Manual D is the ACCA standard for residential duct design. It takes each room's airflow requirement from a room-by-room Manual J, the blower performance of the equipment selected under Manual S, and the duct layout, then sizes supply and return runs so every room gets its CFM at a static pressure the blower can handle. The current version is ANSI/ACCA 1 Manual D - 2016.
On most replacement jobs you are not designing ducts from scratch. You are deciding whether the ducts already in the house can carry the airflow the new equipment needs. That check uses the same Manual D ideas, and it is where many heat pump comfort complaints start.
This guide is in outline form. The full version will include a worked example on a typical two-story house.
In this guide
Where Manual D fits
- The J, S, D sequence and why ducts come last.
- Why a padded Manual J produces the wrong airflow in every room, no matter how good the duct design.
- What carries over from the Manual J guide: room loads and CFM.
Available static pressure and friction rate
- Reading a blower table for external static pressure.
- Subtracting component losses: filter, coil, registers and grilles.
- Turning what is left into a design friction rate.
Effective length and fittings
- Why a fitting can add more equivalent length than the straight duct.
- Sizing trunk and branch runs from friction rate and CFM.
- Flexible duct: compression, sag and real-world losses.
Return air
- Sizing returns as carefully as supplies.
- Pressure balance between closed bedrooms and the return path.
Replacement jobs on existing ducts
- Checking whether old ducts can carry a new heat pump's airflow.
- What a 2014 NIST simulation study found about oversized heat pumps on nominal ductwork and low airflow from undersized ducts.
- When to resize, add returns or change the equipment selection instead.
Ductless and compact-ducted systems
- When a ducted design is not the right answer.
- How room loads size individual heads instead of registers.
Start with the loads
Manual D only works on accurate room loads. Get an estimate with the Manual J calculator, then run the real thing room by room. Free tools give estimates only.
Tradewright runs a room-by-room Manual J and selects equipment from real performance data, so the airflow numbers your duct design starts from are traceable.
Questions
What is Manual D?
Manual D is the ACCA standard for designing residential duct systems. It sizes supply and return ducts so each room receives the airflow its Manual J load requires, at a static pressure the selected blower can deliver. The current version is ANSI/ACCA 1 Manual D - 2016.
Do I need a Manual D on a replacement job?
Often you are reusing existing ducts, so the question becomes whether those ducts can carry the new equipment's airflow. Checking room CFM and available static pressure against the existing duct sizes is a Manual D style check, and it matters most when the new system moves more air than the old one.
What inputs does Manual D need?
Room-by-room heating and cooling CFM from Manual J, the selected blower's performance table from Manual S, the duct layout with lengths and fittings, and pressure drops for components such as filters, coils and registers.
Why do undersized ducts matter for heat pumps?
Undersized ducts reduce indoor airflow and raise fan power. A 2014 NIST simulation study found that low indoor airflow due to undersized ductwork, and an oversized heat pump on nominal ductwork, were among the installation faults most likely to cause significant performance loss.
Is Manual D required by rebate programs?
NYS Clean Heat encourages, rather than requires, the use of Manual D alongside Manual J and Manual S. Mass Save's whole-home verification form says Manual J and/or Manual D documentation may be requested. Check the current program documents for your project.
Sources
- ACCA: Manual D, Residential Duct Systems (ANSI/ACCA 1 Manual D - 2016) (accessed 2026-10-16)
- NIST Technical Note 1848: Sensitivity Analysis of Installation Faults on Heat Pump Performance (2014) (accessed 2026-10-16)
- NYS Clean Heat Heat Pump Program Manual, Version 3 (September 1, 2026), Section 3.2 (accessed 2026-10-16)