DIY How to Install AC Ducts

Map a route through the house on a floor plan from the air conditioning unit to every vent or outlet. Plan a main supply duct through the house with branches forking to individual vents. Locate vents on outside walls, near windows. Space the vents so that all rooms get a supply of cooled air. Size the ducts according to the airflow output of the unit; the sum of individual ducts should balance the total output of the unit. Vent ducts that are too small will restrict air movement; ducts that are too big will waste energy.

Use a basement or insulated crawlspace under the house to run ducts, if possible. Run ductwork through an attic if no under-floor space is available. Make one large duct to the attic from as close as possible to the air conditioning unit, then run branches over joists to room vents, generally placed in a ceiling for attic installation. Route ducts over and between floor joists in a basement or crawlspace. Fasten ducts to joists with metal hangers or straps in either installation.

Start installing ductwork at the supply outlet on the unit. Secure the main duct to the supply plenum or output vent first, then cut openings for side branches with tin snips. Fasten branches with metal sleeves, which fit inside the ducts and are secured with adjustable bands around the duct and seam tape. Use similar connectors if needed to join sections of ductwork for length.

Fasten flexduct to the supply outlet or plenum and to individual vents according to directions of the ductwork or unit supplier. Different units and various types of vents can have different types of connection, but generally, a metal sleeve on the unit or vent fits inside the flexduct, is secured with an an adjustable band and is sealed with tape. Some ductwork bands are like zip-tie, plastic straps, with one end pulled through a slot to tighten the band around the duct.

Lay out a return system to take air from the house back to the unit. Put this return opening in some central area which will collect air efficiently. Install a filter frame between the studs to fit a filter size recommended for the cooling unit. Balance supply and return ductwork so that the return system can handle all the airflow from the unit; an imbalanced system will waste energy.