Sheet Metal & HVAC Glossary

The ratio of a rectangular duct's width to its height — a 24"×12" duct has a 2:1 aspect ratio. SMACNA recommends keeping this at 4:1 or below whenever possible. High aspect ratios increase surface area relative to the cross-section, driving up friction loss and material cost. A lot of coordination problems on job sites get solved by someone agreeing to flip the duct.

A machine used to make straight bends in sheet metal. The cornice brake bends the full length of the sheet in one operation. A box-and-pan brake uses removable fingers so you can bend the sides of a box without hitting already-bent material. Most custom HVAC work still gets shaped on a cornice brake — fast and requiring little setup for standard ductwork.

A longitudinal seam on round and flat oval duct where a folded lock is mechanically staked at intervals by a dimple pressed through both layers. Common on spiral duct and machine-formed round pipe. Not the same as a Pittsburgh. Button punch is for round; Pittsburgh is for rectangular.

The standard unit of airflow volume in HVAC. Every duct, fitting, and piece of equipment is ultimately sized to move a certain number of CFM. Velocity (FPM) × duct area (sq ft) = CFM.

The flat side panels of a rectangular elbow — the two faces parallel to the airflow direction and perpendicular to the turn. Typically the largest panels on the fitting and carry the reinforcement on larger elbows. Throat, heel, and two cheeks — that's your elbow.

A short cylindrical fitting connecting a round branch duct to a main trunk. Can be a simple flanged hole, a stamped spin-in, or a fully fabricated saddle collar. Spin-ins are faster to install; fabricated collars are more structurally sound in high-velocity systems.

A flat strip of sheet metal, formed with two flanges, connecting adjoining duct sections at the top and bottom of a rectangular joint. The drive slides into S-cleat receivers on each duct end and is hammered tight. Standard drives are 3" wide; hemmed drives add rigidity on larger duct. Half of every slip-and-drive connection. The other half is the S-cleat.

Fibrous glass insulation board adhered or mechanically fastened to the inside of metal duct. Provides thermal insulation and acoustic absorption. A 12"×12" duct lined with 1" liner has an effective airway of 10"×10" — liner must be accounted for in sizing. Always size the metal to the outside dimension unless the engineer specifies otherwise.

A duct sizing approach where every section is designed to the same friction rate — typically 0.1" WG per 100 feet. By holding friction rate constant, the system tends toward natural balance. The most commonly used method for commercial HVAC and the basis for most duct sizing calculators.

Expressing the pressure drop through a fitting as a length of straight duct that would produce the same loss. A 90° elbow might have an equivalent length of 30 feet. Used in duct design to account for fitting losses without calculating each one separately. SMACNA and ASHRAE publish equivalent length tables for common fittings.

The static pressure a fan or air handler must overcome — everything outside the unit: ductwork, fittings, dampers, diffusers, filters, and coils. Equipment is rated for a specific ESP; exceeding it reduces airflow.

A duct profile with two flat parallel sides connected by semicircular ends — a round duct compressed in one dimension. Carries more air than a rectangle of the same height and fits tighter vertical spaces than round. More efficient than rectangular, less so than round.

Pressure drop per 100 linear feet of straight duct, in inches of water gauge. The standard design friction rate for most commercial supply duct is 0.1" WG per 100 feet. Higher friction rates mean smaller duct and faster air; lower rates mean larger duct and quieter delivery.

A number indicating sheet metal thickness — counterintuitively, higher gauge numbers mean thinner material. 26 gauge (0.0179") is standard for most low-pressure residential duct. 24 gauge (0.0239") is used for larger or higher-pressure applications. SMACNA tables specify minimum gauges by duct width and pressure class. When someone says "go heavier," they mean a lower gauge number.

The standard material for HVAC ductwork — cold-rolled steel with a zinc coating applied by hot-dipping. The G designation indicates coating weight: G90 has more zinc and better corrosion resistance than G60. Most commercial duct is G90. The zinc coating can outgas when welded and should not be cut with a torch in enclosed spaces.

The outer radius of a rectangular elbow — the long way around the turn, opposite the throat. The heel carries the most material in a radius elbow and is where turning vanes anchor on long-radius fittings. On a flat 90° elbow, the heel is a straight corner with no radius.

A fold made by bending the edge of sheet metal back on itself, creating a doubled-over edge. Used to eliminate raw sharp edges, add stiffness, or create a receiver for a seam. A single hem folds once; a double hem folds twice. Hem widths are typically 3/16" to 3/8" depending on gauge and application.

See Duct Liner above. In shop conversation "liner" almost always means internal acoustic/thermal board. External wrap insulation is usually just called "wrap" or "duct wrap."

