How to Repair Torn or Damaged Air Conditioning Duct

Updated on April 8, 2019
Bert Holopaw profile image

Bert spent 25 years working as a home-improvement and residential construction contractor in central Florida.

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

This air conditioning unit's duct system uses a galvanized sheet metal plenum with flex duct take offs.
This air conditioning unit's duct system uses a galvanized sheet metal plenum with flex duct take offs. | Source

Types of HVAC Duct

All duct systems serve the same purpose. However, local building codes often dictate the type of material used, its installation method and its location. Always wear the appropriate personal protection equipment: such as a dust mask, eye protection, and gloves.

  • Galvanized sheet metal systems: These systems start with a rectangular trunk line or plenum. The system then runs branch lines, made with either round sheet metal or smaller rectangular ducts, to the individual floor penetrations. S-shaped clips and cleats, often called s-locks and drives, hold the pieces of sheet metal ducts together; screws hold round sheet metal duct together. Installers use foil-backed duct tape or sheet metal compatible mastic to seal sheet metal duct. Some applications, such as attic or enclosed systems, need insulation to prevent condensation while using air conditioning.
  • Fiberglass duct board: This type of duct uses compressed fiberglass boards with a foil outer layer. A foil-backed duct tape, mesh, and duct sealing compound hold each joint together. Manufacturers offer 1-, 1.5- or 2-inch thick ridged fiberglass sheets.
  • Flex duct: This easy to install and inexpensive type of duct contains three layers: an inner layer that uses a spiral wire rib to hold its shape, a fiberglass center layer that provides thermal protection and an outer layer that acts as a vapor barrier. When installing a flex duct, a technician slides the inner liner over a sheet metal collar. Then he seals the inner liner with a code-approved duct tape and locks it in place with a large zip tie or worm-gear clamp, called a duct strap. Installers often support flex duct every four feet. This limits sags and keeps connections from pulling away from the collar.
  • Spider systems: A spider system utilizes a centralized plenum that feeds multiple branch lines. Most branch lines connect to a boot which delivers air to an interior room. However, when the system needs more than one plenum, a large branch line feeds each additional plenum.

This sheet metal duct system runs between floor trusses.
This sheet metal duct system runs between floor trusses. | Source

Sheet Metal Duct

Many homes located in northern climates use galvanized sheet metal ducts. These ducts run through the floor joists. Heat radiating from these ducts helps warm the flooring. Occasionally the sealant looses its grip and allows heated air to escape.

  1. Expose the leaking seam: Loosen the hanging straps or brackets on both sides of the repair area. Lower the duct enough to examine the leaking joint. Normally this type of leak only needs additional sealant; however, large gaps should be completely exposed and inspected. If a missing or incorrectly installed S-lock caused the large gap, take apart the joint and reconnect correctly.
  2. Repair the S-lock: When applicable, slice the old mastic from the perimeter of the joint with a utility knife. Remove any screws holding in the S-lock in place. Straighten the tabs on the ends of both drives and pull them from the joint. Separate the joint. Normally tension prevents complete separation. Slip the old S-lock from the joint. Open the S-lock with a screwdriver and slide it back into place. The flat metal on each duct end should slip into the S-lock gap, the gap created by the screwdriver. Install the drives with a hammer.
  3. Seal the duct joint: Cover the joint with a 1/8-inch thick layer of code-approved duct mastic, using a paintbrush to smear the mastic. Manufacturers make mastic designed specifically for galvanized sheet metal ducts.

Cut fiberglass duct board with a non-serrated knife.
Cut fiberglass duct board with a non-serrated knife. | Source

Fiberglass Duct Board

Heating and air-conditioning duct systems using fiberglass duct board rarely needs maintenance or repair beyond an occasional interior cleaning. Professional duct-cleaning technicians use high powered vacuums to evacuate dust buildup from the duct's interior surface. While this type of duct withstands moderate abuse, prolonged exposure to excessive moisture contaminates and destroys the fiberglass. Damp sections often dry without any issues, however, due to mold concerns technicians often recommend replacing sections of saturated duct board.

  1. Drain the duct: The fiberglass layer holds water like a sponge, and an intact foil backing prevents drainage. Place a bucket under the duct and poke a small hole in the duct's foil outer liner with a screwdriver. Keep the bucket under the duct until it stops dripping.
  2. Cut out the damaged section: Force a non-serrated knife blade through the duct and cut out the wet area. Use in and out motions. The notches on a serrated knife grab the foil backing's reinforcement threads, the triangle-shaped lines visible on the outside of the duct.
  3. Create a replacement patch: Either use the old section as a template or measure the opening with a tape measure and transfer the measurements to a new sheet of duct board. Cut the duct board with a non-serrated knife.
  4. Install the patch: Insert the patch into the hole and press on the patch until its foil backing sits flush with the existing duct's surface. Apply a piece of foil-backed duct tape to the seam, keeping the middle of the tape centered over the seam. Press the tape with a squeegee. Cover the tape with a piece of mesh. Coat the mesh with a 1/8-inch thick layer of duct mastic, using a paintbrush as an applicator.

