A VAV (Variable Air Volume) system controls each zone by throttling supply airflow with a box, holding a fixed supply temperature and varying volume from 30% to 100% of design; a typical office zone runs 0.3–1.0 m³/s and trims fan energy 20–40% versus constant-volume reheat. The design hinges on box selection, minimum airflow, and a fan that can follow the pressure signal without hunting or short-cycling.
How VAV Saves Energy
Instead of heating or cooling excess air to hold a setpoint, VAV sends only the air a zone needs. When a south-facing office cools itself through the window, the box closes toward its 30% minimum and the central fan sees less total demand. Because fan power scales roughly with flow cubed, cutting total airflow 25% can cut fan energy 50–60%. Whole-system savings of 20–40% over CAV are typical in mixed-load buildings with real occupancy variation.
The second saving comes from downsizing the cooling coil duty: a VAV system rarely runs at peak in every zone at once, so the AHU can be selected on coincident rather than summed load, often 15–25% smaller than a CAV equivalent. Smaller coil, smaller pump, smaller chiller—the benefits compound through the whole central plant, not just the ductwork.
Single vs Dual Duct and Box Types
A single-duct cooling-only VAV box modulates cold air and adds reheat (electric or hot-water) near the zone when heating is needed—simple but reheat can waste 10–20% of cooling. A dual-duct VAV mixes hot and cold decks and avoids reheat penalty but needs two ducts. Fan-powered boxes add a small 50–200 W fan to keep constant room airflow for better mixing and less draught on glass perimeters.
Comparing VAV Configurations
| Configuration | Min airflow (%) | Reheat use | Added fan energy | Best fit |
|---|---|---|---|---|
| Single-duct, no reheat | 30–40 | None | None | Cooling-dominant zones |
| Single-duct with reheat | 30–50 | 10–20% of cooling | None | Mixed heating/cooling |
| Dual-duct mixing | 30–50 | None | None | Tight simultaneous loads |
| Fan-powered series | 100 (room) | Low | 50–200 W/box | Perimeter, draft-sensitive |
Key Design Numbers
Size the primary air at peak zone load, often 0.5–1.0 m³/s for a 30–50 m² office. Set the minimum at 30–40% to meet ventilation code (typically 8–10 L/s per person) and prevent coil freeze on cooling-only boxes. The duct static pressure setpoint usually lands 250–500 Pa; a pressure-independent box holds airflow within ±5% as the duct swings 200–600 Pa. Allow 15–25% for diversity when summing zones to size the AHU.
Worked Example: A 10-Zone Floor
Ten perimeter offices each peak at 0.6 m³/s cooling but never all peak together; diversity of 0.7 gives a coincident 4.2 m³/s. We select the AHU for 4.2 m³/s at 400 Pa instead of 6.0 m³/s, saving a size class. Each box minimum is 35% (0.21 m³/s) to deliver 8 L/s per person ventilation. A static-pressure reset holds 300 Pa at the most-open box, trimming 15% more fan energy than a fixed 450 Pa setpoint across the occupied day.
Decision Checklist: Which VAV
- Choose single-duct with reheat when zones mostly cool but need occasional heat and you accept modest reheat penalty of 10–20%.
- Choose dual-duct mixing when perimeter and core loads swing in opposite directions and you want to avoid reheat waste entirely.
- Choose fan-powered boxes on glass perimeters where 100% constant room airflow prevents cold-down drafts and improves comfort.
- Choose no-reheat single-duct only in cooling-dominant climates where heating is rare and code minimums still allow a 30% setting.
Control and Commissioning
Tune the static-pressure reset so the fan tracks the most-open box; a well-tuned reset trims 10–20% more fan energy than a fixed setpoint. Balance each box to its measured CFM, verify minimum damper position, and confirm the AHU supply temp holds 12–14°C cooling and 40–45°C heating. Poor commissioning is the top reason VAV systems waste 15–30% of expected savings, so pay for a proper balancing report. A useful commissioning check is to force every box to minimum and maximum and confirm the duct pressure settles within 50 Pa of the reset target at both ends; if it oscillates, the reset schedule is too aggressive for the fan and should be softened before handover to stop the short-cycling that quietly wastes energy.
Standards and References
Follow ASHRAE 90.1 for VAV fan-power limits and demand-controlled ventilation, and ASHRAE 62.1 for the 8–10 L/s per person minimum that sets your box minimums. Pressure-independent boxes should be factory-calibrated to ±5%; request the calibration certificate. Document the design static-pressure band so the controls contractor sets reset limits within the fan’s stable operating range.
Sourcing the Air Handler
Keruite manufactures HVAC fans and air-handling 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. Matching the AHU fan curve to the VAV pressure-reset band avoids short-cycling and keeps boxes within their ±5% airflow window, which is exactly where comfort and the energy savings both live or die.
Adding Demand-Controlled Ventilation
A standard VAV modulates temperature but often runs a fixed outdoor-air fraction sized for peak occupancy, wasting conditioning energy when the floor is half empty. Layering demand-controlled ventilation (DCV) on the same boxes uses CO₂ sensors at 600–1,000 ppm setpoints to trim ventilation to actual need, cutting outdoor-air heating and cooling 10–25% in variable-occupancy spaces like conference floors and auditoriums. The box minimum then becomes a floor set by air quality rather than a fixed percentage, so the two controls must be coordinated in the BMS to avoid fighting.
Implementation is straightforward: the DCV loop raises the effective minimum when CO₂ climbs and the pressure-reset loop keeps the duct stable as total flow shifts. Watch two failure modes—stale air if the sensor sits in dead air away from occupants, and short-cycling if the setpoint band is too tight. A well-tuned DCV on a 4,000 m³/h zone with 30% average occupancy can return its sensor and programming cost in 1–3 years through reduced heating and cooling of unneeded outdoor air, on top of the base VAV saving.
FAQ
How much energy does VAV actually save?
Versus constant-volume reheat, VAV typically cuts total HVAC energy 20–40%. Because fan power scales near flow cubed, a 25% airflow cut can save 50–60% of fan energy. Real savings depend on commissioning quality; poorly tuned systems lose 15–30% of that potential to bad reset schedules and unbalanced boxes left at factory default.
What should the minimum airflow be?
Set single-duct boxes at 30–40% for cooling zones and 30–50% where heating occurs, mainly to meet 8–10 L/s per-person ventilation and protect the cooling coil from freeze. Fan-powered boxes hold 100% room airflow using a small 50–200 W plenum fan instead, which trades a little energy for much better perimeter comfort.
Single or dual duct VAV?
Use single-duct with reheat for mixed but mostly cooling loads; choose dual-duct mixing when perimeter and core need opposite temperatures at once and you want to avoid reheat waste. Dual duct needs two ducts and more ceiling space but removes the reheat penalty entirely, which matters in glass towers with strong simultaneous loads.
Why does my VAV zone feel drafty?
Drafts usually mean the box dropped below its minimum or the supply is too cold and too slow. Keep perimeter boxes at 30–50% minimum, raise supply temperature slightly, or switch to fan-powered boxes that hold 100% room airflow. Verify the box is not hunting due to a faulty pressure sensor or a reset setpoint set above the fan’s capability.