9.3 Plenums, Air Connectors, Sizing & Application

Duct & Plenum Use is 2 percent of the M2, but it is the section that turns a legal product into a legal application. The flex can be a Class 1 air duct, the metal can be SMACNA 006, and the job still fails if the above-ceiling void is treated as a free-for-all, if a stud bay is used as a supply, or if a clothes dryer is tied to the HVAC return.

A plenum, in Chapter 2 language, is an enclosed portion of the building structure — other than the occupiable space being conditioned — that is designed to allow air movement and thereby serve as part of an air-distribution system. A ceiling cavity, a raised floor, an attic, or a mechanical-room enclosure can be a plenum. A corridor used as a giant duct usually cannot. Read 602 for the cavity and 601.2 / 601.5 for the occupied-space air path.

What may serve as a plenum

2024 IMC 602.1: Supply, return, exhaust, relief, and ventilation-air plenums shall comply with Section 602. Fuel-fired appliances shall not be installed within a plenum. 602.1.3 repeats that sentence so it cannot be missed. A gas furnace sitting in a ceiling-return cavity, or a unit heater hung inside a supply plenum, is a stop before you look at cable ratings.

602.1.1 Locations limited. Plenums shall be limited to uninhabited crawl spaces, above a ceiling or below the floor, attic spaces, mechanical equipment rooms, and the framing cavities addressed in 602.2. Occupied rooms, corridors, and tenant spaces are not “bonus plenums.”

602.1.2 Limited to a fire area. Plenums shall be limited to one fire area. Air systems shall be ducted directly from the boundary of the fire area served to the air-handling equipment. A return plenum that freely crosses a fire-barrier line, then shows up at a rooftop unit two fire areas away, is a 602.1.2 fail. Duct the air at the fire-area boundary and apply Section 607 at that penetration.

602.2 Construction of plenums. Enclosure materials exposed to the airflow shall comply with IBC 703.3, or they shall have a flame-spread index not more than 25 and a smoke-developed index not more than 50 when tested to ASTM E84 or UL 723. Gypsum used to form a plenum is limited to systems where air temperatures do not exceed 125°F and the gypsum surface stays above the airstream dew point. Supply-air plenums formed by gypsum boards shall not be incorporated in air-handling systems utilizing direct evaporative cooling. That is the plenum version of the 603.5.1 gypsum-duct rule.

602.2.1 Stud cavity and joist space plenums is the list you will be asked. Those cavities or spaces shall comply with all of the following:

1. They shall not be utilized as a plenum for supply air.
2. They shall not be part of a required fire-resistance-rated assembly.
3. Stud wall cavities shall not convey air from more than one floor level.
4. They shall comply with IBC floor-penetration protection.
5. They shall be isolated from adjacent concealed spaces by approved fireblocking.
6. Stud wall cavities in the outside walls of building-envelope assemblies shall not be utilized as air plenums.

A cold-air return using an interior, non-rated stud bay on a single floor can be legal if fireblocking and floor penetrations are right. A supply “panned joist,” a two-story stud chase used as a return, or an exterior-wall cavity used as any plenum is a 602.2.1 fail.

Ceiling-cavity return plenums are the ordinary commercial arrangement. They are legal when 602.1.1, 602.1.2, 602.2, and 602.3 are met. 601.2.1 adds a corridor-ceiling rule: the space between a corridor ceiling and the floor or roof above may be a return-air plenum only if one of five conditions is true — the corridor is not required to be fire-resistance-rated; the corridor is separated from the plenum by fire-resistance-rated construction; the air handler shuts down on its smoke detectors; the air handler shuts down on sprinkler waterflow in a fully sprinklered building; or the space is part of an approved engineered smoke-control system.

601.2 is the occupied-corridor rule: corridors shall not serve as supply, return, exhaust, relief, or ventilation-air ducts. Exceptions include corridor makeup air for toilet rooms and similar rooms that open onto a corridor that is itself supplied with outdoor air at a greater rate; dwelling-unit corridors used for return; tenant spaces of 1,000 square feet or less; and ASHRAE 170 transfer air in health-care pressurization. “We just use the corridor as the return” is not an exception.

Materials, wiring, and pipe in plenums

2024 IMC 602.3: Materials within plenums shall be noncombustible or shall comply with 602.3.1 through 602.3.10. Exceptions: materials exposed in one- and two-family dwellings; combustible materials fully enclosed in continuous noncombustible raceways, approved gypsum-board assemblies, or materials listed and labeled for plenum installation; and Group H, Division 5 fabrication areas that share a recirculation path.

602.3.3 Wiring is the field favorite. Combustible electrical wires and cables and optical-fiber cables exposed within a plenum shall be listed and labeled as having a peak optical density not greater than 0.50, an average optical density not greater than 0.15, and a flame-spread distance not greater than 5 feet when tested to NFPA 262, or they shall be installed in metal raceways or metal-sheathed cable. Combustible optical-fiber and communication raceways exposed in a plenum shall meet the same numbers to UL 2024. Only plenum-rated wires and cables shall be installed in plenum-rated raceways. A plenum-rated plastic innerduct stuffed with ordinary CM cable is still a fail.

