How to Install Basement Ductwork

Locate the supply plenum, which may be a single point from a combined heating/cooling unit or may be two locations if furnace and air conditioner are separate. Heating and cooling will share ductwork. Draw a rough floor plan of the house and map out the vent locations, typically on walls near windows. Identify a return entry point if one is not built into the house; these typically are on interior walls in hallways or other central locations.

Mark on the map the location of hot water heaters, electrical boxes, any source of heat or any impediment to routing ductwork. Draw a tentative route with a main supply duct from the unit to the far end of the house with branches off to individual vents. Outline a return duct scheme, usually a shorter route but with larger ductwork, from the entry and its air filter to the return opening on the unit.

Start installing ductwork at the supply plenum where air comes out using flexible ducts called flexduct of a size to conform to the airflow output of the unit; this usually will be at least 12 inches in diameter. Place the central duct first. Fasten it to the plenum with adjustable straps, metal or plastic, which tightens around the duct.

Run ducts between floor joists when possible and across joists if necessary. Fasten it to joists with metal hangers about 1 1/2 inches wide under the duct nailed with a hammer and nails to joists on both sides. Avoid bending or compressing the flexduct. Use any natural supports, like gas lines, which have enough space for ducts to run over them without crimping. Keep ducts secure but sagging slightly.

Connect smaller vent ducts at branch locations. Cut into the main supply with tin snips and insert metal connecting sleeves, which insert into both pieces of duct and are fastened with adjustable straps. Put a duct to every vent and connect it to the bottom of the floor vent with straps. Wrap all seams and connections with heat-resistant tape -- not duct tape, which will not hold up.

Install return ducts the same way, starting at the air filter entry point to the return plenum on the unit. Fasten these with straps and tapes, just like supply ducts. Secure them to joists or wall studs; some return ducts go beside walls to the basement return point. Fasten them to joists in the basement with hanger straps.

Test the system once all ducts are installed and sealed. Force the blower on the unit to turn on by adjusting the thermostat. Look for flapping pieces of ductwork and listen for whistles, which indicate air leaks. Repair any leaks with tape.