The seam that runs the length of a duct section, closing the sheet into a tube or box. On rectangular duct this is typically a Pittsburgh seam. On round duct it may be a button punch snap lock or a spiral seam. SMACNA classifies longitudinal seam types by pressure class.

A thick, paste-like sealant used to air-seal duct joints and seams. Applied by brush or glove over the connection and embedded in mesh tape on larger gaps. The correct way to seal duct. Silver cloth duct tape fails over time with heat cycling. UL 181-listed mastic holds indefinitely. If a building inspector is watching, use mastic. If the customer is watching, use mastic. Use mastic.

A fitting that shifts the centerline of the duct in one or two planes without changing size or direction of travel. Used to route around structural members, pipes, or other obstructions. Offsets are one of the most commonly custom-fabricated fittings because the specific dimensions rarely match a catalog item.

The standard longitudinal seam for rectangular sheet metal duct. One edge forms a pocket; the adjacent edge is driven in and hammered flat. Produces a mechanically interlocked, air-resistant seam without fasteners. Named for its development in Pittsburgh-area shops in the early 20th century. The machine that forms the pocket is called a Pittsburgh machine, or just "the Pitts."

A chamber or box that collects or distributes air before it enters or exits the duct system. A supply plenum sits on top of the furnace; a return plenum collects air from multiple return grilles. Plenums operate at lower velocity than branch ducts and are typically fabricated from heavier gauge.

A SMACNA classification determining minimum construction requirements based on operating static pressure. Common classes are ½", 1", 2", 3", 4", and 6" WG. Most residential and light commercial duct is ½" or 1" pressure class.

Methods used to stiffen duct panels against pressure-induced deflection. Cross-breaking (diagonal creases) adds rigidity by triangulating the surface. Hat channels, angle iron, and tie rods are used on larger duct. SMACNA tables specify reinforcement type and spacing by duct width and pressure class. Ignoring reinforcement schedules is one of the most common code violations in duct installation.

A formed strip with an S-shaped cross-section applied to the top and bottom edges of a rectangular duct section to receive the drive cleat. S-cleats and drive cleats together make up the slip-and-drive connection used on the majority of rectangular duct joints.

The dominant joint method for rectangular duct. S-cleats on the top and bottom of each section; a drive cleat slid through both and hammered tight on the sides. Fast to assemble in the field with no fasteners. Not suitable for high-pressure duct without supplementary sealing and mechanical fasteners. On a bad day on a big job, you'll hammer a thousand of these. Your forearm knows when it's done right.

The pressure exerted uniformly in all directions by air inside a duct — the "push" against the duct walls, measured in inches of water gauge. Positive = supply side (pushing out); negative = return side (pulling in). All duct sizing, construction standards, and equipment ratings are based on static pressure.

The inside radius of a rectangular elbow — the short way around the turn. On a flat 90° elbow the throat radius is zero; on a radius elbow it's typically equal to the duct width. Sharp-throat elbows generate significantly more turbulence and pressure drop than radius elbows of the same size.

A fitting that changes the cross-sectional shape or size of the duct. One of the most commonly custom-fabricated fittings in the shop because specific dimensions almost never match a catalog item. Gradual transitions (under 15°) lose less pressure than abrupt ones.

Curved metal blades installed inside a sharp-throat elbow to redirect airflow smoothly, reducing turbulence and pressure drop. A vaned 90° elbow can have an equivalent length 5–10× shorter than an unvaned one of the same size. Installing vanes correctly takes longer than making the fitting. That time is worth it on large duct.

How fast air is moving through the duct. CFM ÷ duct area (sq ft) = velocity (FPM). Recommended velocities: main trunks 700–1,000 FPM; branch ducts 600–900 FPM; final runouts 400–600 FPM. Higher velocity means more noise, more friction loss, and more system pressure required.

The pressure component associated with the kinetic energy of moving air — the "push" in the direction of travel. Total pressure = static pressure + velocity pressure. Velocity pressure is typically small in HVAC duct systems but matters significantly in fan selection and high-velocity applications.

The unit of pressure measurement used throughout HVAC — the height of a column of water (in inches) that a given pressure would support. One inch WG equals approximately 0.036 PSI. Used because HVAC pressures are very low relative to other mechanical systems and PSI would produce inconveniently small decimal numbers.

A fitting that splits a duct trunk into two branches at an angle — as opposed to a tee, which branches at 90°. Wyes produce less turbulence and lower pressure drop than tees because airflow doesn't make an abrupt turn. In high-performance duct design, wyes are preferred at main branch takeoffs wherever geometry allows.