A VAV terminal unit, or VAV box, is the device at the end of a variable-air-volume system that modulates supply airflow to a zone, typically from 100 to 2,000 L/s, to match the real-time heating or cooling load while the central air handler runs at part load for efficiency. Pressure-independent boxes use a flow sensor and actuator to hold the commanded flow despite duct-pressure swings, and fan-powered types add a small fan for mixing or heating. It is the core of modern commercial comfort control.
How a VAV Terminal Works
The box sits in the supply duct to a zone and contains a damper driven by an actuator. A controller sends a flow setpoint from the zone thermostat; the box opens or closes the damper to meet it. In a pressure-dependent box, flow varies with duct pressure, so a pressure drop shifts the actual volume. A pressure-independent box measures flow with a sensor and trims the damper to keep the set value within about 5 percent, giving stable temperature even as the central fan modulates. Minimum flow is usually set to 30 to 40 percent of design to preserve ventilation.
Types of VAV Terminal Units
Single-duct boxes are the simplest, handling cooling and sometimes reheat. Fan-powered boxes come in series and parallel: a series fan runs with the primary air whenever the box is open, inducing plenum air for constant mixing; a parallel fan runs only on heating or ventilation calls, saving fan energy during cooling. Reheat boxes add a coil for perimeter zones that need heat while the core needs cooling. Each type fits a different load profile in the same building.
VAV Box Comparison
| Type | Flow control | Fan | Typical zone use |
|---|---|---|---|
| Single-duct, pressure-dependent | Damper only | None | Small, stable-pressure zones |
| Single-duct, pressure-independent | Sensor and actuator | None | Standard offices, 100 to 800 L/s |
| Series fan-powered | Pressure-independent | In-line, continuous | Open plans needing mixing |
| Parallel fan-powered | Pressure-independent | Parallel, on-call | Perimeter zones with reheat |
The table clarifies where each box type earns its cost through control accuracy or energy savings.
Key Numbers and Sizing
Three numbers guide selection. First, design flow: a typical office zone of 20 to 40 m² needs 100 to 400 L/s, while a large open floor may need 800 to 2,000 L/s across several boxes. Second, minimum ventilation flow, often 20 to 80 L/s or 30 to 40 percent of design to meet code. Third, pressure drop across the box, typically 50 to 250 Pa at design flow; lower drop saves fan energy. The central plant can then run 30 to 60 percent below peak, cutting supply fan power by roughly half versus constant-volume systems.
When to Choose Each VAV Type
- Choose a pressure-independent single-duct box for standard zones needing accurate, stable temperature.
- Choose a pressure-dependent box only for very small systems with steady duct pressure.
- Choose a series fan-powered box for open areas that need continuous mixing and good air distribution.
- Choose a parallel fan-powered box for perimeter zones that mostly cool but occasionally need heat.
- Choose a reheat box for zones with a heating load while the building core is cooling.
Installation and Commissioning
Mount the box with 3 to 5 duct diameters of straight run upstream of the flow sensor for accurate reading, connect the actuator and sensor to the zone controller, and balance the minimum and maximum stops on site. Commission by measuring flow at 25, 50, and 100 percent command and verifying it tracks within 10 percent. Set the minimum to the ventilation requirement, often 30 to 40 percent, and document the values. Over a 15 to 20 year life, proper VAV control typically cuts HVAC energy by 20 to 40 percent versus constant-volume reheat.
Keruite manufactures this equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment.
Flow Measurement and Control Loops
A pressure-independent VAV box holds set flow with a velocity or pressure sensor across a measuring section, trimming the damper to within about 5 percent as the central duct pressure swings from 250 to 750 Pa. The controller converts the zone load to a flow setpoint of 100 to 2,000 L/s and enforces a minimum of 30 to 40 percent for ventilation even at low cooling demand. The box pressure drop at design is 50 to 250 Pa, and keeping it low protects central fan energy. A duct static-pressure reset strategy then lets the air handler fan slow as boxes close, often cutting supply fan power by roughly half versus constant-volume reheat and saving 20 to 40 percent of total HVAC energy over a 15 to 20 year life.
Reheat, Zoning, and Commissioning
Perimeter zones that need heat while the core cools use reheat boxes with a hot-water or electric coil adding 1 to 10 kW, or a parallel fan-powered box that mixes plenum air to lift temperature without a coil. Series fan-powered boxes run the fan whenever open, giving steady mixing and better distribution in open plans but using 50 to 200 W of fan power continuously. At commissioning, measure flow at 25, 50, and 100 percent command and verify tracking within 10 percent, then set the minimum to the code ventilation rate, often 20 to 80 L/s. Document the values and the sensor calibration; a blocked measuring section or a drifting actuator is the usual cause of a zone that cannot hold temperature or meets air-quality complaints.
Duct Static Pressure Reset
A good VAV design resets the central duct setpoint downward as boxes close, so the supply fan slows and saves 20 to 40 percent of HVAC energy versus constant-volume reheat. The reset range is typically 250 to 750 Pa, and the controller raises it only when the most-open box nears 90 percent travel, keeping every zone satisfied while the plant drifts to its lowest stable power.
FAQ
What is the difference between a pressure-dependent and pressure-independent VAV box?
A pressure-dependent VAV box changes flow with duct pressure and needs no sensor, while a pressure-independent box uses a flow sensor and actuator to hold the set flow regardless of upstream pressure swings. Choose pressure-independent for accurate zone control and energy savings, pressure-dependent only for small, stable systems.
How do I size a VAV terminal for a zone?
Calculate the peak heating and cooling loads of the zone, convert to airflow using about 10 to 12 L/s per kW of cooling, then select a box whose minimum and maximum flow cover that range. A typical office zone of 20 to 40 m² uses a 100 to 400 L/s series or parallel box.
What is the difference between series and parallel fan-powered VAV?
A series fan-powered box has the fan in line with primary air and runs whenever the box is open, mixing plenum air continuously; a parallel box has the fan beside the primary path and runs only on heating or ventilation calls. Series gives better mixing and static, parallel saves fan energy when cooling.
What minimum airflow should a VAV box hold?
Most boxes hold a minimum of 30 to 40 percent of design flow for ventilation, often 20 to 80 L/s depending on occupancy and code. Dropping below the ventilation minimum risks indoor air quality, so the controller enforces the set minimum even at low cooling demand.