📈 Turndown Ratio
📏 Duct Connection Size
Sizing Breakdown
Full Calculation Table
| Component | Basis | Value | Result |
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How to Use the VAV Box Sizing Calculator
Choose with reheat if the zone needs heating capability at low airflow, or cooling only for interior zones with no heating requirement.
Enter the zone peak sensible cooling load and supply air temperature difference to calculate maximum airflow.
Enter the zone's minimum outdoor air requirement from the commercial ventilation calculator to confirm minimum airflow setting.
Click Calculate VAV Box Sizing to see maximum and minimum airflow, reheat coil capacity, and recommended duct connection size.
Understanding VAV Box Sizing
A VAV box sits downstream of a central AHU, modulating airflow delivered to its zone based on that zone's actual sensible cooling demand rather than the AHU cycling for the whole building at once. Sizing a box correctly requires setting both a maximum airflow, driven by peak cooling load, and a minimum airflow, driven by ventilation requirements and comfort considerations, with capacity and sound ratings that follow AHRI's VAV terminal certification program.
Maximum Airflow from Peak Cooling Load
Maximum VAV box airflow follows the same sensible cooling relationship used throughout HVAC sizing: divide zone peak sensible load by 1.23 times the supply air temperature difference. This sets the largest airflow the box will ever need to deliver, which in turn determines the box size and duct connection required to handle that peak condition without excessive noise or pressure drop.
Minimum Airflow: Two Competing Requirements
A VAV box cannot close all the way down even at zero cooling demand, since the zone still needs its minimum ventilation airflow satisfied, and very low airflow through diffusers can cause poor air distribution or stuffiness complaints. Minimum airflow is typically set as the larger of two values: the zone's minimum outdoor air requirement in absolute terms, checked against the ventilation code calculator for your provincial minimum, or a percentage of maximum airflow, commonly around 30 percent, chosen to maintain reasonable air distribution and control stability. When ventilation requirement exceeds the percentage-based minimum, ventilation becomes the governing constraint.
When Reheat Coils Are Needed
Reheat becomes necessary whenever a zone needs heating while the box sits at minimum airflow, since that reduced airflow at the AHU's cold supply temperature cannot satisfy heating demand without additional warming at the box itself. Perimeter zones with meaningful envelope heat loss commonly need reheat coils, while interior zones without exterior exposure often skip reheat entirely, since they rarely if ever need heating given their typically internal-gain-dominated load profile discussed in the commercial load calculator.
Duct Connection Sizing for Acoustic Performance
The inlet duct connection size gets selected from manufacturer performance data to keep velocity within an acceptable range at maximum airflow, typically 5 to 8 metres per second, balancing noise control against physical space constraints above the ceiling. A larger connection reduces velocity and noise for the same airflow but takes more room to install, which matters in tight ceiling plenum spaces common in many commercial renovations; check the resulting sound power level against the NC curve calculator for the room's actual noise criteria target.
System-Level Diversity After Individual Box Sizing
Once individual VAV boxes are sized throughout a floor or building, the central AHU and chiller plant should not simply sum every box's maximum airflow, since zones rarely all peak simultaneously, a diversity principle the ASHRAE Handbook, HVAC Systems and Equipment volume addresses directly in its VAV systems chapter. See the VAV system diversity calculator to determine realistic combined demand for central equipment sizing after this individual box sizing step.
Frequently Asked Questions
Maximum VAV box airflow is calculated the same way as any sensible cooling airflow: divide the zone peak sensible cooling load by 1.23 times the supply air temperature difference. This gives the airflow the box needs to deliver at full cooling demand, which sets the box size and duct connection needed to handle that peak flow rate.
A VAV box cannot simply close down to zero airflow, since the zone still needs its minimum ventilation requirement met even at low cooling demand, and very low airflow can cause poor air distribution or comfort complaints in the space. Minimum airflow is typically set to the larger of the zone's minimum outdoor air requirement or a percentage of maximum airflow, commonly 30 percent, whichever governs.
A VAV box needs reheat whenever the zone requires heating while the box is operating at its minimum ventilation airflow, since supply air at that reduced flow rate is too cold to satisfy heating demand without warming it further. Perimeter zones with significant heat loss commonly need reheat, while interior zones with no envelope exposure often do not, since they rarely need heating at all.
Duct connection size is selected from the box manufacturer's published performance data based on maximum airflow and an acceptable duct velocity, typically 5 to 8 metres per second for VAV box inlet connections to control noise. A larger duct connection reduces velocity and noise for the same airflow but takes more physical space above the ceiling, so the selection balances acoustic performance against installation constraints.
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