What Manual D means
Manual D is the ACCA method for designing duct systems: sizing trunks, branches and registers so each room gets the airflow its load calls for.
Also called duct design.
Where it matters on a HVAC job
A new high efficiency unit on undersized ducts runs loud and underperforms. When you replace equipment, check the ducts against the airflow the new unit needs, and quote duct changes as their own line.
What the model code says
- Duct systems are installed per ACCA Manual D, the appliance maker's instructions or other approved methods. (IRC 2021, Chapter 16)
Check which code edition your building department enforces for Manual D before you quote.
What it costs
Jobs that involve Manual D are priced in the ductwork replacement cost guide: about $1,250 typical, usually $450 to $2,200. The guide shows its sources and what moves the price. The duct size calculator prices a job line by line.
Work it out
- Free calculator: Duct size calculator
- Cost guide: Ductwork replacement cost
- Licensing: HVAC license rules by state
- Tools, templates and guides: Free HVAC resources
Sources: 2021 International Residential Code, Chapter 16, Duct Systems (ICC). Checked October 9, 2026.