Multi-unit mechanical ventilation: single duct or branched network

Understanding the two mechanical ventilation architectures for multi-unit buildings

Single-duct multi-unit ventilation: principle, operation and limits

With a single duct, all dwellings connect to a shared vertical duct, with an extractor on the roof that puts the network under negative pressure. Fresh air enters through the air inlets of dry rooms, then is extracted from wet rooms. Common limits in multi-unit buildings: tricky balancing between floors, pressure losses, noise, and a risk of odour back-draught if the dampers or settings are unsuitable.

Branched network: organisation of connections and air circulation

The branched network distributes extraction via several branches and connections, then a header duct to the fan. This gives better control of flow rates per riser or per stairwell, with more accessible adjustment components. In return, it requires space in technical shafts, good network airtightness, and ongoing maintenance.

Single flow, humidity-controlled, heat-recovery: what changes in multi-unit buildings

With single-flow systems, mechanical ventilation extracts continuously. With humidity-controlled systems, extract points and air inlets modulate according to humidity, often more efficient without changing the architecture. With heat-recovery systems, supply air and heat recovery are added. It's efficient on paper, but heavier to implement in an existing multi-unit building, since it requires two networks, filters, and regular maintenance.

Choosing between single duct and branched network based on the building

Building type: height, number of dwellings, available technical shafts

In a building with a continuous technical shaft across several floors, the single-duct solution often fits the existing structure well. Fewer penetrations, a clear path, and multi-unit ventilation that's simpler to balance. Conversely, when dwellings are laid out on large open floor plates, with few vertical provisions, a branched network makes it possible to work around obstacles and limit duct lengths.

Renovation or replacement: routing constraints, reworking extract points and fan units

In a replacement project, the challenge is to reuse existing routing and extract points as much as possible to avoid redoing finishes. Check the space available for the fan unit, accessibility for maintenance, and diameter compatibility. In major renovation, a branched network offers more freedom but requires precise layout planning and room-by-room adjustments.

Nuisance and continuity of service: interventions in common areas and in dwellings

A single duct often concentrates interventions in common areas. It's faster, but shutoffs and tests need to be organised. A branched network multiplies access points inside dwellings. To limit nuisance, plan by stairwell and by floor, and secure a minimum level of ventilation between two phases of the project.

Key installation points on site (quality, safety, durability)

Sizing and balancing: avoiding under-ventilated dwellings

For a mechanical ventilation system, start from the regulatory flow rates, then adapt to the dwelling. Check air passages between rooms (undercut doors, transfer grilles). At the end of the project, measure each extract point, adjust the settings, and record the measured flow rates in a simple report.

Ducting and connection airtightness: reducing leaks, odours and flow-rate losses

A leak in a loft means lost flow rate in the rooms. Take care with fittings, clamps and sleeves. Use adhesives and sealants compatible with ventilation. Limit flexible-duct lengths, avoid crushing, and secure airtight connections all the way to the fan unit.

Acoustics and vibration: limiting complaints and taking care with fixings

Nuisance often comes from the fixings. Suspend the fan unit on anti-vibration mounts, plan for a flexible connection, and avoid any rigid contact between duct and structure. Add a silencer if needed. Good acoustic decoupling makes the installation last and avoids callbacks.

Maintenance and operation: what weighs on the choice

Network access: hatches, connections, cleaning and dust removal

In renovation, the operating cost of a mechanical ventilation system often comes down to access. Plan inspection hatches at direction changes, at connections, and near the fan unit. Without this, cleaning and dust removal become a project in themselves, and get postponed. Removable extract points, accessible ducting and measurement points avoid working "blind". Goal: easy access for quick maintenance, without damaging finishes.

Common faults: dampers, extract points, fan unit and duct fouling

Common faults come from everyday wear. Stuck dampers, fouled extract points, a worn or noisy fan unit, poorly managed condensate. When ducts get fouled, flow rates drop and odours return. Checking fixings, airtightness and cleaning extract points limits heavy interventions. With a legible network, simple faults get fixed faster.

Multi-unit mechanical ventilation diagnosis: when to propose a full upgrade

In a co-owned building, propose a diagnosis when complaints pile up (noise, humidity, odours), when measured flow rates don't hold, or after several one-off repairs. If the network is inaccessible, punctured, poorly sized, or if the fan unit is at the end of its life, a full upgrade becomes more cost-effective than a patch job. It's also the time to revisit balancing and air inlets. A full upgrade when the building needs to start again on a sound basis.

Financial aid and obligations in 2026: how to secure your file for multi-unit mechanical ventilation

CEE and 2026 schemes: documents to provide, proof of works and inspections

For multi-unit mechanical ventilation funded through CEE, the rule stays simple. Everything comes down to evidence. A dated and signed quote before the works, a sworn statement, a detailed invoice, technical sheets and references for extract points, fan units, ducting and controls. Add before and after photos, plus a record of settings and flow rates. In 2026, documentary and on-site inspections remain frequent. Anticipate them.

RGE requirements and traceability: best practices to avoid a rejected file

Check the RGE qualification for the right trade, the right SIRET number, and the date of signature. On the quote and invoice, show the brand, model, performance data and the exact scope of the ventilation trade. Keep a clear file. Zero ambiguity. Commissioning report, measurement records, and correction reports in case of discrepancies.

Coordination with the co-ownership/property manager: votes, quotes, scheduling and handover of works

In a co-owned building, secure the decision-making chain. A vote at the general assembly, compared quotes, a schedule for access to dwellings and common areas, then a formal handover. Plan for an as-built file (DOE), the handling of reservations, and delivery to the property manager. One single chain of documents, from the vote to the invoice.