How to Run Heat Ducts in the Ceiling

Run a main supply duct from the furnace to the attic above the ceiling as directly as possible, preferably through the ceiling in a utility room on one end of the house. Fasten the vertical duct to wall studs with 1 1/2-inch wide metal straps put around the duct and nailed to joists with galvanized nails and a hammer. Skip this step if a furnace is installed in the attic.

Lay a central supply duct the length or width of the house, from the attic entry point to the opposite wall. Position this in the center of the house, between ceiling joists if possible, across them if necessary. Lay this on top of any loose fill or other insulation, avoiding any hot water pipes, other sources of heat or electrical fixtures. Fasten it with straps to joists enough to keep it from moving or being blown in the wind.

Link the central duct to the ceiling vents or outlets in each room or space with smaller ducts. Keep these away from walls so the ceiling duct is not exposed to cold air through eaves or vents. Connect smaller ducts to the main supply with sheet metal tee collars. Cut the main duct with tin snips and slide two ends over the collar and the vent duct to the T end. Secure the ducts with adjustable straps tightened around the collars and seal the seams with heat-resistant tape.

Connect the room ducts to the round connection collars on the ceiling vents. Fasten these with straps and seal with tape. Secure vent ducts with metal straps nailed to joists; put fasteners close to the outlet and elsewhere enough to keep ducts from blowing. Cover the end of the main duct with a cap once all ceiling supply ducts are in place.

Install a return system to collect "used" air and return it to the furnace. Use a built-in return on the furnace if it has one, otherwise install a separate return entry either in a hallway in the house or in the attic at the end opposite from the supply entry. Fasten a return frame with an air filter enclosure to joists or studs, typically with screws through a metal frame into joists or studs.

Tie the return ductwork together just like supply ducts, if more than one section is required to reach from the entry to the furnace. Fasten the main supply and the return duct systems to the appropriate plenums or collars on the furnace. Follow furnace directions for these connections, typically putting sheet metal screws through the duct into the metal collar and sealing the seams with tape.