Inputs
Example data table
| Scenario | Method | Key inputs | Typical output |
|---|---|---|---|
| Office interior zone | Sensible load | Qs=6,800 W, Tr=24°C, Ts=14°C | ~560 L/s (≈1,190 CFM) |
| Small meeting room OA | Ventilation | Pz=10, Az=35 m², Rp=2.5, Ra=0.3, Ez=1.0 | ~39.5 L/s (≈84 CFM) |
| Supply branch estimate | Duct velocity | Rect 500×250 mm, v=4.0 m/s, leak=1.05 | ~525 L/s (≈1,110 CFM) |
| Storage ventilation | ACH | Volume=180 m³, ACH=6, k=1.0 | ~300 L/s (≈636 CFM) |
Formula used
- V̇ (m³/s) = Qs / (ρ·cp·ΔT)
- L/s = V̇ × 1000
- CFM = V̇ × 2118.88
- Qs in Btu/h, ΔT in °F
- L/s = CFM × 0.471947
- Use a safety factor for contingency
- Round: A = π·(d/2)²
- Rect: A = w·h
- Apply leakage factor for field conditions
- ACH: V̇ = ACH · Volume / time
- Vent: Vbz = Rp·Pz + Ra·Az
- Outdoor air: Voz = Vbz / Ez
How to use this calculator
- Select SI or IP unit system.
- Choose the method matching your available data.
- Enter required inputs; keep defaults when appropriate.
- Optionally enter design maximum and minimum fraction.
- Click Calculate to see results under the header.
- Use CSV or PDF buttons to save your calculation.
Design intent for variable airflow zones
Use this calculator to size supply airflow for a zone under changing load profiles. For cooling, airflow rises as sensible load or temperature difference increases. A design maximum sets an upper bound that supports diffuser selection, terminal sizing, and upstream duct checks. The minimum fraction models turndown for ventilation and pressurization. When enforce minimum is enabled, results reflect practical VAV limits during low-load operation for safety.
Method selection and data quality checks
Pick the method that matches available inputs and measurement reliability. Sensible load works well when a load model provides watts or Btu/h and you know room and supply temperatures. Duct velocity is best for field verification using measured velocity and duct dimensions. ACH applies to storage, labs, and special exhaust spaces. Ventilation supports people and area outdoor air calculations with effectiveness adjustments in real projects.
Setpoint logic and damper position estimation
The tool converts each method to a common airflow, then compares it to the optional design maximum. Minimum airflow equals design maximum multiplied by the minimum fraction. Damper position is estimated as required divided by design maximum, capped at 100%. If required airflow exceeds design maximum, the notes flag a sizing mismatch. This supports quick coordination between load calculations, terminal selection, and duct constraints during design.
Unit handling and practical conversions
Calculations can be performed in SI or IP units while preserving engineering meaning. SI uses Qs = ρ·cp·V̇·ΔT and reports L/s, m³/s, and m³/h for schedule coordination. IP uses the sensible approximation CFM = Qs/(1.08·ΔT) with ΔT in °F. Duct sizing uses velocity times area, with round or rectangular geometry and a leakage factor for installed performance as needed.
Documentation, exports, and design workflow
After each successful run, the calculator stores the last result for reporting and downloads. Export CSV when you need a tabular audit trail for reviews or commissioning notes. Export PDF for quick attachments in submittals and meeting minutes. Use the example table as a reasonableness check, then refine inputs using project criteria. Confirm final values against local codes, sequences, and specifications before construction at every stage.
FAQs
Which method should I use for a typical office VAV box?
Use the sensible load method when you have modeled zone load and supply temperature. If you are balancing or verifying on site, the duct velocity method is usually faster and uses measured duct data.
Does the calculator size outdoor air or total supply air?
It depends on the method. Ventilation returns outdoor air based on people and area rates. Load, duct, and ACH methods estimate total airflow needed for cooling, distribution, or air change targets.
What does the minimum fraction represent in VAV operation?
It represents the lowest controllable airflow as a fraction of the design maximum. It helps maintain ventilation, mixing, and pressurization when the zone load is low and the damper is near minimum.
Why might required airflow exceed the design maximum?
Inputs can be inconsistent, or the terminal/duct is undersized for the stated load, ΔT, or ventilation targets. Verify temperatures, sensible load assumptions, and duct dimensions, then update the design maximum to match equipment selection.
How are SI and IP results kept consistent?
The calculator uses standard unit conversions. SI load uses ρ·cp·V̇·ΔT, while IP uses the 1.08 factor for sensible cooling. Both are reported with equivalent L/s and CFM values for coordination.
Can I use the PDF and CSV exports for submittals?
Yes, for preliminary documentation. Exports capture the latest successful calculation inputs and results. For formal submittals, pair the report with project schedules, code references, and manufacturer data for the selected terminal unit.