A Constant Air Volume (CAV) system delivers a fixed supply airflow while the temperature is modulated to hold setpoint; it is simplest and lowest-cost for spaces with steady, uniform loads, such as warehouses, retail floors, and single-zone offices running 3,000–30,000 m³/h at 250–600 Pa. Choose CAV when zoning complexity would cost more than the energy it saves, which is surprisingly often in single-purpose buildings.
When CAV Makes Sense
CAV shines where the load barely varies: a warehouse at 18–24°C, a supermarket sales floor, or a factory bay with steady process heat. Because the fan runs at one speed and one volume, controls are a thermostat and a heating/cooling valve—no boxes, no pressure reset, no per-zone balancing beyond the initial duct design. First cost can be 20–35% below a comparable VAV build-out, and reliability rises simply because there are fewer moving and sensing parts to fail.
Operationally, CAV is forgiving. A facilities team can troubleshoot it with a thermometer and a pressure gauge rather than a BAS laptop. For small sites without a controls engineer on staff, that simplicity is worth more than the energy difference, and it avoids the classic failure where a VAV system is never commissioned and runs like an expensive CAV anyway.
How It Holds Comfort
With volume fixed, the only lever is supply temperature. On a cooling day the unit may deliver 12–14°C air at a steady 10,000 m³/h; as the room approaches setpoint the cooling coil modulates and reheat or less cooling holds the line. The risk is simultaneous heating and cooling—where one part of a space needs warm air and another cool—which CAV cannot fix without wasteful reheat of 15–30% of total flow, the exact scenario where you should have specified VAV.
Comparing CAV and VAV
| Aspect | CAV | VAV | Implication |
|---|---|---|---|
| Airflow | Fixed | 30–100% variable | VAV saves fan energy |
| Control complexity | Low (1 thermostat) | Medium–high | CAV cheaper to install |
| Multi-zone comfort | Poor | Good | CAV needs open mixing |
| Typical energy | Baseline | 20–40% lower | VAV wins mixed loads |
| Best use | Steady single load | Mixed/variable load | Match to duty |
Key Design Numbers
Size the fan for peak block load, not average, so steady airflow meets the hottest hour; a 500 m² retail floor often needs 15,000–25,000 m³/h at 300–500 Pa. Keep supply velocity in ducts at 6–10 m/s and face velocity at coils under 2.5 m/s to limit pressure drop to 150–300 Pa across the AHU. Reheat, if used, should stay below 20% of airflow to avoid paying to cool and heat at once, which destroys the economy case.
Worked Example: A Distribution Warehouse
A 5,000 m² warehouse holds 18–24°C with steady internal load of 40 W/m². Design airflow is 5,000 × 40 / (1.2 × 10) ≈ 16,700 m³/h at 350 Pa. A single-speed 4-72 fan delivers this continuously; a thermostat modulates the cooling coil between 12 and 14°C. No boxes, no reset. First cost is roughly 30% under a VAV equivalent, and the load never varies enough for VAV’s savings to pay back the extra capital within the building’s life.
Decision Checklist: Choose CAV When
- Choose CAV for single-zone spaces with a steady load profile and few internal gains swings, like warehouses and shops.
- Choose CAV when first cost and simplicity beat 20–40% energy savings you would not realize anyway because the duty is essentially constant.
- Choose VAV instead when you have many zones with opposite heating/cooling needs that CAV cannot serve without reheat.
- Choose CAV with careful mixing for open plans where one setpoint serves the whole volume and zoning adds no comfort benefit.
Avoiding Reheat Waste
The classic CAV failure is a building carved into zones that fight each other, forcing reheat of 15–30% of airflow. If you see that pattern, either open the space for one setpoint or move to VAV. Keep supply-air temperature reset by outdoor air (14°C cooling in summer, 40°C heating in winter) and verify duct balance within ±10% at commissioning so no branch is quietly over- or under-supplied.
Standards and Commissioning
Design to ASHRAE 62.1 for ventilation rate per occupant, applied to the fixed volume, and to ASHRAE 90.1 fan-power allowances. Because CAV has no variable box, commissioning is mostly airflow balance and coil control sequencing. Log supply temperature against room temperature for a week; a stable 1–2°C offset confirms correct modulation and rules out simultaneous heat/cool waste.
Equipment Sourcing
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. For a CAV plant, a steady single-speed 4-72 or 4-79 centrifugal fan matched to the fixed 250–600 Pa duty gives the simplest, most reliable result, and the G2.5 balance means the unit arrives ready to run without field re-balancing that CAV systems rarely get budgeted for.
Filter, Coil, and Economizer Choices for CAV
Because CAV runs a fixed volume, the filter and coil see a constant face velocity, which simplifies selection: pick a G4 or F7 panel rated to 150–250 Pa clean and size it so loaded resistance stays under 300 Pa, keeping total AHU drop within the 150–300 Pa target. A constant 2.5 m/s coil face velocity also keeps heat-transfer coefficients stable, so the cooling capacity you calculate is the capacity you get, unlike VAV where part-load coil performance shifts with flow.
Add an air-side economizer to a CAV plant and the steady-volume nature becomes an advantage: when outdoor air is cool (below about 12°C dry-bulb), the controller can flush the space with 100% outdoor air at the fixed flow, harvesting free cooling for 800–1,500 hours a year in many climates. Since the fan volume never changes, there is no minimum-ventilation conflict to manage, only a mixing damper and a freeze-stat to set. For steady-duty buildings this free-cooling dividend is often the largest single energy win available without touching the ductwork at all.
FAQ
What is a CAV system?
A Constant Air Volume system supplies a fixed airflow and modulates only supply temperature to hold setpoint. It suits steady, uniform loads such as warehouses, retail floors, and single-zone offices. Controls are just a thermostat and a heating/cooling valve, keeping first cost 20–35% below VAV and making it easy to operate without a building-management system.
When should I avoid CAV?
Avoid CAV when spaces have opposite heating and cooling needs at the same time, because it can only fix one with wasteful reheat of 15–30% of airflow. Multi-zone offices, mixed perimeter/core buildings, and labs with varying loads are better served by VAV or CAV with careful open mixing. If you need per-zone setpoints, CAV will fight you.
How much airflow does a CAV unit need?
Size for peak block load, not average. A 500 m² retail floor typically needs 15,000–25,000 m³/h at 300–500 Pa; a warehouse bay scales with heat gain. Keep duct velocity 6–10 m/s and coil face velocity under 2.5 m/s to limit AHU pressure drop to 150–300 Pa and protect the fan from unnecessary static-pressure demand.
Is CAV cheaper than VAV?
Yes for steady loads: CAV saves 20–35% in first cost by dropping boxes, pressure reset, and per-zone balancing. It gives up 20–40% fan-energy savings, but those savings only matter where loads vary. For single-zone steady duty, CAV is usually the smarter buy, with higher reliability and far simpler troubleshooting for small facilities teams.