Many duct board systems need the seam around the air handler or furnace repaired or sealed. This normally occurs when the installer neglected to wipe all of the manufacturing oil and debris from the air handler's surface, or he failed to seal around the entire outside perimeter of the plenum.

  1. Remove old sealant: Cut away the old sealant, mesh, and tape with a knife. Wipe the exposed connection with a rag, removing any debris or oil.
  2. Replace missing or damaged insulation: The duct-board's insulation must touch the furnace. Otherwise, condensation forms and becomes a breeding ground for mold and mildew.
  3. Cover the joint with foil-backed duct tape: The tape should bond the plenum to the furnace.
  4. Apply mesh: Cover the duct tape with mesh. In tight spaces use a paint stick to press the mesh against the tape.
  5. Seal the joint: Apply a 1/8-inch thick layer of duct mastic to the mesh with a paintbrush. Attach a paint stick to the brush's handle to reach hard to get to areas. Let the mastic harden, using the manufacturer's recommended dry time, then turn on the air handler and use a hand to feel for drafts.

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

This air conditioning system uses single pieces of flex duct to span across the attic
This air conditioning system uses single pieces of flex duct to span across the attic | Source

Flex Duct

Occasionally a home inspection reveals an exposed air-conditioning duct collar. This happens when pressure forces the outer liner and insulation away from the collar. The exposed collar greatly reduces the system's energy efficiency.

  1. Solve the issue causing the exposed collar: Add a hanger strap next to the slipping connection, relieving any tension. Place additional hanging straps throughout the duct run.
  2. Disconnect the loose liner: Cut the duct strap with wire cutters or pliers. Pull the insulation away from the connection. Adjust the outer liner, so it properly covers the insulation.
  3. Attach the outer liner: Apply mastic to the outer layer's butt edge. Slide the connection together and mash the material together until the mastic seals the connection, then lock it in place with a duct strap. Cover the outside of the joint with mastic.

Another common type of air conditioning duct damage is torn flex. Tears in the outer liner often happen when an installer drags a section of flex duct across roof trusses, and the outer liner catches on a nail or truss connector plate. Usually, the installer notices the damage and repairs it. However, occasionally the damage slips through the inspection and after the warranty period the problem becomes yours.

  1. Close the outer liner's rip: Close the center of the rip with a 6-inch section of duct tape. Seal each side of the rip with duct tape, starting at the center and working out.
  2. Seal the repair: Apply a piece of mesh across the length of the repair. Cover the mesh with duct mastic. The mesh holds the mastic together, which prevents cracks after it drys.

A knife cuts the flex duct's three layer and wire cutters snips the wire rib.
A knife cuts the flex duct's three layer and wire cutters snips the wire rib. | Source

Replacing Flex Duct

In certain situations, such as a crushed duct or animal damage, it makes more sense to replace a section of flex duct than it does to repair it. The new section uses the original system's duct collars and hangers. The cost of materials and the location of the repair makes this determination.

  1. Remove the damaged flex: If disassembling a flex connection, peel away the old duct seal and cut the duct strap. Pull the insulation away from the connecting, exposing the inner liner. Remove the duct strap and tape holding the liner in place. Carefully slide the damaged duct off the collar. Skip to step three. When removing a small section of flex duct, technicians slice through the duct with a knife. The blade cuts all three layers at once and follows the inner liner's wire rib around the perimeter. Remove the knife once it makes a full revolution, snip the wire rib with wire cutters.
  2. Install a Splice Connector: Fold the existing duct's outer liner over the insulation and slide it between the insulation and inner liner, leaving about 4 inches of the liner exposed. Slip a splice collar into the end of the duct's inner liner and wrap the connection with duct tape. Lock the inner liner against the splice collar with a duct strap. Push the collar into the duct until the collar's rib rests flush against the insulation roll.
  3. Calculate the length of the replacement section: Measure the distance between the existing ducts' collars and add two feet. Increase this calculation an additional one foot for each bend.
  4. Cut the new flex to the appropriate length: Stretch out the new flex duct to its full length. Roll the outer liner over the insulation and tuck it between the insulation and the inner liner. Measure from the end of the insulation roll to the appropriate place on the duct and cut with a knife. Roll the outer liner over the insulation.
  5. Install new flex: Slide the inner liner over the splice collar, leaving about 1 inch of the collar exposed. Seal the connection with code-approved duct tape. Slip a duct strap over the inner liner and tighten it with the appropriate tool. Butt the new section's outer liner and insulation against the existing duct's. Lock it in place with a duct strap. Seal the joint with duct tape or code-approved mastic.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2017 Bert Holopaw


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