Industry Articles From Kaempf and Harris

The Difference Between Flexible And Metal Ductwork


Kaempf & Harris' sheet metal fabricators give 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 a commercial HVAC system. Galvanized steel and aluminum ductwork last longer. This is especially true when proper maintenance occurs on a regular schedule. 
It requires computerized design, measurements, and careful ductwork installation. Experts consider the existing components of the commercial building's heating and cooling system. These considerations include the furnace, heat pump, air conditioner, and more. 
The width-to-height ratio in a commercial building is also a consideration. It should provide 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. This is comprised of grills and registers that are mounted in a commercial building's ceiling or walls.
Flex duct is convenient for attaching supply air outlets to more rigid ductwork made from sheet metal. It's often attached with long zip ties or metal band clamps. 
Rubber, PVC, stainless steel, silicone, polyurethane, and neoprene-dipped polyester fabric are popular types of flexible ductwork material
Rubber, PVC, stainless steel, and silicone are best for commercial buildings in extreme climates. This is because of an inherent resistance to heat and cooling ensures long-lasting productivity. Rubber and polyurethane offer the best abrasive resistance.
Silicon can't handle interior damage during extreme weather resistance. Yet, silicon and PVC offer the highest quality for air handling environments. This is because each offers flexibility for any heavy duty hose. 
How do these ductwork designs differ?
  • Price. Because metal ductwork lasts 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. This is 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. Yet, this leaves more room for contractor error. Contractor error can cost you in high energy bills, excess maintenance, or sudden repairs and replacements

    Metal duct is a pricey upfront investment for a commercial building, yet it saves 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 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
  • Poor fastened and sealed connections
  • Unsupported butt joints
While flex ducts work best for short runs in a trunk-and-branch system, metal duct works best for entire air distribution systems.
Metal ductwork needs design, measurement, and installation by or with the help of an HVAC professional. Metal also offers a lower resistance to airflow, and many of the problems listed above are irrelevant to sheet metal rigidness
The choice depends on what you're looking for in an HVAC system. Factors include location, number of building occupants, square footage, and more. Talk to a sheet metal fabricator or HVAC technician about your commercial building. 
For more information, download Kaempf & Harris' Guide To The Commercial Ductwork Standards. Click on the button below to get started:
Download The Guide To Commercial Ductwork Standards

Topics: Metals, Sheet Metal Fabrication

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