HVAC Duct System Installation – Part 3


Duct Sealing

  • The largest loss of efficiency in a ducted air system comes from air leaks.
    • The biggest leak source in many older systems is found when building cavities are used for return air.
    • The largest sources of leaks are around plenums and takeoffs.
    • The use of garden-variety duct tape with no mastic is the next most often encountered problem.
  • Sealing the duct system during installation is relatively simple.
  • Sheet-metal ductwork is most commonly sealed with mastic.
  • Fiberglass ductboard is sealed with UL-181 aluminum foil tape.
  • Flexible duct is sealed with both UL-181 tape and mastic.
  • All duct sealants should be UL-181 approved.
  • Duct tape was the primary method of sealing metal duct, but tape use is less common today.

Duct Cleaning

  • The National Air Duct Cleaners Association establishes duct-cleaning standards and duct-cleaning technician certifications.
  • System components, including ducts, accumulate a fine layer of dust over time.
    • In most systems there is more dust accumulation in the return than the supply.
  • Mold can also sometimes grow in ductwork.
    • The mold can be cleaned, but if the root cause is not addressed, the mold will return.
  • A visual inspection is necessary to determine the need for cleaning.
  • A visual inspection after cleaning is necessary to assess the results.
  • Tools are available that enable inspection of the duct system via remote video cameras.
  • Most duct-cleaning equipment is some form of specialized vacuum cleaner.

Flexible Duct and Flexible Connectors

  • Both the UL 181 standard and the NFPA 90 standard differentiate between flexible air duct and flexible air connectors.
    • A flexible air duct must pass all fifteen of the UL 181 tests.
    • Air connectors are not required to pass the flame penetration, puncture, or impact tests in Standard UL 181.
      • Air connectors are limited to 14 ft.
      • Air connectors are limited to temperatures less than 250°F.
      • Air connectors are not allowed to pass through floors.
      • Air connectors are not allowed to pass through walls that are required to have a fire rating.
    • The markings on the duct indicate whether it is rated as a flexible duct or a flexible connector.
      • Flexible ducts have a square or rectangular label.
      • Flexible connectors have a circular label.
    • Flexible duct is available in insulation R values of 4.2, 6.0, and 8.0.
    • Flexible duct is the easiest ducting to install.

Connecting Flex Duct

  • The basic method of connecting flex duct is to slide the inner core at least 2 in. over a metal beaded connector, roll back the outer insulation, fasten the inner core, and then roll the insulation back in place and fasten it.
  • Two approved methods are used for fastening the core and insulation:
    • Using UL 181B duct tape
    • Using UL 181B mastic and straps

Stretching Flex Duct

  • Flex duct by its very nature does not want to stay straight.
  • It is necessary to pull it straight to eliminate sags.
  • The maximum allowable sag between supports is 0.5 in./ft.

Trimming Flex Duct

Flex duct should be trimmed to the length needed and no more, leaving extra duct leads to sags, bends, and crimps that severely reduce airflow.

Supporting Flex Duct

Flex duct is not heavy, but it needs more support than metal duct because it has no structural strength of its own.

Flex duct should be supported every 5 ft with straps or hooks that are at least 2 in. wide. Do not support by wire.

A common abuse of flexible duct is squeezing and twisting it to fit through places that are simply not large enough for the duct.


Underwriters Laboratories

Standards 181, 181A, and 181B are probably the most referenced duct standards. Any manufactured duct should comply with UL 181 standards.

The flame spread index is a number relating to ASTM E 84 flammability testing. Class 0 ducts have a flame spread index of 0. Class 1 ducts have a flame spread index not over 25 and a smoke developed index of not over 50. The smoke developed index compares the amount of smoke produced.

Many mechanical codes require that mastic and tape be UL 723 rated for metal duct.

UL Standard 181A addresses closure and sealing systems for manufactured rigid ducts, such as fiberglass ductboard. UL Standard 181 B addresses closure and sealing systems for flexible ductwork.

Sheet Metal and Air Conditioning Contractors National Association

  • SMACNA publishes several duct and installation standards.
  • The most commonly referenced is HVAC Duct Construction Standards—Metal and Flexible.
    • It covers a lot of the nuts-and-bolts aspects of constructing and installing ductwork.
    • Topics include:
      • Duct joining
      • Duct sealing
      • Duct hanging



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