Imagine a huge glass office building. The central Air Handling Unit (AHU) on the rooftop sends chilled air to every floor. Office 1 (south side) is overheating at 28 °C - sunlight, computers, 5 workers. Office 2 (north side) is empty, shaded, at 19 °C.
If the AHU sends the same volume of chilled air everywhere, Office 2 turns into an igloo! How can a single central machine deliver different temperatures to 50 offices simultaneously? The answer lies in the battle: CAV versus VAV. For decades, buildings were air-conditioned in a wasteful way - modern technology, however, changed everything.
In a CAV system, the central unit continuously delivers a fixed volume of air through the ductwork. The fan runs permanently at 100%. The only way to adjust the temperature is to change the supply air temperature at the source - the AHU sends colder or warmer air to the entire building simultaneously.
Think of it as central heating without thermostatic valves: you turn on the heating (or cooling) and every room gets exactly the same. The fan produces e.g. 10,000 m³/h constantly - regardless of whether a room needs cooling or not. Result: zero zone autonomy.
No per-room adjustment. In Office 1 (28 °C) people sweat, in Office 2 (19 °C) they shiver - and no one can do anything about it. Complaints like "it's freezing here / it's boiling there" are the number-one plague of CAV buildings.
In single, large open spaces with uniform loads: cinemas, indoor gyms, industrial warehouses, supermarkets. Where all positions have virtually identical thermal needs, CAV remains the simplest and cheapest solution.
The fan runs always at maximum - even if the building is half-empty. Power consumption remains constant 24/7. In a 5,000 m² office building, the CAV fan can waste 30,000-50,000 kWh/year more compared to VAV.
The philosophy is completely reversed: the AHU delivers air at a fixed, chilled temperature (13-14 °C). What changes continuously is the volume of air reaching each office through the VAV Box - a metal device above the false ceiling of each zone with a motorised damper that works like an air "tap."
On each office wall sits a thermostat (preset 24 °C). If the temperature reaches 28 °C: "Open 100%!" - a massive volume of chilled air (14 °C) floods in and cools instantly. Once it hits 24 °C: "Close" - the damper closes to 10-20% (ASHRAE minimum outdoor air).
Every office, meeting room or corridor has its own VAV Box and thermostat. It adjusts its temperature completely independently from the adjacent room. In a 50-zone building, 50 independent air "taps" operate simultaneously.
For higher budgets, each VAV Box can incorporate an electric resistance or hot water coil (reheat coil). If an office needs heating while the AHU sends cold air, the reheat warms it locally - without affecting anyone else.
Single-duct VAV: one duct (cooling) + optional reheat. Fan-powered VAV: built-in small fan for mixing plenum air - ideal for interior zones without windows. Dual-duct VAV: two ducts (hot + cold) - maximum flexibility, higher cost.
What happens when afternoon comes, employees leave, and 40 out of 50 VAV Boxes close? If the huge AHU fan continued at 100%, the pressure would skyrocket - the ducts would whistle or "burst". That is why VAV systems are inseparably linked to Inverter fans (VFD - Variable Frequency Drive).
Inside the main duct, a static pressure sensor is installed (setpoint e.g. 250 Pa). When VAV Boxes close, pressure rises above 250 Pa. The BMS commands the Inverter: "reduce speed!" The fan drops to e.g. 30%.
Energy consumption changes with the cube of the speed: if you reduce speed to 50%, consumption drops to 12.5% (0.5³ = 0.125). This means a VAV fan at half load consumes just 1/8 of the electricity of a CAV fan!
At partial speed the fan operates almost silently. Ducts don't whistle, diffusers don't hum. Meanwhile, mechanical stress (friction, heat) decreases dramatically, extending the fan's lifespan by 40-60%.
A VFD costs €800-2,000 depending on power rating. In a 3,000 m² office building, annual savings can exceed €5,000-8,000 in electricity. Payback occurs in 3-8 months - one of the fastest returns in the HVAC industry.
If you are designing HVAC for a space with many rooms and varying thermal loads, VAV is the undisputed king. The initial installation cost is higher (due to boxes, thermostats and Inverter), but the payback from electricity savings and increased productivity is incomparable.
CAV: single open space, uniform loads, low budget. VAV: multiple rooms, varying loads, office buildings, hotels, hospitals. Under the Greek KENAK (Building Energy Performance Regulation), adopting VAV earns points towards higher energy ratings (class A/A+).
A CAV system costs approximately €25-40/m² installed. A VAV system rises to €50-80/m² (VAV boxes, thermostats, VFD, DDC controllers). The difference is recovered in 2-4 years through reduced operating expenses.
In hospitals, CAV is often used in critical areas (operating theatres, ICU) where ventilation must be constant and strictly controlled, while VAV is used in offices, semi-public areas and examination rooms - creating a hybrid system.
In a typical office building, switching from CAV to VAV reduces fan consumption by 50-70% and total HVAC consumption by 20-30%. Over a 15-year lifecycle, savings can exceed €100,000 for a 5,000 m² building.
🏢 VAV is the "king" of modern building HVAC. Every room has its own air "tap" - no one freezes, no one sweats, and bills drop dramatically thanks to the Inverter.
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