24.1 Installing Sheet Metal, Fiberglass & Flexible Duct

24.1 Installing Sheet Metal, Fiberglass & Flexible Duct

Trade Area D — Equipment Component Installation — is 14 percent of both the Class A and Class B trade exams. After Chapter 23 set condensers, compressors, and air handlers, the outline next asks you to install sheet-metal duct, fiberglass (fibrous-glass) duct, and flexible duct. Chapter 19 was the shop: gauge, Pittsburgh locks, seal class. This section is the field hang. The 2026 Air Conditioning CBT books that matter here are SMACNA HVAC Duct Construction Standards, Metal and Flexible, 4th Edition (2020), SMACNA Fibrous Glass Duct Construction Standards, 8th Edition (2021), Florida Building Code — Mechanical, 2023 Chapter 6, Florida Building Code — Energy Conservation, 2023 (attic R-8), and NFPA 90A (2015) / NFPA 90B (2024) for occupancy and fire. Class B still hangs every product in this section; the Class B outline does not drop Trade D. It drops 25–100 ton, over-100 ton, and centrifugal system installs (Chapter 21). A 16-ton strip-center trunk is still a SMACNA hang on a Class B ticket (F.S. 489.105(3)(g) caps 25 tons / 500,000 Btu in any one system).

Quick Answer: Metal follows FBC Mechanical 603.4 and SMACNA hangers. Round metal needs three screws and a 1-inch crimp lap with the male end in the direction of airflow. Fibrous-glass board is UL 181 Class 0 or 1, closed with 181A tape or mastic, air not over 250°F. Flexible air ducts are not length-limited by 603.6.1.1. Flexible air connectors are limited to 14 feet and shall not pass through any wall, floor, or ceiling. SMACNA still wants flex fully extended, supported at least every 5 feet, sag ≤ 1/2 inch per foot, and a centerline bend radius of at least one duct diameter.

Why install is a different question from fabricate

Pearson VUE will hand you a hang spacing, a flex label, a three-screw joint, or a 14-foot connector and ask whether the field assembly is legal. A perfect Pittsburgh in the shop that is then hung from a ceiling grid, crushed into an 8-inch flex accordion, or taped to a collar with no clamp is a failed Trade D item. FBC Mechanical 603.1 says the air-distribution system shall be installed to supply the required air and shall not damage the fire protection the Building Code assigned to the wall you just punched. 603.10 sends metal hangers to SMACNA; factory-made flex follows the manufacturer and SMACNA’s flex pages.

Installing sheet-metal duct

FBC Mechanical 603.4 constructs metallic ducts to SMACNA HVAC Duct Construction Standards — Metal and Flexible. The dwelling-unit thickness exception (Table 603.4) still stops near 20 inches and 1 in. w.g. — Chapter 19 already used that table. In the field you still:

1. Match the pressure class on the shop ticket (½ through 10 in. w.g., positive or negative).
2. Fasten round joints with not less than three sheet-metal screws or rivets spaced equally (603.4.1). If the joint is partly inaccessible, three fasteners go on the exposed portion so the pipe cannot hinge.
3. Lap crimp joints not less than 1 inch, male end extending into the adjoining duct in the direction of airflow (603.4.2). A female-into-male “backwards crimp” is a dirt dam and a leak.
4. Seal every longitudinal seam, transverse joint, and connection per 603.9 and the energy spec. Metal and flex tapes/mastics are UL 181B (181 B-FX tape, 181 B-M mastic). Mechanical fasteners on flexible nonmetallic duct are 181 B-C. The < 2 in. w.g. exception for locking joints does not excuse snap-lock and button-lock — those still need the extra closure.
5. Support per SMACNA hanger tables (603.10). Typical rectangular strap or trapeze spacing is on the order of 8 to 10 feet depending on the largest side; smaller round often stretches farther. Do not hang from a lay-in ceiling grid. Do not use hanger wire as a choke around insulated flex. Strap width and trapeze design come from the SMACNA table for that size, not from leftover #9 wire.
6. Isolate the first connection at the air handler with a listed flexible connector so fan vibration does not fatigue the first transverse joint. That connector is still a 603.6.2 product if it is a flexible air connector — 14 feet max, no wall penetrations.

Underground metal without an approved coating is completely encased in not less than 2 inches of concrete (603.8), sloped 1/8 inch per foot to an accessible drain point (603.8.1), and leak-tested before burial. 603.14 keeps ordinary duct out of the earth and 4 inches away from it unless you are on the 603.8 detail. Outdoor duct gets weather protection (603.16). FBC Energy R403.3.1: supply and return ducts in attics, 3 inches diameter and greater, insulated to R-8 (smaller attic ducts R-6). Construction class and insulation class are different rows on the takeoff.

