How to Use Flexi Duct for Heaters

Connecting Duct to Collars and Vents

• 1

  Check your furnace plenum to see if it has sheet metal collars protruding from the plenum. Some furnaces are already equipped with 2-inch sheet metal collars that protrude from the plenum. If your furnace does not, install collars. Measure the diameter of the plenum hole and purchase a compatible sheet metal collar. Bend every other tab on the end of the collar to a 90-degree angle. Insert the collar with the tabs pointing toward the furnace, into the hole. Reach inside the collars and firmly press the remaining tabs down to press down and around the inside of the plenum. The tabs will hold the collar in place. Repeat this installation for every opening in the furnace plenum.

• 2

  Measure the first run of duct from the sheet metal collar in the furnace plenum to your heat register. Measure a run of flexible duct and cut the duct with scissors and the wire with wire cutters. Fold back the layers of insulation and the outer jacket of the duct approximately 4 to 5 inches.

• 3

  Slide a metal clamp onto the sheet metal collar. Slide 1 inch or more of the wire-supported core of the duct over the sheet metal collar. Apply two or more wraps of duct tape to secure the core to the collar. Pull the metal clamp onto the duct core, over the duct taped area but behind the beaded end of the collar. Tighten the clamp with the screwdriver until the clamp tightly grips the collar. The beaded end of the collar will prevent the clamp from slipping off.

• 4

  Unroll the insulation and outer jacket of the duct so that they cover the clamped area and abut the furnace plenum where the collar protrudes. Apply at least two layers of duct tape over the jacket to hold the insulation and jacket in place.

• 5

  Have an assistant extend the first 4 to 5 feet of flexible duct toward the heater output vent duct. Loop a vinyl plastic cable tie or a flexible support strap under the duct and connect the tie or strap to the wooden joist above the duct. Ensure that the duct is fully extended with no kinks, bends or bunched up areas. Avoid sags greater than 1/2-inch per foot by adding more supports, if necessary. Do not bend the duct greater than 90 degrees and keep the centerline radius of the duct under one duct diameter width. You may hang the duct across joists but do not allow the duct to drape over them, as this will create friction loss resulting in diminished efficiency. Always support the ducts with ties or support straps.

• 6

  Continue extending and supporting the flexible duct as you approach the heater output vent duct. Install the duct to the heater output vent in the same manner as the sheet metal collar on the furnace plenum. Roll back the outer layer, insert a metal clamp to the vent duct, slide the flexible duct onto the heat duct, tape securely, tighten the clamp, tape securely, replace the outer layer, tape securely again.

Connecting Duct to Duct

• 7

  Roll back the insulation and outer jacket layers on both ends of the ducts approximately 4 to 5 inches.

• 8

  Insert a 4-inch sheet metal sleeve inside the first flexible duct and position the sleeve so that roughly 3 inches extend from the flexible duct core. Tape the core to the sleeve with at least two layers of duct tape. Insert a metal clamp over the sleeve and duct tape, and securely tighten the clamp with a flathead screwdriver.

• 9

  Slide a second metal clamp over the sleeve. Do not tighten the second clamp yet, but push it back next to the tightened clamp for now. Slide the flexible duct core over the protruding end of the sheet metal sleeve until the duct core abuts the duct core of the first duct. Tape the second core onto the sheet metal sleeve with at least two layers of duct tape. Pull the second clamp over the duct taped area and tighten.

• 10

  Roll back the layers of insulation and outer jackets of both ducts. Connect and secure both sides with at least two layers of duct tape.