How to Repair HVAC Flex Duct

The energy efficiency of a heating and air conditioning system is greatly reduced when the duct system is damaged. Every ounce of conditioned air should travel directly from the furnace to the living area. In extreme cases, the cost of energy lost due to a torn or damaged duct in a single season far outweighs the cost of repair materials.


The Different Types of HVAC Ducts

The purpose of all duct systems is the same. Local building codes, on the other hand, frequently dictate the type of material used, as well as its installation method and location. Wear appropriate personal protective equipment, such as a dust mask, eye protection, and gloves, at all times.


  • A rectangular trunk line or plenum is the starting point for galvanized sheet metal systems. The system then connects the individual floor penetrations with branch lines made of round sheet metal or smaller rectangular ducts. Sheet metal ducts are held together by S-shaped clips and cleats, also known as s-locks and drives; round sheet metal ducts are held together by screws. Sheet metal duct is sealed with foil-backed duct tape or sheet metal compatible mastic by installers. To avoid condensation while using air conditioning, some applications, such as attics or enclosed systems, require insulation.
  • Fiberglass duct board: Compressed fiberglass boards with a foil outer layer are used in this type of duct. Each joint is held together with foil-backed duct tape, mesh, and duct sealing compound. Ridged fiberglass sheets come in thicknesses of one, 1.5, or two inches.
  • Flex duct: This low-cost, easy-to-install duct has three layers: an inner layer that maintains its shape with a spiral wire rib, a fiberglass center layer that provides thermal protection, and an outer layer that serves as a vapor barrier. A technician slides the inner liner over a sheet metal collar when installing a flex duct. He then seals the inner liner with code-approved duct tape and secures it with a duct strap, which is a large zip tie or worm-gear clamp. Flex duct every four feet is commonly recommended by installers. This prevents connections from pulling away from the collar and causing sags.
  • Spider systems make use of a centralized plenum that feeds multiple branch lines. The majority of branch lines are connected to a boot that supplies air to an interior room. When the system requires more than one plenum, each additional plenum is fed by a large branch line.


Sheet Metal Duct

Galvanized sheet metal ducts are used in many homes in northern climates. These ducts are located between the floor joists. The heat that radiates from these ducts aids in the warming of the flooring. The sealant loses its grip on occasion, allowing heated air to escape.


  • Loosen the hanging straps or brackets on both sides of the repair area to expose the leaking seam. Reduce the duct’s height to inspect the leaking joint. Normally this type of leak only needs additional sealant; however, large gaps should be completely exposed and inspected. If the large gap was caused by a missing or incorrectly installed S-lock, disassemble the joint and reconnect it properly.
  • Repair the S-lock by slicing the old mastic from the joint’s perimeter with a utility knife, if necessary. Take out any screws that are holding the S-lock in place. Straighten the tabs on both drives’ ends and pull them out of the joint. Disconnect the joint. Tension normally prevents complete separation. Remove the old S-lock from the connection. With a screwdriver, open the S-lock and slide it back into place. Each duct end’s flat metal should slide into the S-lock gap created by the screwdriver. With a hammer, install the drives.
  • Glue the duct joint shut: Apply a 1/8-inch thick layer of code-approved duct mastic to the joint, smearing it with a paintbrush. Mastic made specifically for galvanized sheet metal ducts is available from manufacturers.


Fiberglass Duct Board

Aside from an occasional interior cleaning, fiberglass duct board heating and air-conditioning duct systems require little maintenance or repair. High-powered vacuums are used by professional duct-cleaning technicians to remove dust from the duct’s interior surface. While this type of duct can withstand some abuse, prolonged moisture exposure contaminates and destroys the fiberglass. Although damp sections of duct board often dry without issue, technicians frequently recommend replacing saturated duct board sections due to mold concerns.


  • Drain the duct: The fiberglass layer acts like a sponge, absorbing water and preventing drainage. Place a bucket under the duct and use a screwdriver to make a small hole in the foil outer liner. Keep the bucket underneath the duct until it no longer drips.
  • Remove the damaged area as follows: Cut out the wet area with a non-serrated knife blade forced through the duct. In and out motions should be used. A serrated knife’s notches grab the foil backing’s reinforcement threads, which are visible on the outside of the duct as triangle-shaped lines.
  • Make a patch to replace it: Measure the opening with a tape measure and transfer the measurements to a new sheet of duct board, or use the old section as a template. Using a non-serrated knife, cut the duct board.
  • To apply the patch, follow these steps: Insert the patch into the hole and press it down until the foil backing is flush with the surface of the existing duct. Apply a piece of foil-backed duct tape to the seam, keeping the middle of the tape centered over the seam. Using a squeegee, apply pressure to the tape. A piece of mesh can be used to cover the tape. Using a paintbrush as an applicator, apply a 1/8-inch thick layer of duct mastic to the mesh.