Worked hang. A 16-ton Class B rooftop in Pinellas feeds a 24×12 2 in. w.g. supply (over 2 feet on the long side — Chapter 19 already picked TDC) and 14-inch spiral runouts. Field: TDC gasket compressed, hangers on the SMACNA rectangular row (not 12-foot “whatever the bar joist is”), 14-inch couplings with three screws and 181 B-M mastic, attic wrap R-8, and a listed connector at the curb — not 20 feet of flex used as the trunk.

Installing fibrous-glass (fiberglass) duct board

FBC Mechanical 603.5: nonmetallic ducts are Class 0 or Class 1 material complying with UL 181. Fibrous construction shall conform to SMACNA Fibrous Glass Duct Construction Standards (the exam book is the 8th Edition, 2021) or NAIMA. Air temperature inside shall not exceed 250°F. Closures are UL 181A, marked 181 A-P (pressure-sensitive tape), 181 A-M (mastic), or 181 A-H (heat-activated tape). 181B flex tape on board is the wrong listing. Cloth “duct tape” is neither.

Board is not interior-lined metal (Chapter 19.2). Typical SMACNA fibrous-glass rows keep the product on the order of 2 in. w.g. class and about 2,400 fpm. Support is closer than metal: commonly 6-foot intervals with 2-inch-wide saddles so the board is not pinched into a restriction. Shiplap joints are stapled in the flap, then closed with the 181A system. Screws that crush the facing the way you screw a drive cleat are not a fibrous-glass joint. Do not use board for Type I grease duct, clothes-dryer exhaust, dishwasher steam, or outdoor runs without a weather jacket. Do not bury it as 603.8 underground duct.

Installing flexible duct — ducts versus connectors

This is the highest-yield hang item on Trade D.

Both products: design air temperature < 250°F (603.6.3); clearance to an appliance per the appliance listing (603.6.4). Connection: inner core pulled over the collar at least 1 inch, listed 181 B-C clamp or strap, then 181 B-FX tape or 181 B-M mastic — tape alone is not the mechanical fastener (603.9). Fully extend the helix. A 25-foot compressed bag used as a 12-foot run is extra equivalent length, extra leakage, and extra sag. Do not rest flex on ceiling grid, do not staple through the core, and do not make a 90-degree fold against a truss (that inner radius is near zero, not one diameter).

Worked flex example. An 8-inch runout needs a 2-foot drop, a 90, and 6 feet to the boot. Buy listed air duct, stretch it, support at the takeoff, at midspan, and at the boot (all < 5 ft), keep the inner radius ≥ 8 inches (centerline ≥ 8 inches for a 1×ID rule), clamp and 181B-seal both ends. Substituting a connector for that run and passing it through a rated corridor wall is two independent failures: 14-foot / no-penetration on the connector, and a missing UL 555 fire damper on the rated wall.

Manual D and ACCA still prefer short metal plus a short flex runout because flex friction is higher than stretched metal of the same ID. That is design. It is not a secret FBC 5-foot length cap on air ducts. The exam trap is applying 603.6.2.1’s 14 feet to a product labeled air duct.

Florida HVAC scenario

Citrus Ridge Air, a certified Class B shop in Polk County, change-outs a 4-ton air handler in a 130°F attic. The helper uses one 25-foot connector (round 14-foot label) as the supply trunk, leaves it compressed, supports it at the unit and at the far boot 18 feet apart, folds it 90 degrees over a truss with a 2-inch inner radius, and tapes the collar with cloth tape and no clamp. Four independent Trade D failures: (1) 603.6.2.1 / 603.6.2.2 — a connector over 14 feet and it should not be the trunk through framing cavities treated as “walls”; (2) SMACNA 5-foot support and 1/2-inch-per-foot sag; (3) bend radius < 1 diameter; (4) 181 B-C clamp plus 181B closure, not cloth tape. Class B is legal on 4 tons. The license does not rewrite SMACNA or 603.6. If the same attic needed R-8 wrap on metal runouts (Energy R403.3.1), skipping insulation is a fifth failure that the hang still has to carry.

Traps: (1) Treating every gray spiral as an unlimited “flex duct.” Read the UL 181 label: duct versus connector. (2) Hanging flex from wire that pinches the core. (3) Three screws on round metal but the male crimp facing upstream. (4) 181B tape on fibrous-glass board. (5) Using Table 603.4 dwelling gauges on a commercial 2-inch-class hang.