Flexible duct connectors reduce noise without sacrificing efficiency, says John Guthrie, and provides a checklist to help select the appropriate material to ensure high indoor air quality.

The importance of finer details is something the HVAC professional appreciates, and a proper understanding of how these details interact with one another results in the highest performance possible. “High performance” in this instance means both enhancing the efficiency of a system and ensuring optimal comfort for a building’s occupants. Proper sealants and coatings ensure high indoor air quality. Proper cleaning and remediation result in optimal airflow and consistent temperatures. A properly designed HVAC system will also take into account the noise the system might produce, making flexible duct connectors one detail that simply cannot be overlooked.

What are flexible duct connectors?

Flexible duct connectors are the airtight flexible joints comprising a sheet of “fabric” which is secured on both sides by sheet metal, typically G-60 (or in the case of higher quality systems, G-90) galvanised steel. These are inserted between HVAC equipment and ductwork as a way to reduce vibrations created by the equipment. Without flexible duct connectors, vibrations created by the rattling of equipment would be generated directly to the metal ducts and amplified throughout the ductwork of a facility. Naturally, such a system would create a tremendous amount of noise, disrupting a building’s occupants.

Types of material

One of the goals in designing an HVAC system is reducing noise without sacrificing energy efficiency. In terms of materials, great strides have been made in this area over the past few decades. For more than half a century, canvas duct connectors were the standard. These were porous and often contained asbestos, which was outlawed in the late 1970s due to health risks associated with the material. Alternative materials were developed that offered enhanced strength, increased leak protection, and better value. Synthetic rubbers, compounds, and canvas-like materials treated with synthetic compounds, are the “fabrics” used today in flexible duct connectors.

Making the right selection

Codes: Selecting a suitable connector is first and foremost, a matter of codes. For instance, the International Mechanical Code allows for flexible ducts to be used for certain exhaust fans, but prohibits its use to vent clothes dryers. Mechanical and Residential codes may differ. Installation instructions often specify where a connector may be used. But be aware that you may have to refer to the Mechanical Code if you are unfamiliar with a particular type of connector.

Material: The next consideration that must be made is which material is best suited to the needs of the job. For example, indoor or outdoor, high or low temperature use, and application type could be deciding factors in selecting the material. Neoprene is the industry standard, suitable for commercial, residential and industrial use. It is suitable for indoor and outdoor applications and has fair degrees of chemical and temperature resistance. Vinyl is often more economical, and is considered a replacement for the old canvas connectors, but it lacks the UV stability for outdoor use. When strong direct sunlight is a concern, or in instances of UV filtration, Hypalon is a great material with similar temperature and chemical resistance to Neoprene. With a temperature range nearly twice that of the others, Silicone is often used in high temperature applications.

Seam: There is a final consideration that makes a difference in terms of flexible connector strength and energy efficiency – the seam. Regular seams place the fabric in direct contact with the clamp of the brake, which sandwiches the duct connector as it is being folded. Certain types of double-fold seams employ an offset, which prevents the brake from crushing the fabric when the connector is folded, offering better holding power, tear (and therefore, air leakage) resistance and fabric protection. Eliminating or minimising fabric contact with the brake will typically enhance the life of the fabric.

Flexible duct connectors are yet another detail that can have a huge effect on the comfort level of a facility’s occupants. Finding the suitable connector to reduce noise, determining the best fit for the application, and looking for the double-seam advantage, will all help move the design of an HVAC system to its highest possible level of efficiency.

The writer is Regional Sales Manager, Carlisle HVAC. He can be contacted at john.guthrie@ carlislehvac.com