Nobody has written a good residential ductwork guide — the field is PDFs and forum fragments. This is it: trunk-and-branch vs radial layouts, Manual D explained in plain language, honest rules of thumb, takeoffs, sealing, and return air, written for the helper and the installer.
Ductwork is the delivery system the equipment depends on: a perfectly sized furnace connected to bad ducts performs like a bad furnace. Yet most installers learn duct design by copying the last job, which is how undersized returns and noisy supplies propagate for decades.
This guide gives you the fundamentals the trade usually skips: how air actually moves through ducts, how Manual D sizes them, and the rules of thumb that work when you understand their limits. Learn the why and the how follows.
Ductwork installation must meet the mechanical code as adopted locally, including construction standards, sealing, and insulation requirements. This guide teaches the fundamentals — it does not replace Manual D training or supervised field experience.
Ductwork falls under the mechanical code (IMC) as adopted locally, including duct construction standards (SMACNA), sealing requirements, and insulation R-values. References are to current model codes — check your local amendments, since energy code requirements vary significantly by jurisdiction and climate zone.
Start from the equipment: every ton of cooling needs roughly 400 CFM of airflow, and the duct system's job is delivering that air to each room in proportion to its load. A 3-ton system needs about 1,200 CFM total — and if the ducts can only move 900, the equipment cannot perform no matter how good it is. Airflow is the foundation; everything in duct design serves it.
Trunk-and-branch runs a large main trunk with smaller branches tapping off — compact, familiar, and efficient in basements with open joists. Radial (spider) systems run individual ducts from a central plenum to each register — more ductwork, but each run is independently sized and balanced. Extended plenum is the hybrid most homes actually get. Match the layout to the house's structure, not to habit.
Manual D is just friction accounting: every foot of duct and every fitting adds resistance, and the blower can only overcome so much. The design picks a friction rate (typically around 0.08–0.10 inches of water column per 100 feet for residential), then sizes each duct section so its friction loss matches. Software does the arithmetic now — your job is understanding that undersized ducts mean high friction, high static, and low airflow.
The honest rules of thumb: 400 CFM per ton, 6-inch round handles roughly 100–120 CFM, 7-inch about 150–200, 8-inch about 200–250 (at typical residential friction rates). Trunk ducts step down as branches leave. These get you in the neighborhood — Manual D gets you to the address. Never present a rule-of-thumb size as engineered, and never skip the static pressure check that proves the system works.
Takeoffs should scoop air in the direction of flow — spin-in takeoffs with dampers are the pro standard for round branch connections. Keep branch runs as straight and short as possible, support flex duct every 4–5 feet with wide straps (never wire that cinches it), and avoid sharp bends: a kinked flex duct delivers a fraction of its rated airflow. Every unnecessary fitting is friction you chose.
Seal every joint, seam, and connection with mastic or approved foil tape — not cloth duct tape, which dries out and fails. Unsealed ducts in unconditioned spaces leak 20–30% of the system's air, which is energy paid for and never delivered. Sealing is the highest-return hour in the whole duct job, and inspectors in energy-code jurisdictions check it.
Size returns generously — undersized return is the single most common duct defect, causing noise, high static pressure, and starved equipment. Every bedroom with a supply needs a return path: dedicated return, transfer grille, or undercut door with a clear path. A system with great supplies and no returns is like breathing through a straw.
Measure external static pressure against the equipment's rated maximum — this single number tells you whether the duct system can breathe. Then balance: adjust the dampers at each takeoff so every room gets its design CFM, verified with a flow hood or anemometer. A duct system that is installed, sealed, balanced, and verified performs; one that is just installed merely exists.
Any duct running through an attic, crawl space, or garage needs insulation — typically R-6 to R-8 depending on your energy code and climate zone — with an intact vapor barrier on the outside. Uninsulated supply ducts in a hot attic lose a shocking amount of capacity before the air ever reaches the room, and in humid climates the cold duct surface condenses moisture that drips onto ceilings. Insulate after sealing, keep the vapor barrier continuous and undamaged, and support insulated ducts so the insulation does not compress and lose its R-value.
Every one of these has cost someone a callback. Learn them here instead of on the job.
The most common duct defect in existence. Starved returns cause noise, high static, and equipment-killing airflow.
Flex duct installed like a garden hose — sharp bends and sagging runs that choke airflow to a fraction of rated capacity.
Unsealed ducts in attics and crawl spaces leak 20–30% of the air. Mastic every joint — it is the cheapest efficiency in the house.
Throwing air at windows that do not exist or behind furniture. Register placement follows the room's heating and cooling loads.
Installing without measuring external static pressure means never knowing if the system can actually move its rated airflow.
Ductwork itself is tin — but the changeout it comes with is copper. Duct jobs ride along with full system replacements: the condenser coil, the line set, the old air handler's blower motor with copper windings, and the control wire. The sheet metal goes to the tin pile; the copper goes in your bucket.
Strip the motors, coils, and line sets off every changeout. No minimums — every bucket counts. When the pile builds up, call Sonny at 312-880-8174.
No minimums — every bucket counts. On bigger lots, we handle the freight so you don't lift a finger.
Work to code. Hire a licensed pro where required. This guide explains how the work is done — it does not replace training, apprenticeship, or a license.