How do I select a VAV terminal unit for a zone?

Question

Accepted Answer

## The VAV Selection Process

Selecting a VAV terminal unit requires balancing multiple interdependent parameters: airflow capacity, sound power levels, heating coil performance, inlet size, and control strategy. The process begins with the zone peak cooling airflow, determined by a proper load calculation per ASHRAE Standard 62.1 ventilation requirements and ASHRAE 90.1 energy limits. The VAV box should be sized so this peak airflow falls between 60% and 80% of the unit's rated maximum capacity.

### Why 60–80% Matters

Running a VAV box near its maximum airflow pushes velocity through the inlet and damper assembly into the region where aerodynamic noise increases exponentially. A box sized at 100% of peak airflow will generate sound power levels 5–10 NC points higher than one sized at 70%. Conversely, a unit sized too low (peak flow at 40% of maximum) limits future flexibility and may cause the damper to operate in its unstable low-flow region where airflow measurement accuracy degrades.

### Inlet Sizing

VAV inlet diameters typically range from 100 mm (4-inch) to 400 mm (16-inch) round. The inlet size determines the maximum airflow and also affects the accuracy of the integral pitot-tube airflow sensor — the signal strength from a velocity pressure sensor is proportional to the square of velocity, so small inlets at high velocity produce stronger, more reliable signals. As a practical rule, select the smallest inlet that accommodates the required peak airflow while keeping the inlet velocity below 2500 FPM (12.7 m/s) for NC-35 spaces and below 2000 FPM for NC-30 spaces.

### Heating Coil Considerations

For series fan-powered or reheat boxes, the heating coil must be sized to offset the zone heating load at the minimum primary airflow setting — not the peak. A 2-row hot water coil with entering water at 80°C and a ΔT of 20°C typically delivers 5–15 kW of heating capacity depending on coil face area and airflow. Electric reheat coils provide a lower-first-cost alternative but carry higher operating costs and may be restricted by energy codes.

### Control Type

ASHRAE 90.1-2019 mandates DDC pressure-independent VAV control for most commercial applications. This means the controller uses the integral airflow sensor to maintain a commanded flow setpoint regardless of upstream duct pressure changes — a significant advance over older pressure-dependent controllers that drifted with system conditions.

Parameter	Recommendation	Verification Method	Consequence of Missing
Peak airflow (% of max)	60–80% of unit rated max	Manufacturer selection software	Noise exceeds NC rating if >90%; poor turndown if <50%
Inlet size	Smallest that meets peak flow	Verify velocity <2500 FPM for NC-35	Oversized inlet reduces airflow sensor accuracy
Sound power (discharge)	NC-30 to NC-35 per space type	Manufacturer radiated + discharge sound data	Occupant complaints in open-plan offices
Minimum airflow	20–30% of peak (or ASHRAE 62.1 minimum)	Verify ventilation rate at minimum primary flow	Underventilation and IAQ complaints
Heating coil capacity	Sized at minimum primary airflow	Manufacturer coil selection software	Inadequate heating on cold mornings
Controller type	DDC pressure-independent, BACnet MS/TP	Points list verification; BMS integration test	Drift, poor temperature control, no trending data
Inlet condition	≥3 duct diameters straight run	Visual inspection during installation	Airflow sensor reading errors

How do I select a VAV terminal unit for a zone?

VAV Terminal Unit Selection Parameters

🔑 Key Takeaways