2021 IMC Chapter 6
Ducts move air, but they can also move smoke and flame if nothing stops them. Chapter 6 of the 2021 IMC does three jobs. It says where air may and may not travel, including corridors and plenums. It says how ducts and their insulation are built and sealed. And it keeps smoke and fire from riding the duct system through a building, using duct smoke detection and dampers. The figure follows one air handler: the return plenum, the duct smoke detector, the outdoor air connection, the filter, and a supply duct crossing a rated wall through a fire damper. Local adoption governs which mechanical code applies (Lesson 1).
A corridor may not serve as a supply, return, exhaust, relief or ventilation duct, with exceptions such as makeup air for exhausted rooms that open onto a corridor with its own outdoor air supply, and return air in tenant spaces of 1,000 square feet or less (601.2). Exhaust ducts under positive pressure, chimneys and vents may not pass through ducts or plenums (601.4). Return air openings have their own rules (601.5): at least 10 feet from an open combustion chamber or draft hood in the same room, no more return than the supply delivered to the room, and no return from a closet, bathroom, toilet room, kitchen, garage, boiler room, furnace room or unconditioned attic. A kitchen may give return air if the opening is at least 10 feet from the cooking appliances.
A plenum is a space that carries air, and 602.1 limits plenums to uninhabited crawl spaces, areas above a ceiling or below a floor, attics, mechanical rooms and certain framing cavities. A plenum is limited to one fire area, air systems are ducted from the boundary of the fire area served to the air handler, and fuel-fired appliances never sit inside one. Surfaces exposed to the airflow are noncombustible or have a flame spread index of 25 or less and a smoke-developed index of 50 or less (602.2). Gypsum board may form a plenum only where the air stays at or below 125 degrees F and the board stays above the dew point. Other materials inside a plenum follow the same 25 and 50 limits, and combustible cable is plenum-rated or run in metal raceway or metal sheathed cable (602.2.1.1). Stud cavities and joist spaces may act as plenums only if they carry no supply air, are not part of a rated assembly, serve one floor level, and are not in exterior walls (602.3).
Ducts outside single dwelling units are sized by the ASHRAE Handbook of Fundamentals or an equivalent method (603.2). They are classified by the pressure they can carry, from 0.5 to 10 inches of water column, and the class must at least equal the system's design pressure (603.3). Metal ducts follow SMACNA; the code's own gauge table applies only inside single dwelling units (603.4). Round metal ducts get at least three screws or rivets per joint, and a crimped joint laps at least 1 inch with the male end pointing with the airflow (603.4.1, 603.4.2). Joints and seams are sealed with welds, gaskets, mastics or tapes marked to UL 181A or 181B (603.9). Flexible ducts are listed Class 0 or Class 1 and, in the 2021 code, are not limited in length. Flexible air connectors are limited to 14 feet and may not pass through a wall, floor or ceiling (603.6). Underground ducts are approved for it and sloped at least 1/8 inch per foot to an access point, and unprotected metal is encased in at least 2 inches of concrete (603.8); otherwise ducts stay at least 4 inches out of the earth (603.14). Branch ducts or outlets get balancing dampers with access (603.18).
Duct coverings and linings, adhesives included, are listed and labeled with a flame spread index of 25 or less and a smoke-developed index of 50 or less (604.3). A duct running above 120 degrees F is insulated so its exposed surface is no hotter than 120 degrees F (604.2). External insulation is marked at intervals of 36 inches or less with the maker, R-value and flame and smoke ratings (604.7). Linings stop at a fire damper and 6 inches either side of an electric or fuel-burning heater (604.8), and coverings do not pass through a wall or floor that is fire-rated or fireblocked (604.6).
Detectors used in ducts are listed for that service. Section 606.2.1 requires a detector in any return air system with a design capacity above 2,000 cfm, in the return duct or plenum upstream of any filters, exhaust air connections, outdoor air connections or decontamination equipment. Where several air handlers share supply or return ducts or plenums and their combined design capacity is above 2,000 cfm, the shared return gets detectors too (606.2.2). A return air riser serving two or more stories and any part of a return system above 15,000 cfm gets a detector at each story (606.2.3). Duct detectors are not required in the return air system where area smoke detectors, tied to a fire alarm system, protect every part of the building the system serves (exception to 606.2.1). Detectors monitor all of the airflow, follow NFPA 72 and have access for maintenance (606.3). When one trips, the system shuts down as its listing directs, or a smoke control system switches to smoke mode (606.4). Where a fire alarm system is required, duct detectors connect to it and give a supervisory signal (606.4.1).
