Manual D
Also called: ACCA Manual D, duct design, Manual J, duct sizing calculation
The ACCA standard for sizing residential ducts: it takes the room-by-room heating and cooling loads from a Manual J calculation and the equipment's airflow, and works out the diameter, length and layout each run needs. A design is $500–$3,000 and is what fixes a room that never gets enough air.
Three ACCA manuals govern a residential HVAC design. Manual J calculates the load — how much heating and cooling each room needs, from its size, windows, insulation, orientation and climate. Manual S selects equipment to meet that load. Manual D designs the ducts: the airflow each room needs (in CFM), the duct sizes and lengths that deliver it at the blower’s available static pressure, the trunk layout, and how much return air is required. The output is a drawing with a size on every run.
Most existing houses never had one. Ducts were sized by rule of thumb — 6 in to every bedroom — and the rooms at the ends of long runs, over garages or with big west windows have been uncomfortable since. Replacing those ducts like for like copies the mistake. That’s the line between a DIY flex duct replacement and a contractor’s job: if the system worked and just wore out, copy it; if it never worked, it needs a design.
A Manual J/D from an HVAC contractor or an independent designer is $500–$3,000 depending on house size, and many jurisdictions require Manual J and S (and sometimes D) with a permit for new equipment. Ask for the report, not just the assurance that “we sized it.”