Terminology

• Supply / return / exhaust / outdoor air duct: duct functions
• Pressure class: design static pressure rating, in. w.g.
• Seal class: joint sealing level per SMACNA
• Spiral duct: round duct with helical lock seam; efficient and quiet
• Flexible duct: short connectors to diffusers; limited lengths
• Duct liner / external wrap: acoustic lining or thermal insulation
• Fire damper / smoke damper / combination damper: dampers at rated barriers and smoke control boundaries
• Plenum return: ceiling space used as return air path
• Diffuser / register / grille: supply outlets with or without dampers, and return inlets
• Throw / NC: distance air travels from a diffuser, and noise criteria rating

Standards

Ratings

• Duct construction by pressure class per SMACNA standards
• Materials exposed in plenums limited to flame spread 25 and smoke-developed 50
• Nonmetallic and flexible ducts listed to UL 181
• Fire and smoke dampers listed to UL 555 and UL 555S

Test standards

• SMACNA HVAC Duct Construction Standards, Metal and Flexible
• UL 181: factory-made air ducts and connectors
• UL 555 / UL 555S: fire dampers and smoke dampers
• ASTM E84 / UL 723: surface burning characteristics
• ASHRAE Handbook of Fundamentals: duct design
• ACCA Manual D: residential duct design

Components

• Galvanized steel ducts: standard for most systems
• Stainless steel or aluminum ducts: kitchen, lab, and corrosive exhaust
• Fibrous glass duct board: low-pressure residential and light commercial where allowed
• Flexible ducts and connectors: short final connections
• Dampers: volume, fire, smoke, and combination
• Terminal devices: ceiling diffusers, linear slots, sidewall registers, return grilles

Detailing

• Reserve ceiling zones and shaft sizes for main ducts during schematic design
• Locate fire and smoke dampers with access panels at rated walls and shafts
• Keep flexible duct runs short and straight
• Coordinate diffuser layout with ceiling grid, lighting, and sprinklers
• Use round or flat-oval ducts and acoustic lining near quiet spaces
• Seal ducts to the specified seal class and test leakage where required

Uses

Specific uses

• Supply and return air for AHU and RTU systems
• Exhaust from toilets, kitchens, and labs
• Outdoor air ducts from DOAS and ERVs
• Smoke control systems
• Residential forced-air systems

Pros and cons

Pros

• Delivers ventilation, heating, and cooling through one system
• Centralized filtration and conditioning
• Flexible diffuser layouts for open spaces
• Long service life

Cons

• Consumes ceiling and shaft space
• Leakage and poorly sealed joints waste energy
• Noise without proper sizing and attenuation
• Fire and smoke dampers need access and testing

Manufacturing and cost

Construction

• Ducts fabricated in sheet metal shops from coil; spiral duct formed on spiral machines
• Hung from structure, sealed, insulated, tested, and balanced

Main manufacturers and suppliers

• Ductmate (duct connectors and accessories)
• Hart and Cooley
• Titus
• Price Industries
• Greenheck (dampers)

Trade associations: SMACNA and AMCA.

Sources

• UpCodes: 2024 IMC Chapter 6 duct systems, https://up.codes/viewer/department-of-defense/imc-2024/chapter/6/duct-systems
• Newfoundland and Labrador: Division 23 master specification index (23 31 13 metal ducts), https://www.gov.nl.ca/ti/works/masterspec/div23
• SMACNA: technical standards, https://www.smacna.org/
• Wikimedia Commons: HVAC duct installation in the future LIRR passenger concourse. (CM014B, 03-21-2019) (33565522128).jpg, by MTA Capital Construction Mega Projects (CC BY 2.0), https://commons.wikimedia.org/wiki/File:HVAC_duct_installation_in_the_future_LIRR_passenger_concourse._(CM014B,_03-21-2019)_(33565522128).jpg