Worked example. Three air handlers in a school wing share one return plenum, with design capacities of 900, 800 and 600 cfm. Each alone is under 2,000 cfm, but the combined capacity is 900 + 800 + 600 = 2,300 cfm, which is above the limit. The shared return needs detectors under 606.2.2, upstream of any filter or outdoor air connection.
| Damper | Tested to | Rating in the 2021 IMC |
|---|---|---|
| Fire damper | UL 555 | 1 1/2 hr where the assembly is under 3 hr; 3 hr where it is 3 hr or more (Table 607.3.2.1) |
| Smoke damper | UL 555S | Leakage Class I or II; elevated temperature at least 250 degrees F (607.3.2.2) |
| Combination fire/smoke damper | Both | Both ratings |
| Ceiling radiation damper | UL 555C, or the tested assembly | Protects the ceiling membrane of a rated floor/ceiling or roof/ceiling assembly |
| Corridor damper | UL 555 and 555S | 1-hour rating, Class I or II leakage (607.3.2.4) |
A fire damper closes when a heat-responsive device operates at about 50 degrees F above the normal duct temperature and not less than 160 degrees F (607.3.3.1). A smoke damper closes on a signal from a listed smoke detector. Where the damper is in a duct, the detector is in the duct, or has sampling tubes in it, within 5 feet of the damper, and no air inlet or outlet sits between them (607.3.3.2). Dampers are installed to their listing and the manufacturer's instructions, and a damper listed only for static systems goes where the system shuts down automatically in a fire (607.2.3).
Listed fire dampers protect duct and transfer opening penetrations of fire walls, fire barriers, fire partitions and rated exterior walls that need protected openings (607.5.1 to 607.5.3, 607.5.6). Shafts that ducts may penetrate get fire and smoke dampers (607.5.5). Smoke dampers protect smoke barriers, corridors that need smoke and draft control doors, and smoke partitions (607.5.4, 607.5.7). Where both are required, a combination damper or both devices are used. Ducts through the ceiling membrane of a rated floor/ceiling or roof/ceiling assembly need a shaft or a ceiling radiation damper (607.6.2).
Exceptions save many dampers. In a fire barrier or fire partition rated 1 hour or less, in a building sprinklered throughout and outside Group H, a fully ducted system of sheet steel at least 26 gage, continuous from the air handler to the terminals, needs no fire damper (607.5.2 and 607.5.3). Other exceptions cover tested assemblies and ducts in an approved smoke control system. Read the exceptions against the wall type before demanding a damper, and again before waiving one.
A damper needs an access door big enough for inspection and service; one with a fusible link or internal operator needs a door at least 12 inches square, or a removable duct section (607.4.1). The opening must not reduce the wall's rating. Access points are labeled on the outside in letters at least 1/2 inch high: FIRE DAMPER, SMOKE DAMPER or FIRE/SMOKE DAMPER (607.4.2).
In the field
A church education wing remodel runs a new supply duct through the 1-hour wall between the fellowship hall and the classrooms. The plans show a fire damper, but the wall sits above a finished ceiling, so nobody planned an access door, and the damper's label would be hidden. Check the damper's rating (1 1/2 hour for this wall), its access door and label, and whether the duct really qualifies for the sprinklered, fully ducted exception before accepting or waiving it. On the return side, confirm the detector sits ahead of the outdoor air connection and filter, and add up shared systems: three small units can total more than 2,000 cfm.
1. An inspector reviewing a Group B office building's air handler finds no duct smoke detector on its return. Under 2021 IMC 606.2.1, detectors are required in return air systems with a design capacity greater than:
2. Where is a return air duct smoke detector placed relative to the filters and outdoor air connection?
3. Three air handlers in a Group E school wing share one return plenum with design capacities of 900, 800 and 600 cfm. Do detectors apply under 606.2.2?
4. A duct passes through a 1-hour fire barrier and needs a fire damper (no exception applies). What is the minimum fire damper rating under Table 607.3.2.1?
5. An inspector finds a duct smoke detector 12 feet from a smoke damper in the same duct. Under 607.3.3.2, the detector or its sampling tubes must be within:
6. A fire damper's heat-responsive device should operate at about how much above the normal duct temperature, but not less than 160 degrees F (607.3.3.1)?
7. Plenum surfaces exposed to airflow must be noncombustible or have flame spread and smoke-developed indices of not more than:
8. A Group E school design uses the corridor itself as a duct to deliver supply air to the classrooms. Under 601.2, a corridor may not serve as:
9. Under 602.3, a stud wall cavity used as a plenum may NOT be used for:
10. A damper with a fusible link or internal operator requires an access door of at least what size (607.4.1)?
11. A supply duct in a sprinklered Group B building crosses a 1-hour fire partition. The system is fully ducted 26 gage sheet steel from the air handler to the terminals. Is a fire damper required at the partition?
12. Fire, smoke and fire/smoke damper access points are labeled with letters at least how high (607.4.2)?
13. A flexible air connector in a Group B building may not exceed which length under 603.6.2.1?
14. Unprotected metal duct installed underground must be encased in at least how much concrete (603.8)?