6.6002.4 Ducts (Exhaust Fans)

Desired Outcome: 
Installed ducts effectively move the required volume of air and prevent condensation
Duct design and configuration

Consideration will be given to:

  • Vent termination location
  • Amount of space for duct run
  • Roof condition, type, and access (e.g., metal, shingle, bow string, flat)
  • Duct insulation

When applicable, pitch duct to remove condensation to outdoors

Ducts will be as straight as possible, fully extended, and have the shortest run possible

Turns will be made so the radius at the centerline is no less than one duct diameter

Duct diameter will be equal to or greater than the exhaust fan outlet

Fan flow will be verified by flow measurement to meet ASHRAE ASHRAE American Society of Heating, Refrigerating and Air-Conditioning Engineers, www.ashrae.org Standard 62.2

Effectively move the required volume of air

Duct insulation

Ducts installed outside of the thermal envelope Envelope The separation between the interior and exterior environments of a building that includes a combination of air and thermal barrier will be insulated to a minimum of R-8 or in accordance with local codes

Prevent condensation from forming or collecting inside or outside of the ductwork

Duct support

Horizontal runs will be supported in accordance with flex duct manufacturer specifications and local codes

Supports with a width of at least 1 ½" will be used or adequate metal support

Effectively move the required volume of air

Preserve the integrity of the duct system

Duct connections

Metal-to-metal or metal-to-PVC connections will be fastened with a minimum of three equally spaced screws

Flexible duct-to-metal or flexible duct-to-PVC connections will be fastened with tie bands using a tie band Tie band A strap, often made of nylon, that mechanically squeezes a flexible duct to a fitting. Must have a minimum performance temperature rating of 165° (per UL 181A-type test) and a minimum tensile strength rating of 50 pounds tensioning tool

PVC-to-PVC connections will be fastened with approved PVC cement

Other specialized duct fittings will be fastened in accordance with manufacturer specifications

In addition to mechanical fasteners, duct connections will be sealed with UL UL Underwriters Laboratories 181B or 181B-M listed material

Effectively move the required volume of air

Preserve the integrity of the duct system

Duct materials

Flexible materials will be UL UL Underwriters Laboratories 181 listed or Air Diffusion Council approved

Rigid, smooth metal of 30-gauge wall thickness or thicker will be used

PVC material may be used

Effectively move the required volume of air

Preserve the integrity of the duct system

Total exhaust airflow

Total exhaust system ventilation airflow will be measured

Ensure air flow is as designed