The seam around the air handler or furnace on many duct board systems needs to be repaired or sealed. This usually happens when the installer didn’t wipe all of the manufacturing oil and debris off of the air handler’s surface, or he didn’t seal the plenum around the entire outside perimeter.


  • Cut away the old sealant, mesh, and tape with a knife to remove the old sealant, mesh, and tape. Wipe any debris or oil from the exposed connection with a rag.
  • Replace any insulation that is missing or damaged: The insulation on the duct board must come into contact with the furnace. Condensation forms otherwise, providing a breeding ground for mold and mildew.
  • Apply foil-backed duct tape to the joint: The plenum should be taped to the furnace.
  • Make a mesh: Use mesh to cover the duct tape. Use a paint stick to press the mesh against the tape in tight spaces.
  • Close the gap: Using a paintbrush, apply a 1/8-inch thick layer of duct mastic to the mesh. To reach hard-to-reach areas, attach a paint stick to the brush’s handle. 


Allow the mastic to harden according to the manufacturer’s instructions, then turn on the air handler and check for drafts with your hand.
In some cases, such as when the air-handler or furnace is located in a small closet, the installer is unable to seal portions of the duct’s outer surface due to a lack of space. The installer must open an access hole and seal the joint from the inside in these cases. Use caution to avoid dripping mastic onto the evaporator coil of the system.


Flex Duct

An exposed air-conditioning duct collar is occasionally discovered during a home inspection. When the outer liner and insulation are pushed away from the collar by pressure, this happens. The system’s energy efficiency is greatly reduced by the exposed collar.


  • Fix the problem that’s causing the exposed collar: Next to the slipping connection, add a hanger strap to relieve any tension. Additional hanging straps should be placed throughout the duct run.
  • Disconnect the strewn-about liner: Wire cutters or pliers can be used to cut the duct strap. Removing the insulation from the connection is a good idea. Adjust the outer liner to cover the insulation completely.
  • Attach the outer liner as follows: Apply mastic to the butt edge of the outer layer. Slide the connection together and mash the materials together until the mastic seals the connection, then use a duct strap to secure it in place. Mastic should be applied to 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 and the outer liner catches on a nail or truss connector plate, tears in the outer liner are common. The installer usually notices the damage and fixes it. However, occasionally, damage escapes inspection, and the problem becomes your responsibility after the warranty period has expired.


  • Close the rip in the outer liner with a 6-inch section of duct tape in the center. Duct tape each side of the rip, starting in the center and working outward.
  • Complete the repair: Apply a piece of mesh across the repair’s length. Duct mastic should be used to cover the mesh. The mesh holds the mastic together while it dries, preventing cracks.


Replacing Flex Duct

In some cases, such as a crushed duct or animal damage, replacing a section of flex duct makes more sense than repairing it. The original system’s duct collars and hangers are used in the new section. This is determined by the cost of materials and the location of the repair.


  • Remove the damaged flex by peeling away the old duct seal and cutting the duct strap when disassembling a flex connection. Remove the insulation from the connection, revealing the inner liner. Remove the duct tape and duct strap that is holding the liner in place. Slide the damaged duct off the collar with care. Step three can be skipped. Technicians use a knife to cut through flex duct when removing a small section. The blade cuts through all three layers at the same time, following the wire rib of the inner liner around the perimeter. After the knife has completed a full revolution, remove it and snip the wire rib with wire cutters.
  • Splice Connector Installation: Fold the outer liner of an existing duct over the insulation and slide it between the insulation and the inner liner, leaving about 4 inches of liner exposed. Wrap the connection with duct tape after inserting a splice collar into the end of the duct’s inner liner. A duct strap is used to secure the inner liner against the splice collar. Insert the collar into the duct until the rib of the collar is flush with the insulation roll.
  • Calculate the replacement section’s length: Add two feet to the distance between the collars of the existing ducts. For each bend, add an additional foot to the calculation.
  • Trim the new flex to the desired length: Extend the new flex duct to its maximum length. Tuck the outer liner between the insulation and the inner liner after rolling it over the insulation. Cut with a knife from the end of the insulation roll to the appropriate location on the duct. The outer liner should be rolled over the insulation.
  • Set up a new flex: Cover the splice collar with the inner liner, leaving about 1 inch of the collar exposed. Duct tape that is code-approved should be used to seal the connection. Use the appropriate tool to tighten a duct strap over the inner liner. Butt the outer liner and insulation of the new section against the existing ducts. Using a duct strap, secure it in place. Duct tape or code-approved mastic can be used to seal the joint.


If you feel overwhelmed and need help with HVAC Repair, give us a call.


Smart Living Home Repair Services

244 Madison Avenue , #1019

New York, NY 10016

(888) 758-9103

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