How to Work With Round Flex Duct

Flex duct is an integral part of an air conditioning system. The flex duct connects the HVAC system ductwork to the air distribution. The air distribution is the grills and registers that are mounted in your ceiling or walls. If the flex duct is not properly installed, the air conditioning system will not perform efficiently. This inefficiency will cause your air conditioning unit to work harder to heat and cool your home, which can result in premature wear to the unit's internal components.

Things You'll Need

• Flexible duct
• Flex duct scissors or long sharp knife
• Wire cutters
• Non-metallic flexible duct clamps
• Duct clamping tool
• Duct tape

Instructions

1. Open the box of flex duct. You will notice that the flex is compacted. Remove the flex from the box. Grab the inner liner of the flex. Quickly jerk the inner liner up and down to line up the inner liner with the insulation and outer shell. Hold the inner liner, the insulation, and the outer shell together. Jerk the flex hard to fully stretch out the flex duct. With one hand, hold the inner liner. Tuck the outer shell down and under the insulation. You should see no exposed insulation, just the outer protective shell and the inner flex duct.

2. Measure the distance between the air distribution inlet and the round tap on the ductwork. Allow two times the diameter of the flex duck at all bends that are 45 degrees or over. For example, if the distance between the tap and grill is 6 feet and you are installing 12-inch round flex, you need to add at least 24 inches to the length for the bend. It is important that the flex have enough room to bend without restricting airflow. Round up the length to the nearest foot. Count the 12-inch markings on the flex ducts outer sleeve until you reach your cut length.

3. Pierce the flex duct with the flex duct scissors or a long sharp knife. Cut around the entire circumference of the flex duct. Pull the cut pieces of flex apart, and you will notice a metal support wire connecting the pieces. Cut the support wire with the wire cutters. Tuck the outer shell down and under the insulation.

4. Place the flex duct between the round ductwork tap and the grill inlet. Insert the tapered end of the non-metallic duct clamp into the clamps large gripping end. Leave the clamp open enough to allow you to easily slide the clamp over the outer shell of the flex duct. Perform the same operation on another clamp. Slide one clamp over both ends of the flex duct.

5. Slide one end of the flex duct over the round ductwork tap. Pull the clamp over the flex's inner liner. Tighten the clamp with the duct clamping tool. While you have pressure on the duct clamping tool, pull the small lever near your index finger to cut the strap. Wrap duct tape around the duct clamp. Continue to wrap the duct tape over the end of the inner liner of the flex duct. Install the flex to the grill inlet, using the same steps as the connection to the round tap. Pull the insulation and outer sleeve over each connection, and fasten the outer layer with another duct clamp and a wrap of duct tape.

6. Support the flex duct on long runs. The support strap must be at least 1 inch thick to avoid pulling the inner liner into the airflow. If the hanger support obstructs the airflow, turbulence will result. The turbulence increases friction loss in the duct.

References

• SMACNA -- HVAC Duct Construction Standards Metal and Flexible Second Addition, Chapter 3, pages 3.15 thru 3.21