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Tech Tip: Flex Duct

Flex duct is much easier, quicker, and less expensive to install in a building than hard duct.  Flex duct is soft and flexible making it easy to squeeze it though tight places, bend it around corners, and compress excessive lengths at fittings.  The ability to bend and compress makes the flex duct a popular choice among contractors. These are the very attributes that make flex duct restrictive.  Flex duct has the highest friction lost when compared to sheet metal duct or fiberglass duct board.

Safety First.  You will be working with sharp objects and fiberglass.  PPE is always recommended.

Airflow is the #1 issue in our industry for component and unit failures.  Poorly installed and not properly sized duct is one of the main reasons.  There are several things that will help improve air flow through the flex duct.

* Pull the flex tight.  Excessive duct should always be removed.  If only 15 feet is needed, don’t bunch up the extra flex at the connections or loop extra flex.  Support flex duct every 4 to 6 feet with wide straps and never pinch the duct.

* Keep bends to a minimum and use hard elbows or elbow supports when possible.  Every 90-degree flex duct bend is the equivalent of adding 20 feet of flex.  A 10-foot flex duct run with two 90-degree flex duct turns has the same friction loss as a 50-foot straight flex duct run.

* Size the flex duct for the Job, one size does not fit all applications.  Both under sizing and oversizing of flex duct will affect the volume and velocity of air.  When replacing hard duct, do not use the same size flex.  Hard duct and flex duct have very different characteristics.

* Return boxes and plenums are a must.  These will reduce the turbulence of the air as it enters or exits an air conditioning system.

*Duct leakage will occur when duct is not sealed properly.  Take care to seal start collars, elbows, and connectors.  When connecting the inter core, secure the flex core with a flexible duct strap or metal band and then use a high-quality tape overlapping the core and metal to insure an airtight connection.  It is highly recommended to use a screw to secure the core wire to help prevent the flex duct from pulling away.  Properly sealing the insulation jacket is often overlooked.  Secure the insulation jacket with a flexible duct strap or metal duct clamp and tape.  Note: the metal duct clamp will not become brittle and break over time.

**Check local building codes for Flex Duct R Values.  Per local code, R-8 is now required in many municipalities.

Flex duct can be used as an alternative to hard duct when sized and installed properly.  No short cuts.  Take the time to do a quality install.