Field — combustible cable in a return-air plenum. NM-B (Romex), ordinary CM communications cable, or unlisted thermostat wire laid on the ceiling grid of a return plenum is a 602.3.3 fail unless it is in metal raceway or metal-sheathed cable. CMP / plenum-rated cable that actually carries the NFPA 262 listing is the intended exposed product. Metallic-enclosure electrical equipment is the 602.3.6 exception; a plastic junction body is not.

Other 602.3 listings to recognize on a stem:

• 602.3.4 plastic fire-sprinkler piping: wet-pipe only, listed to UL 1887 at the same 0.50 / 0.15 / 5-foot numbers.
• 602.3.5 combustible pneumatic tubing: UL 1820, same numbers.
• 602.3.6 discrete electrical, plumbing, and mechanical products with exposed combustibles: UL 2043 (exception: electrical equipment with metallic enclosures).
• 602.3.8 plastic plumbing piping: flame spread 25 / smoke 50, or UL 2846 at the 0.50 / 0.15 / 5-foot numbers.
• 602.3.9 pipe and duct insulation in plenums: flame 25 / smoke 50 to E84 or UL 723 using ASTM E2231, plus ASTM C411 at not less than 250°F, listed and labeled. Insulation cannot be used to “fix” a pipe’s flame/smoke numbers unless the pipe and insulation are tested as a composite under 602.3.8.

Supply versus return versus exhaust; connectors versus ducts

All five plenum types — supply, return, exhaust, relief, ventilation — sit under 602.1. The extra limits are application limits. 602.2.1 bans stud/joist cavities as supply. Return-air openings still follow 601.5 (10 feet from an open combustion chamber or draft hood; no return from a bathroom, toilet room, kitchen, garage, boiler room, furnace room, or unconditioned attic; closet returns regulated, not banned). That list is taught with heating equipment; do not re-learn a different 601.5 on a plenum stem.

601.4 Contamination prevention: Exhaust ducts under positive pressure, chimneys, and vents shall not extend into or pass through ducts or plenums. Limited exceptions exist for certain Chapter 4 recirculation rooms and for listed positive-pressure vents or vents run in a sealed enclosure. A positive Type I riser, a dryer exhaust, or an unlisted Category IV vent joint sitting in a ceiling return is a 601.4 / Chapter 5 problem.

Air connectors versus air ducts is still the 603.6 split. A listed flexible air duct is not length-limited (603.6.1.1) but still cannot pass a fire-resistance-rated assembly (607.7). A flexible air connector is capped at 14 feet, shall not pass through any wall, floor, or ceiling (603.6.2.1 / 603.6.2.2), and also cannot pass a fire-resistance-rated assembly (607.7). Where 607 describes a “fully ducted HVAC system” for a damper exception, that system is continuous 26-gage (0.0217-inch) sheet steel from the air handler to the inlet and outlet terminals. Flexible air connectors are allowed in that fully ducted definition only as (1) a nonmetallic connection from a duct to equipment in a mechanical room, or (2) a 603.6.2 connector from an overhead metal duct to a ceiling diffuser in the same room.

Sizing: approved drawings, not a memorized CFM table

2024 IMC 603.2 is the sizing sentence. Ducts installed within a single dwelling unit shall be sized in accordance with ACCA Manual D, the appliance manufacturer’s installation instructions, or other approved methods. Ducts installed within all other buildings shall be sized in accordance with the ASHRAE Handbook of Fundamentals or other equivalent computation procedure.

The M2 does not ask you to recite a cfm-per-ton table. The inspector verifies the installed sizes against the approved drawings and the named method. 601.5 Item 4 says the same thing for return and transfer openings: size them from the manufacturer’s instructions, ACCA Manual D, or the registered design professional. 601.5 Item 3: the amount of return air taken from any room shall be not greater than the supply air delivered to that room.

Field fail — undersized return. The supply trunk matches the drawing; the return is a single 14-inch flex on a 7½-ton rooftop. Static pressure is high, doors slam, and a fuel-fired furnace in the same system starts spilling. You do not need a memorized 400-cfm-per-ton number to fail that job. You need the approved drawing, 603.2, 601.5 Items 3 and 4, and 608.1: air-distribution, ventilation, and exhaust systems shall have a means to adjust to the design airflow rates and shall be balanced by an approved method. Ventilation balancing shall verify the system can supply and exhaust the Chapter 4 rates.

Prohibited uses

Do not let a Chapter 6 product become a Chapter 5 system:

• Clothes-dryer exhaust is Chapter 5. It is not a return-air plenum, not a flexible air connector to an HVAC trunk, and not a gypsum shaft.
• Type I kitchen grease is Chapter 5. Slope, clearance, cleanouts, and welded steel are not 603.8.1 / 603.9 mastic rules.
• Hazardous and product-conveying exhaust is Chapter 5.
• 601.2: a corridor is not a duct (except the listed exceptions).
• 602.1 / 602.1.3: no fuel-fired appliance in a plenum.
• 601.5 Item 7: no HVAC return from a bathroom, toilet room, kitchen, garage, boiler room, furnace room, or unconditioned attic.
• 601.4: no positive-pressure exhaust, chimney, or vent through a duct or plenum except the written exceptions.

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