the liner exposed to about 4 inches. Slip the splice collar into the end of the inner liner of the duct and wrap the duct tape connection. Using a duct strap, lock the inner liner against the splicing collar. Until the collar's rib rests flush against the insulation roll, push the collar into the duct.
  • Calculate the replacement section's length: Measure the distance between the collars of the existing ducts and add two feet. For each bend, increase this calculation by an additional 1 foot.
  • Cut the new flex to the relevant length: Stretch out to the full length of the new flex duct. Roll the outer liner and tuck it between the insulation and the inner liner over the insulation. Measure from the end of the roll of insulation to the suitable location on the duct and cut with a knife. Over the insulation, roll the outer liner.
  • Install new flex: Slide the inner liner over the splicing collar, exposing about 1 inch of the collar. Seal the link with duct tape that is code-approved. Over the inner liner, slip a duct strap and tighten it with the appropriate tool. Butt the outer liner and insulation of the new section against the existing duct. With a duct strap, lock it in place. Seal the joint with duct tape or with mastic approved by code.


If you feel overwhelmed and need help with your Raleigh HVAC Repairs, give us a call.


Smart Living Home Repair Services

244 Madison Avenue , #1019

New York, NY 10016

(888) 758-9103