The Best AC Duct Installation

Map the house for ductwork routes, using a basic floor plan with the air conditioning unit and all outlet vents marked. Use a basement or crawl space for ductwork, if possible; otherwise, draw a route from the unit to an attic to use for running ducts. Use as many straight runs as possible and avoid any sharp turns or bends. Go up to an attic through a closet or similar large wall space.

Test the route map to locate any serious barriers, like hot water heaters, electrical boxes or structural elements, that would interfere with a straight duct run. Modify the plan to work around those obstacles. Locate return vents to take "used" air back to the unit. Place supply vents on outside walls, preferably near windows, and return openings either in the same space as vents or in a central location, like a hallway, where air will flow naturally.

Size the ductwork to the output of the unit, which should describe the maximum air flow. Make a central supply duct the length of the house with as direct a route as possible and put forks to individual vents. Use Y connectors where possible for two vent ducts to run from one main supply opening, to reduce the number of cuts in the main duct. The total air flow capacity of all ducts should at least equal the maximum airflow output of the unit.

Make return ducts large enough to equal the maximum airflow from the supply side. Locate return openings between wall studs, which can be used to fasten frames to hold air filters, to trap dust and dirt before the air goes back into the unit. Put central returns close to the unit so as much foreign matter as possible is removed.

Install ductwork under a floor with metal straps around the flexduct, fastened to joists on either side. Place ducts between joists in an attic and fasten with enough straps to prevent it from blowing with any wind that gets in. Keep ductwork as straight and even as possible, but allow enough sag so the flexduct is not stretched. Avoid any tight spaces, which might crimp or bend flexduct. Make alternative ducts of rigid foamboard if necessary in spaces too cramped for round ducts.

Cut flexduct for joints with tin snips. Fasten joints with metal sleeves, which slide inside each duct element. Secure seams with tape recommended by the manufacturer. Put tape all the way around the flexduct so no air escapes. Test the system by forcing the air conditioner to turn on, either with a switch or by adjusting the thermostat. Listen for any whistles, which indicate air leaks, or look for flapping tape on seams.