Guidelines For HVAC Duct design and


Based on California guide lines


Layout duct system on floor plan and determine register positions and duct paths to optimize room air circulation and minimize duct length;



The following are minimum materials specifications recommended to achieve a
substantially tight installation that will last:


All Materials

  • Shall have a minimum performance temperature ratings per UL181 (ducts), UL181A (closure systems for rigid ducts), UL181B (closure systems for flexible ducts) and/or UL 181BM (mastic);
  • Shall have a flame spread rating of no more than 25 and a maximum smoke developed rating of 50 (ASTM E 84)

Factory-Fabricated Duct Systems

  • All factory-fabricated duct systems shall include UL 181 listed ducts with approved closure systems including collars, connections and splices;
  • All pressure-sensitive and heat-activated tapes used in the manufacture of rigid fiberglass ducts shall be UL 181A listed;
  • All pressure-sensitive tapes and mastics used in the manufacture of flexible ducts shall be UL 181B (tape) or UL 181BM (mastic) listed.

Field-Fabricated Duct Systems

  • Ducts:
    • Factory-made ducts for field-fabricated duct systems shall be UL 181 listed.
  • Mastic sealants and mesh:
    • Sealants shall be UL 181BM listed, non-toxic, and water resistant;
    • Sealants for interior applications shall pass ASTM tests C 731 (extrudability after aging) and D 2202 (slump test on vertical surfaces);
    • Sealants and meshes shall be rated for exterior use;
    • Sealants for exterior
      applications shall pass ASTM tests C 731, C 732 (artificial weathering test), and D 2202.
  • Pressure-sensitive tapes:
    • Pressure sensitive tape shall be that recommended by and meet the requirements of the flexduct manufacturer;
  • Drawbands:
    • Shall be either stainless-steel worm-drive hose clamps or uv-resistant nylon duct ties;
    • Shall have a minimum performance temperature rating of 165 degrees Fahrenheit (continuous, per UL 181A-type test) and a minimum tensile strength rating of 50 pounds;
    • Shall be tightened as recommended by the manufacturer with an adjustable tensioning tool.




The following are design, fabrication and installation guidelines that, if
carefully followed, will provide a duct installation that is substantially


General Issues


  • Ducts, plenums, and fittings should be constructed of galvanized metal, or flexible duct. Building cavities may not be used as a duct or plenum without a sealed duct board or metal liner.
  • The air handler box should be air-tight;
  • Air filters should be easily accessible for replacement, and evaporator coils should be easily accessible for cleaning;
  • Ducts should be configured and supported so as to prevent use of excess material, prevent dislocation or damage, and prevent constriction of ducts below their rated diameter;
  • Flexible duct bends should not be made across sharp corners or have incidental contact with metal fixtures, pipes, or conduits that can compress or damage the ductwork;
  • Sheet metal collars and sleeves should be beaded to hold drawbands.




All Duct Types


  • All joints and seams, including the seams of boots and Y connections of duct systems and their components should be sealed with tape and mastic, mastic and embedded mesh, or pressure-sensitive tape approved for use by the duct manufacturer and meeting UL181 specifications ("approved tape"); this includes around junctions of collars to distribution boxes and plenums;
  • At plenum and air handler joints seal with silicon caulking or foil tape. This is beneficial for possible future modifications and maintenance (furnace replacement etc.)
  • All sealants should be used in strict accordance with manufacturer's installation instructions and within sealants moisture and temperature limitations;
  • All tapes used as part of duct system installation should be applied to clean, dry surfaces and sealed with manufacturer's recommended amount of pressure or heat. If oil is present, taped surfaces should be prepared with a cleaner / degreaser prior to application;
  • It is recommended that all register boxes should be sealed to the drywall or floor with caulking or mastic.

Flexible Ducts

  • Flexible ducts should be joined by a metal sleeve, collar, coupling, or coupling system. At least 2 inches of the beaded sleeve, collar, or coupling must extend into the inner core while allowing a 1 inch attachment area on the sleeve, collar, or coupling for the application of tape;
  • The inner core should be mechanically fastened to all fittings, preferably using drawbands installed directly over the inner core and beaded fitting. If beaded sleeves and collars are not used, then the inner core should be fastened to the fitting using #8 screws equally spaced around the diameter of the duct, and installed to capture the wire coil of the inner liner (3 screws for ducts up to 12" diameter, and 5 screws for ducts over 12" diameter);
  • The inner core should be sealed to the fitting with mastic or approved tape;
  • Tape used for sealing the inner core should be applied with at least 1 inch of tape on the duct lining, 1 inch of tape on the fitting of flange, and wrapped at least three times;
  • The outer sleeve (vapor barrier) should be sealed at connections with a drawband and/or three wraps of approved tape;
  • The vapor barrier should be complete. All holes, rips, and seams must be sealed with mastic or approved tape.

Metal Ducts and Plenums

  • Metal-to-metal connections should be cleaned and sealed in accordance with manufacturer's specifications;
  • Openings greater than 1/16 inch should be sealed with mastic and mesh, or butyl adhesive tape;
  • Openings less than 1/16 inch should be sealed with mastic or UL-181A listed tape;
  • Special attention should be paid to collar connections to duct-board and/or sheet metal; seal around the connection with mastic;
  • Connections between collars and distribution boxes should be sealed with mastic or approved tape;
  • At least three equally-spaced #8 screws should be used to mechanically fasten round ducts (3 screws for ducts up to 12" diameter, and 5 screws for ducts over 12" diameter);
  • Crimp joints should have a contact lap of at least 1-1/2 inches;
  • Square or rectangular ducts should be mechanically fastened with at least one screw per side.

Duct Support

  • Supports should be installed per manufacturer's specifications or per UMC requirements;
  • Supports for flexible ducts should be spaced at no more than 4 foot intervals;
  • Flexible ducts should be supported by strapping having a minimum width of 1-1/2 inches at all contact points with the duct;
  • Supports should not constrict the inner liner of the duct;
  • Flexible ducts should have maximum of 1/2 inch sag per foot between supports;
  • Flexible ducts may rest on ceiling joists or truss supports as long as they lie flat and are supported at no more that 4 foot intervals.


  • After mechanically attaching the register boot to floor, wall, or ceiling, all openings between the boot and floor, wall, or ceiling should be sealed with caulk or mastic.

Note: that after rough duct install, all duct openings should be temporarily sealed against dust and debris.