Dual-flow mechanical ventilation (with heat recovery)

A dual-flow mechanical ventilation system (with heat recovery) renews the air by supplying filtered fresh air and extracting stale air, while recovering heat (and sometimes some humidity) from the extracted air using a heat exchanger. The result: improved indoor air quality, significantly reduced ventilation losses, and more stable comfort, both in winter and summer. When well-designed, balanced, and maintained, it becomes a key factor in complying with the RE2020 regulations, as well as in offices, schools, shops, and multi-family housing.

Dual-flow mechanical ventilation: principle, components and settings

A typical installation includes: a dual-flow unit (two fans), a heat exchanger (counter-flow plates, rotary, sometimes enthalpy), a bypass (for free/night-cooling), filters (G4–F7 on fresh air, G4 on return air), sealed and balanced (supply/extraction) networks, acoustic attenuators, condensate trays (winter), probes (T°, RH, CO₂/TVOC) and a BMS/automation system.

Key performance indicators:

- Exchanger efficiency η ≈ 75–90% depending on technology and flow rate.

- Balancing of N-1 / N flow rates (isopressure) to avoid overpressures/underpressures.

- On-demand regulation (CO₂/presence/TVOC) and time-based laws.

- Bypass for free/night cooling when the outside air is cooler/drier.

- Filtration adapted to the context (urban/pollen), ΔP control filters.

- Anti-frost treatment (pre-heating, flow modulation) and condensate drainage.

- Sealing/insulating networks to limit losses and noise.

Advantages, limitations and points to consider of a dual-flow mechanical ventilation system

Interests

• Energy: high ventilation losses avoided (kWh heating ↓).
• Comfort & IAQ: filtered air, gentler blowing temperature, controlled CO₂.
• Acoustics: better distributed flow rates → reduced noise levels at the vents.
• Control: bypass and regulation for the off-season and night-cooling.

Boundaries

• CAPEX and footprint are higher than in single flow.
• Demanding networks (traces, attenuation, sealing).
• Essential maintenance (filters/condensates/wheels) to avoid drift.

Points to consider

• Sizing based on actual hygiene flow rates (offices/schools ≠ housing).
• Networks: moderate speeds (noise), precise balancing, well-positioned connections.
• Filtering: classes adapted to the site, replacement plan (ΔP, schedule).
• Bypass & anti-frost: hygro-thermal logic (T°/RH), not just T°.
• GTB: time-based scenarios, alarms (filters, frost, fans), CO₂/T° trends.
• Accessibility: hatches and clearances for maintenance (condensate trays, batteries).

Anecdote — “CO₂ levels have decreased in Lille”

In Lille, a primary school was reaching 1,700 ppm of CO₂ in the afternoon. The existing dual-flow ventilation system was not providing enough airflow, the filters were clogged, the bypass was stuck, and the flow rates were unbalanced. After upgrades (F7 filters, balancing, CO₂-controlled logic, and a bypass for night cooling), the classrooms stabilized below 900 ppm, the airflow became gentler, and the teachers noted "less oppressive end to the day"... without a surge in energy consumption thanks to the finally efficient heat recovery system.