HVAC ductwork carries cooled air from the air handler to living spaces and provides a route for air to return to the system. Its design, condition, and connection to the equipment affect whether the installed AC can deliver the intended airflow and comfort. A properly selected unit alone cannot compensate for ducts that are poorly sized, leaky, or mismatched to the blower.
What ductwork does in a central AC system
Supply ducts carry conditioned air from the air handler toward rooms. Registers or grilles release that air into the spaces being cooled. Return grilles and return ducts bring room air back to the equipment, where it can be filtered and cooled again. Together, these parts form the system’s air-distribution path.
If that path is restricted, leaky, or poorly arranged, some rooms may receive less airflow than intended while others receive too much. Duct problems can contribute to uneven comfort and wasted energy, though equipment selection, refrigerant charge, the building envelope, and other factors also affect performance.
How duct design matches the AC equipment
Residential duct sizing is an engineering calculation, not a choice made by looking at a room or copying a neighbor’s duct diameter. ACCA’s Manual D® Residential Duct Design describes a method that uses room heating and cooling loads, duct-system resistance, and the selected equipment’s manufacturer blower data. The aim is to provide appropriate airflow to rooms while keeping the system within the equipment’s operating range.
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ACCA identifies its Third Edition, ANSI/ACCA 1 Manual D—2016, as its current technical-manual page edition. In a February 23, 2024 announcement, ACCA said Manual D is referenced by the International Residential Code and Uniform Mechanical Code. Which code edition applies to a particular job depends on local adoption; the reference does not make one edition universally binding everywhere.
There is no reliable universal duct diameter or quick sizing formula for every home. The design depends on the room loads, equipment, duct routes, and system resistance.
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Why duct leaks and weak connections matter
ENERGY STAR says a typical house loses about 20 to 30 percent of the air moving through its duct system through leaks, holes, and poorly connected ducts. That is a general consumer-guidance estimate, not a measurement for every home; the current page does not state a publication year. Leakage can waste conditioned air and contribute to higher bills or rooms that remain difficult to cool. See ENERGY STAR’s duct-sealing guidance.
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Sealing work should be followed by performance checks where appropriate. DOE’s measure guideline for existing homes recommends measuring airflow before and after sealing: closing leaks in a very leaky system can raise pressure and reduce total airflow, so a seal alone does not prove that delivery is correct. The guideline is available at DOE EERE.
Insulating ducts outside conditioned space
Ducts routed through attics, crawlspaces, garages, or other unconditioned areas can gain or lose heat through their walls. That can reduce the usefulness of the cooled air before it reaches the rooms. DOE Building Science describes metal ducts insulated on the exterior with duct wrap, such as a fiberglass blanket with a foil-faced vapor barrier. Its overview is HVAC Sealed and Insulated Metal Ducts.
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The appropriate insulation level depends on climate, duct location, the building assembly, and applicable local requirements. The cited guidance does not establish one R-value for every installation, so ask the contractor to specify the insulation selected for the actual route and code requirements.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Registers, grilles, returns, and airflow verification
Supply and return terminals are part of the design, not finishing details. The size, quality, and placement of grilles can contribute to drafts, stagnant areas, noise, or comfort problems. DOE Building Science explains these considerations in HVAC Duct Terminal Sizing.
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Airflow should be checked rather than inferred from the AC’s nameplate or the fact that air comes out of a register. DOE notes that more than half of U.S. home HVAC systems are believed to perform poorly because of one or more installation issues; examples include oversizing and duct systems that are improperly designed, sized, or installed. The page does not state a date for that estimate. See HVAC Quality Installation.
For duct leakage testing, ASHRAE’s 2025 duct-design chapter places tests after duct sections are assembled but before insulation and concealment, with allowable leakage criteria set by the design engineer. DOE also provides training material on duct leakage testing. The test method and acceptance criteria for a project should be established for that system, not assumed from a generic target.
Questions to ask before the AC installation is finished
- Was the duct design based on room-by-room heating and cooling loads and the selected equipment’s blower data?
- Does each room have an appropriate supply path and a clear return-air path?
- Which ducts run inside the conditioned envelope, and which run through unconditioned spaces?
- What sealing and insulation products and locations are specified?
- What airflow or leakage checks will be performed, and what results will be recorded?
- Which locally adopted code, program requirements, and test protocol apply to this project?
Do not assume that older program targets are current code. For example, the ENERGY STAR V3 HVAC Quality Installation Guidebook, last updated March 23, 2011, listed 6 CFM25 per 100 square feet for total duct leakage and 4 CFM25 per 100 square feet for leakage to outdoors. Those were checklist limits for that guidebook version, not universal present-day legal thresholds. The dated guidebook is available here.
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