Assisted natural ventilation (ANV)

Assisted natural ventilation (ANV) — also known as hybrid ventilation — combines natural mechanisms (thermal draft and wind pressure) with minimal mechanical assistance (low-energy fans, damper actuators) to renew the air in buildings. The idea is to let nature do most of the work and only activate assistance when needed (no wind, small temperature fluctuations, peak occupancy), in order to ensure good indoor air quality, summer comfort, and energy efficiency.

VNA: principle, components and control

A VNA relies on vertical chimneys/shafts (draft), sensors (CO₂, temperature, humidity, wind speed/direction), controlled air inlets (facades, motorized frames, grilles with attenuators), and roof outlets (skylights, static ejectors, cowls).

When external conditions are sufficient, the air is naturally renewed: warm air rises, and fresh air enters through the facades. If the temperature gradient or wind is insufficient, variable-speed auxiliary fans (VFDs) take over (often in extraction mode) to ensure adequate airflow.

The building management system (BMS) is controlled by CO₂/RH setpoints, time-based systems, free/night cooling, and bypass of the heat exchangers if an air handling unit (AHU) is present for seasonal support. Motorized dampers regulate airflow (zoning, draft prevention); rain/frost sensors ensure safe window opening. Acoustics are addressed by attenuators and controlled air velocities in the ducts.

Advantages, limitations, and points of attention of a well-designed VNA

Interests

• Energy efficiency: fans under little use → very low kWh for fans and SFP.
• Summer comfort: massive night-cooling via draft, reduction of DH (RE2020) without a cooling unit.
• Air quality: generous fresh air intake when conditions allow (CO₂/odours ↓).
• Resilience: passive operation possible in case of electrical failure (excluding motors).

Boundaries

• Dependence on climate/location (wind, ΔT): variable performance.
• Fine piloting is necessary to avoid drafts or excessive flow rates in winter.
• Acoustics and outdoor pollution to be controlled (filtered/positioned air inlets).
• Safety: fire protection and smoke extraction compatibility to be checked (separate networks).

Points to consider

• Installation: sizing chimney sections, emerging heights, air intakes far from polluted sources; add filters (ePM) if urban context.
• Regulation: CO₂/HR/T° sensors, predictive weather algorithms, opening hysteresis; limit air speeds in occupied area (≤ 0.15–0.2 m/s).
• Acoustics: attenuators, moderate speeds in ducts, choice of silent dampers.
• Sealing & thermal performance: frames/shutters with thermal break, effective seals to prevent heat loss and cold infiltration.
• System coupling: VNA + CTA double flow (winter/inter-season support mode), exchanger bypass for free-cooling, anti-conflict interlocks.
• Operation: remote CO₂/openings reading, seasonal tests, nighttime instructions to discharge inertia (exposed slabs).

Anecdote — “The school that breathed better in Toulouse”

In Toulouse, a renovated school has adopted a natural ventilation system (NVS) with solar chimneys on the roof, motorized air inlets on the facade, and CO₂ control. In winter, natural ventilation covers 70–80% of the time; during the shoulder seasons and summer, the night-cooling system opens the windows wide to remove heat from the ceiling slabs. On windless days, VFD (Variable Flow Dynamic) extractors take over. The results over a year: CO₂ < 1,000 ppm on average, complaints of discomfort plummeting, and fan consumption reduced by two-thirds compared to the old single-flow system. The building manager sums it up: "We let the sky do the ventilation... and kept the fans just for the unpredictable days."