How to Repair Damaged Flex Duct

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How to repair damaged HVAC flex duct.

Ripped or damaged air conditioning flex ducts reduce the HVAC systems energy efficiency. This can add several dollars to each month's electricity bill. Calling an air conditioning duct mechanic to fix rips can be expensive. This type of service call is almost all labor and profit. Even in the worst cases damaged air conditioning flex duct can be repaired by a homeowner for far less money.

The materials you need include HVAC duct mastic, paint brush, duct tape, duct mesh, knife, wire cutters, and panduit straps. Panduit straps are the large zip ties that hold the flex in place. Slice connections will also require a metal splice collar. Sections of flex duct that has been completely crushed will need that section replaced with the same size flex and two collars. Flex has its size printed on the outside covering.

Flex Connections

Inspect the area where each flex run connects to a mixing box or a vent boot. If the flex connection has completely fallen off the collar then you must remove the old panduit straps and duct tape. There should be a strap on the inner layer and another around the outer layer.

Pull back the outer layer and the insulation. Reveal 8 to 12 inches of the inner layer. Slide the inner layer back onto the collar. Leave 1 inch of collar showing. Attach the inner liner in place with duct tape. Use a pair of pliers to install a new panduit strap over the inner liner. Pull the outer layer over the connection and install a panduit strap. Use a brush to apply duct sealing mastic to the connection.

Connections that are still connected but have small air leaks need the outer layer pulled back for inspection. If the inner is still attached simply seal the connection with mastic. Push the insulation back over the wet mastic and reattach with a panduit strap. Unattached inner layers will need to be completely redone. While you are at the connections make sure that the hanging straps are supporting the weight of the duct. Every place that there is a collar needs a hanging strap near-by.

Ripped Flex

This is the most common flex duct repair. During installation tiny rips go unnoticed. Through time these small tears become larger. The outer layer over the insulation acts as a vapor barrier. This keeps the cold air in and the hot air out. Close each rip with a piece of duct tape. place a section of duct sealing mesh over the tape. Cover the mesh with duct mastic. Large rips that require more than one layer of mesh should have a hanging strap close.

Crushed Flex Duct

There is not good way to repair crushed flex. All methods of rebending the wires will fail. The crushed section must be replaced. This does not mean that the whole duct run is bad. A new piece of flex can be spliced into place.

Cut the outer liner and insulation about six inches past the crushed section. Roll the outer liner over the insulation. Cut the inner liner 4 inches away from the rolled insulation. Wire cutters and a knife will be needed. Do this on both sides of the crush. Cut a section of new flex 1 foot longer than the piece that you removed. Roll both ends of the insulation. Install collars on both sides. To install the collars you will need to duct tape and strap the inner liner to both the original and the splice pieces of flex. Then pull the rolled ends together. Duct tape the seams. Wrap the seam with flex mesh and paint with mastic. A hanger must be installed close to each collar.

Repairing damaged flex duct will help lower your electric bill. You can save even more by doing the work yourself. The best part of this project is the money you invest now will come back to you each month from now on.