BuildMeter guide · Energy & Electrical
Duct Airflow and Heat-Loss Planning Guide
Coordinate airflow, velocity, insulation and route length as one distribution system. Includes measurement workflow, a worked example, decision table, ordering checkpoints, common failures and professional boundaries.
Direct answer
What to know first
Coordinate airflow, velocity, insulation and route length as one distribution system.
Open Duct Air Velocity CalculatorAirflow and duct heat loss answer different questions
Duct airflow determines air velocity and pressure behavior; duct heat loss/gain depends on duct surface area, insulation, temperature difference and surrounding conditions. A duct can have acceptable airflow yet still waste energy if it runs through an attic or crawlspace with inadequate insulation or leakage.
| Check | Inputs | Output |
|---|---|---|
| Velocity | Airflow and internal duct area | ft/min or m/s |
| Surface heat transfer | Duct area, insulation/R-value, ΔT | Heat loss/gain rate |
| Distribution | Branch airflow and fittings | Balancing/pressure implications |
| Leakage | Measured/tested leakage | Air lost before registers |
Worked airflow example
A 12 in diameter round duct has an internal area of about 0.785 ft². At 600 cfm, average air velocity is about 600 ÷ 0.785 = 764 ft/min. That velocity calculation does not tell you the duct heat loss. For that, the Duct Heat Loss Calculator needs duct surface area, insulation and temperature difference. Use the Duct Air Velocity Calculator for the airflow geometry.
Recommended workflow
- Confirm actual internal duct dimensions.
- Calculate velocity from airflow divided by internal cross-sectional area.
- Calculate exposed duct surface separately for heat-transfer estimates.
- Use insulation data appropriate to the installed duct and environment.
- Evaluate fittings, static pressure and balancing with proper HVAC design methods.
Common mistakes
- Using duct outside dimensions to calculate internal airflow area.
- Confusing cfm with velocity.
- Using duct length alone for heat loss without circumference/surface area.
- Assuming insulation fixes leakage or poor balancing.
Professional limits
Duct sizing and balancing require fan performance, total external static pressure, fitting losses and equipment requirements. These calculators isolate individual planning relationships and do not replace a complete duct design.