Things You'll Need
Rigid duct board (optional)
Planning ductwork installation for an HVAC (Heating, Venting, Air Conditioning) system starts with a house plan, showing locations of ceiling and floor joists, wall spaces capable of containing ducts and obstacles to be avoided, such as water heaters and pipes and electrical boxes. It helps to have measurements; otherwise, measure potential routes with a tape measure to determine how much ductwork of each size is needed, where connectors will be located and how many are required. Use flexible duct, flexduct, wire cylinders covered with fiberglass insulation and sealed outside with a metallic barrier.
Design a system with as many straight runs a possible, starting with a main supply duct to take hot or cold air through the center of the house from the heating/cooling unit outside to the opposite wall. Plan branches off to each side to reach vents on outside walls, typically near windows. Size the main duct according to the air output from the unit, according to the manufacturer. Use smaller branch ducts sized so the total air flow is approximately the same as the unit output.
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Build a return system to collect "used" air from inside the house, pass it through a filter and return it to the unit. The return system must have the same air flow capacity as the supply system. Ideally, returns would be located in each room with an output vent but on an opposite wall. Practically, most return systems are built in a center hallway or other interior space where air flows naturally. Use several return entry points, if necessary, to collect enough air, with a filter at each one. But they must join to a single duct to connect to the unit.
Plan duct runs to avoid any sharp turns or bends or small spaces where the duct cylinder would be compressed. Use rigid duct board to make rectangular ducts to fit those spaces, typically within wall studs. Connect rigid and flexduct with connectors recommended by the duct board manufacturer. Route ducts between floor and ceiling joists as much as possible. Run ducts through an attic if there is no basement or other suitable under-floor space. Plan a single main duct to carry air up to the attic and use a trunk and branches to distribute air to vents, either in the ceiling or at floor level in walls.
Route return ducts below floors wherever possible. Return air will be colder than heated air and so will flow naturally downward; cooled air coming from the unit also will trend downward. Size the total return ductwork to match the output of the supply system, so the total airflow is balanced. Follow the guidelines on the unit for airflow capacity.
Create a system with as few cuts and connections as possible, since every seam is a potential air leak. Use Y connectors when possible to tie two smaller vent supply ducts to the central duct, instead of two separate connections. Attach branches with metal connectors, which fit inside each duct and are fastened with heat-resistant tape.