Duct Design and Airflow Delivery
Sizing, friction, fittings and the reasons a duct system fails to deliver what the equipment can produce.
What it is
Duct design turns a room by room load calculation into the airflow each room needs and then into duct sizes that can deliver it at acceptable pressure and noise levels. Equipment capacity is meaningless if the ducts cannot carry the air.
Key ideas
- Room airflow comes from the room load, not from the size of the room.
- Friction rate is the pressure budget per hundred feet of duct, derived from the available static pressure and the total effective length of the longest run.
- Fittings add effective length, often far more than the straight duct itself.
- Velocity drives noise. Trunks, branches and grilles each have practical velocity limits.
- Flexible duct must be pulled tight and supported. Compressed flex can lose a large fraction of its capacity.
- Return air paths matter as much as supply. Closed doors with no transfer path pressurize rooms and starve the return.
Friction rate = (available static pressure x 100) / total effective lengthAvailable static pressure is what remains after subtracting the pressure drops of the coil, filter and other components from the rated external static.
What can go wrong
| Symptom | Common duct cause |
|---|---|
| One room never comfortable | Undersized branch, long compressed flex run, or the branch damper closed |
| Whole house weak airflow | Undersized trunk or return, high total external static pressure |
| Noise at registers | Velocity too high, undersized grille, or sharp fittings near the outlet |
| High bills and short cycling | Duct leakage to unconditioned space |
| Cold air on the floor in winter | Poor throw or register selection, or supply location fighting the room |
Local requirements
Duct materials, insulation levels, sealing methods, leakage testing and documentation requirements vary by jurisdiction, adopted code edition and the manufacturer instructions.
Local Code Notice
Symptoms
Practice & Mastery
- Duct Design and Airflow Delivery Troubleshooting Scenarios2
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