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Flex Duct Sizing

Lowe's Editorial Team

By Lowe's Editorial Team
Published September 25, 2024

Understanding flex duct sizing is essential to having an efficient HVAC system. Our guide will help you choose the right one by explaining the available types and sizes.

What is a Flex Duct

An HVAC system needs ductwork and one type of ductwork used is called a flex duct, which is short for flexible duct. It moves air from the HVAC system to different areas of a building. Flex ducts are made of plastic. They are known for being very adaptable and easy to install. They can fit into small spaces where rigid ducts cannot. Flex ducts can also be insulated to save energy and reduce noise. There are many different HVAC flex duct sizes to choose from, according to your needs.

Types of Flex Ducts

Flexible Ducts

Flexible air duct sizes are made from bendable plastic with wire reinforcement, which is perfect for tight spaces. They come in various sizes to meet airflow needs and are easy to install. These versatile connectors are ideal for joining air supply and return ducts in HVAC systems.

Semi-rigid Ducts

Semi-rigid ducts are built from sturdy materials like aluminum. They offer more stability but less flexibility compared to flexible ducts. These ducts are commonly used in areas where durability is essential, such as dryer vents. Available in a range of sizes, they provide a balance between rigidity and flexibility, making them suitable for specific ventilation needs.

Why Size Matters

Choosing the right size for your flexible ductwork ensures your ventilation system works well. To find the right size, consider a few key factors. First, figure out the airflow needs of your space, usually measured in cubic feet per minute (CFM). You can get this information from your HVAC system's specs or by asking a professional. Next, measure the length of the duct run and note any bends or turns, as these can affect airflow. It's also important to check the available space in walls, ceilings or floors where the ducts will go. By measuring these aspects carefully and understanding your system's needs, you can easily select the best option for your next flex duct project.

Common Flexible Duct Sizes

Flexible ducts are versatile and come in various sizes, making them suitable for different applications.

Insulated Polyester Flexible Duct

This insulated flexible duct size is commonly used for medium-sized HVAC systems, providing a balance of airflow and insulation. It is suitable for connecting air supply and return ducts to grills and diffusers.

  • 8-in x 300-in Insulated Polyester Flexible Duct

This size is great for medium to large systems, offering a good balance between size and insulation. It's commonly used in residential and light commercial applications.

  • 12-in x 300-in Insulated Polyester Flexible Duct

Ideal for larger systems needing more airflow, this duct size is often used in commercial or large residential buildings where significant air volume is required.

  • 7-in x 300-in Insulated Polyester Flexible Duct

Suitable for various applications, this size is versatile and can be used in both residential and commercial settings.

  • 10-in x 300-in Insulated Polyester Flexible Duct

For larger systems with high airflow requirements, offering good insulation and durability.

  • 4-in x 300-in Insulated Polyester Flexible Duct

Small and efficient, suitable for compact systems needing moderate insulation.

Foil Flexible Duct

Ideal for smaller HVAC needs, such as venting exhaust fans in bathrooms or kitchens.

  • 6-in x 300-in Foil Flexible Duct

Versatile for various uses, this duct size can handle medium airflow needs in residential and commercial settings.

  • 4-in x 96-in Foil Flexible Duct

Best for small spaces like bathroom exhausts, this size is compact and easy to install in tight areas.

  • 6-in x 240-in Foil Flexible Duct

A shorter option for medium applications, providing flexibility in installation without compromising on airflow.

Caution

To preserve airflow efficiency, make sure there are no abrupt bends or kinks while installing flex ducts.

Common Semi-Rigid Duct Sizes

Semi-rigid flexible ducts offer more stability and are often used in specific installations requiring durable materials.

  • 7-in x 8-in Aluminum Semi-rigid Flexible Duct

Great for specialized applications requiring a unique size for specific installations.

  • 6-in x 96-in Aluminum Semi-rigid Flexible Duct

Ideal for tighter spaces needing sturdy material, commonly used in high-heat areas like dryer vents.

  • 4-in x 8-ft Silver Semi-Rigid Flexible Duct

Perfect for bathroom exhaust systems, providing reliable venting with a sturdy design.

  • 3-in x 96-in Aluminum Semi-rigid Flexible Duct

Compact and sturdy for specific uses, it is often utilized in smaller ventilation applications.

  • 4-in x 96-in Aluminum Semi-rigid Flexible Duct

Versatile for various installations, suitable for venting exhaust fans and small HVAC systems.

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