Industry Articles From the Kaempf and Harris Blog

The Difference Between Flexible And Metal Ductwork

Posted by Kaempf & Harris on November 30, 2017


Following the format of our popular sheet metal blog about spiral and rectangular ductwork, Kaempf & Harris' metal fabricators are giving you an inside look on the basics and differences between metal and flexible ducts:

What's a metal duct?

Metal ducts (also called rigid or hard ducts) offer a more permanent solution to the HVAC system. Galvanized steel and/or aluminum ductwork lasts longer than its counterparts, especially when the proper maintenance is performed on a regular schedule. 

It requires computerized design, measurements, and careful ductwork installation by experts with consideration for the existing components of the commercial building's heating and cooling system, including the furnace, heat pump, air conditioner, and so on. 

The width-to-height ratio in your commercial building also must provide adequate space for metal ducts to optimize energy efficiency and performance. 

What's a flexible duct?

Flexible or flex duct is an integral part of an air conditioning system. The flexible ducting connects the heating and cooling system ductwork to the air distribution, which is comprised of grills and registers that are mounted in your commercial building's ceiling or walls.

Flex duct is convenient for attaching supply air outlets to more rigid ductwork made from sheet metal and is commonly attached with long zip ties or metal band clamps. 

Rubber, PVC, stainless steel, silicone, polyurethane, and neoprene-dipped polyester fabric are the most popular types of flexible ductwork material available. 

The first four materials are best for commercial buildings located in extreme climates because of their inherent resistance to heat and cooling ensures long-lasting productivity. Rubber and polyurethane offer the best abrasive resistance, while silicon typically can't handle interior damage during extreme weather resistance. 

However, silicon and PVC do offer the highest quality for air handling environments because they offer flexibility for any heavy duty hose. 

How do these ductwork designs differ?

Price. Because metal ductwork tends to last longer than flex duct, it's more costly to create, transport, and install.

Flexibility. Flex ducts are better for existing trunk-and-branch heating and cooling systems because they're more versatile and flexible. Metal ducts are more rigid due to the nature of steel, making them ideal to build an entire HVAC system. 

Installation. In comparison to metal duct, flex duct is easier and faster to install. However, this leaves more room for contractor error, which can cost you in high energy bills, excess maintenance, or sudden repairs and replacements. 

Metal duct is a pricey upfront investment for your commercial building, but it can save you money by maintaining high indoor air quality and regulating indoor temperature no matter the outdoor climate. 

Which is best for your commercial building?

A lot of contractors have issues with flex duct, but that's mainly because a lot of ductwork installation jobs with flex are haphazard. According to Green Building Advisor, the most common problems are the following: 

  • Kinks and sharp turns
  • Lengthy and unsupported duct runs
  • Radial systems with an excess number of ducts coming off the plenum
  • Unmeasured takeoffs that are too close
  • Extra duct length
  • Poorly fastened and sealed connections
  • Unsupported butt joints

While flex ducts work best for relatively short runs in a trunk-and-branch system, metal duct works best for entire air distribution systems.

Metal ductwork must be designed, measured, and installed by or with the help of an HVAC professional. They also offer a lower resistance to airflow, and many of the problems listed above are irrelevant to sheet metal rigidness. 

Overall, it largely depends on what you're looking for in an HVAC system. Factors include location, number of building occupants, square footage, and more. It's best to talk to a sheet metal fabricator or HVAC technician for insight on your specific commercial building. 

For more information, click on the button below to download Kaempf & Harris' Guide To The Commercial Ductwork Installation Process:

Download The Ultimate Guide to Commercial Ductwork

Topics: Sheet Metal Fabrication, Metals

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