Design Standard - Ductwork - Version 1 - 2011.9.30

Oct 24, 2026

Design standard for ductwork within the South San Francisco Unified School District HVAC systems, defining requirements for design, specification, installation, and testing of ductwork and related accessories. Covers supply/return, outside air, and exhaust systems, including hangers, plenums, gas vents, and flexible ducts. Establishes material and performance criteria such as pressure classification, thermal transmission, vapor transmission, airflow velocity, flame/smoke ratings, seismic restraints, and outdoor waterproof/slope design. Provides installation constraints and approved manufacturers, plus allowed substitutes based on documented equivalency.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages3

Reading Time8 min

AccessFree to download

Purpose

Scope and Design Requirements

Systems Covered

Material and Compliance Standards

Flexible Ductwork Requirements

Thermal, Vapor, and Fire/Acoustic Criteria

Installation Constraints for Flexible Ducts

Gas Vent Requirements

Positive Pressure Gas Vents

Gravity Gas Vents

Approved Manufacturers and Substitutes

Associated Design Standards

South San Francisco Unified School District Section 23 31 00 Version 1, 2011.9.30 Design Standard Ductwork Purpose: Ductwork is an essential element of the mechanical space cooling and heating systems. This design standard has the purpose of creating a consistent application of ductwork requirements throughout the South San Francisco Unified School District therefore achieving a standard of quality for maintenance, energy efficiency, and reliability throughout all renovation and new building projects. Design Standard: Design and specify work to include materials, installation and testing of HVAC ductwork and accessories, including the following: Heating and air conditioning supply and return systems Outside air systems Exhaust systems Ductwork hangers Plenums Gas vents 􀁸 Galvanized Steel Ductwork: Carbon steel, lock-forming quality, hot-dip galvanized, with spangle-type zinc coating, double seam without showing fracture. Conform to ASTM A525 and A527 . 􀁸 ASHRAE and/or SMACNA shall be used as a guide. 􀁸 Design medium pressure ductwork at 0.1”/100’ pressure drop 􀁸 Design low pressure ductwork at 0.08”/ 100’Pressure classification shall be specified on the drawings. 􀁸 All metal ductwork shall be cross broken to ensure rigidity. 􀁸 Inlet and discharge ductwork configuration shall conform to the SMACNA HVAC Duct Design Manual. 􀁸 Seismic restraints shall be designed per SMACNA requirements 􀁸 All ductwork located outdoors shall be designed to be waterproof and sloped for water run-off 􀁸 Flexible ducts: 􀀐 Standard factory fabricated product, construct an inner wall of impervious vinyl or chlorinated polyethylene, permanently bonded to a vinyl or zinccoated spring steel helix. Cover the assembly with fiberglass blanket insulation covered by an outer wall of vinyl or fiberglass-reinforced metalized vapor barrier. UL 181 listed Class 1 flexible air duct material. South San Francisco Unified School District Section 23 31 00 Version 1, 2011.9.30 Overall thermal transmission no more than 0.25 (BTU/in)/(hr/sq.ft./deg. F) at 75F differential, per ASTM C335 . Vapor transmission value no more than 0.10 perm, per ASTM E96 . Rated for a minimum of 4-inch w.g. positive pressure and 1-inch w.g. negative pressure. Air friction correction factor of 1.3 maximum at 1000 FPM. Working air velocity of at least 2000 FPM. Flame spread rating no more than 25. Smoke development rating no more than 50 as tested per ASTM E84 . Must have cataloged data on insertion loss characteristics, minimum attenuation of 29 DB for 10-foot straight length at 8-inch diameter and 500 Hz. 􀀐 Install flexible duct with bend radius equal to 1.5 times the diameter. Minimum length 2 feet. Maximum length 5 feet. 􀀐 Provide round neck grilles/diffusers or square-to-round transitions. No flex duct connections directly to square neck allowed. 􀀐 Flex duct allowed only for vertical drops to diffusers. Maximum offset angle from vertical: 30 degrees. 􀀐 Approved for use on supply ducts only; not allowed for return or exhaust. 􀀐 Flex duct allowed in concealed spaces above lay-in ceilings only 􀁸 Positive pressure gas vents: 􀀐 Doublewalled insulated piping system. Construct the inner wall of at least 0.035-inch-thick Type 304 stainless steel. Construct the outer wall of at least 0.025-inch-thick aluminized steel. Fabricate duct to provide space between the inner and outer walls of at least 1 inch. List system by UL as 1400F Factory Built Chimneys with 2-inch clearance to combustibles for use with No. 2 fuel oil-fired equipment. Join sections by means of a system, capable of sealing gastight to pressure up to at least 60-inch w.g. for temperatures up to 600F. Fittings ofthe same manufacture and construction as the straight sections 􀁸 Gravity gas vents: 􀀐 Type "B" factory fabricated, UL listed, doublewall flue, with aluminum inner wall, galvanized steel outer wall and 1/2-inch air space between unless noted otherwise. Provide t

FAQs

What ductwork systems are included in the design standard?

The standard specifies heating and air conditioning supply and return systems, outside air systems, and exhaust systems, along with related components such as hangers and plenums.

What performance limits are required for ductwork?

It sets limits for overall thermal transmission and vapor transmission, requires ratings for minimum pressure, specifies an air friction correction factor and minimum working air velocity, and includes flame spread and smoke development ratings.

Under what conditions is flexible ductwork allowed?

Flexible ducts are restricted by layout and application: bend radius and length limits apply, round neck grilles/diffusers or proper transitions are required, flex duct is limited to vertical drops to diffusers with offset angle limits, and it is approved only for supply ducts, not for return or exhaust.

Oct 24, 2026

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