Heating HVAC Ductwork Layout Plan

Get or make a floor plan of your house, with all vent locations marked and the entry point of the heating/cooling unit shown. Mark any hot water heaters or pipes, any chimneys or other sources of heat and all electrical boxes or other impediments. Indicate the location of floor joists, and attic joists if ductwork will have to be run through an attic.

Put a sheet of clear plastic over that plan for temporary marking, and start drawing a layout to route ductwork. Use a marker that will rub off the plastic so you can easily adjust routings. Draw a "trunk and branch" design, with a central supply duct running from the unit to the farthest wall as directly as possible while avoiding spaces like water heaters, fireplaces and electrical boxes.

Mark branches off that trunk to each individual vent. Locate all vents on outside walls, preferably under windows. Make these branches as straight as possible from the trunk to the vent, but avoid any hot water pipes or other sources of external heat. Route ducts to go between floor joists as much as possible.

Draw a return system, using one or two large inlets on inside walls in areas where air will flow naturally, like hallways. Route return ducts to the return opening on the unit; these typically will be larger than the supply ducts, because one or two return ducts must have the same airflow capacity as the supply ducts combined.

Route ducts through an attic if there is no suitable access under the floor, through a basement or crawlspace. Route the duct up to the attic from the unit as directly as possible, through a closet, a utility room wall or other opening large enough to accommodate the duct. Use a trunk-and-branch system within the attic, routing ductwork over joists to ceiling vents or down walls for wall vents.

Check your final plan by inspecting the routings to make sure they avoid all hot water pipes, heaters or electrical boxes; ducts may go over or under shielded wires but should not be adjacent to junction or breaker boxes. Make sure there are joists or framing members that duct supports can be nailed to. Use natural supports, like gas pipes or cross braces under joists, wherever possible.