Three different solutions can be built on the same existing vertical duct, and France's 3CL-DPE 2021 method scores them a long way apart. Passive stack ventilation is taken at 2.23 m³/(h.m²) of reference floor area, mechanical extract on an existing duct at 2.24 then 1.97 depending on whether it predates 2012, and hybrid ventilation at 1.52, then 1.33, then 1.17 on the same timeline. In other words, motorising a duct without revisiting the air inlets is the least favourable line in the whole table, while the same intervention treated as hybrid nearly halves the conventional flow.
Understanding the principle of assisted natural ventilation through a duct
Difference between natural ventilation, mechanical ventilation and stack draught through a duct
Natural ventilation relies on thermal draught and wind, with no motor, and rates that move with the weather and with the airtightness of the home. Mechanical ventilation imposes a rate through a fan, at the cost of running power and maintenance. Assisted natural ventilation sits between the two: the duct does the work most of the time, and a fan takes over when the draught is no longer sufficient. That intermittency is what defines the hybrid category, and it is what the method puts a figure on.
How the duct creates the draught: pressure, height and temperature difference
The principle is that of the chimney effect. Warmer indoor air is lighter: it rises in the duct and creates a negative pressure that draws fresh air in through the inlets. The pressure difference grows with the height of the duct and with the indoor-outdoor temperature gap, and wind at roof level can either reinforce or oppose it. It is the same driver behind summer night purge, where the vertical stack doubles the achievable air change rate. The practical consequence is that draught is weakest in the shoulder seasons, exactly when demand is high, and that is precisely the window mechanical assistance exists to cover.
What the French DPE method assigns to each of these configurations
The table below gathers the six relevant lines, with the conventional air-inlet modulus that feeds the permeability calculation alongside the extract rate.
| Category in the method | Conventional extract rate | Air-inlet modulus |
|---|---|---|
| Passive stack ventilation through a duct | 2.23 m³/(h.m²) | 4 m³/(h.m²) |
| Passive stack ventilation with humidity-controlled inlets | 2.23 m³/(h.m²) | 3 m³/(h.m²) |
| Mechanical extract on an existing duct, up to 2012 | 2.24 m³/(h.m²) | 4 m³/(h.m²) |
| Mechanical extract on an existing duct, after 2012 | 1.97 m³/(h.m²) | 4 m³/(h.m²) |
| Hybrid ventilation, after 2012 | 1.17 m³/(h.m²) | 3 m³/(h.m²) |
| Hybrid ventilation with humidity-controlled inlets, after 2012 | 1.17 m³/(h.m²) | 2 m³/(h.m²) |
Two readings come out of it, and they shape the quote. First, fitting humidity-controlled inlets on a duct does not change the extract rate taken, but drops the modulus from 4 to 3, or from 3 to 2 on a hybrid: a real gain, carried entirely by the permeability term. Second, the gap between the "mechanical extract on an existing duct" line and the hybrid line is large even though the installations look alike on site. What separates them is the nature of the assistance and whether the inlets were revisited, so it is what you describe and evidence, not simply what you wire in. The category adopted also weighs on ventilation heat losses, which feed the sizing note to NF EN 12831-1 that Argile generates.
Choosing and sizing the duct for effective ventilation
What the draught has to achieve, and what is not negotiable
Article 3 of the order of 24 March 1982 applies in full: systems, whether mechanical or passive, must be able to reach the same extract rates, 75 to 135 m³/h in the kitchen depending on the number of main rooms, and 15 or 30 m³/h in bathrooms and sanitary accommodation. No text grants stack draught a tolerance on the grounds that it depends on the weather. That is why sizing a duct is not validated by calculation but by measurement, under unfavourable conditions.
Height, diameter, routing: the criteria that really make a difference to draught
The duct must limit pressure losses and keep a usable draught across the whole season. Keep the routing as straight as possible, with few bends, wide radii and a constant cross-section. An undersized duct drops the draught and raises noise; an oversized one lets air stagnate and encourages condensation on cold surfaces. The cross-section adopted is justified by the terminal's data sheet or by a draught study, not by copying the existing diameter.
Materials and installation: masonry duct, metal duct, ducting and airtightness
In renovation, favour a continuous rigid metal liner, more consistent and easier to seal than flexible ducting, and reachable for maintenance. Insulated ducting limits condensation in cold lofts. Treat every joint with tape rated for airtightness. On an old flue, the preliminary diagnosis covers clearance, airtightness, continuity and cross-section, and the use is never mixed with a combustion appliance still in service.
Carrying out the installation on site without bad surprises
Preparing the substrate and securing penetrations (lofts, floors, roof)
Before running anything, survey the route, the access, the fixing points and the cold zones. In lofts, avoid crushing within the insulation. At floor and roof penetrations, treat the sleeves, seals and protections carefully, with airtightness and vapour-barrier continuity as the goal. A rushed penetration turns the duct into a thermal bridge and a condensation point.
Key locations: air inlets, extract points, roof outlets and recirculation risk
Extract points go in service rooms and air inlets in main rooms, as article 2 requires. Article 15 adds a constraint that is easily forgotten on this kind of job: air inlets may be self-regulating or occupant-adjustable, but they may not be closable. The roof outlet must clear turbulence zones and stay away from openings and other discharges, otherwise extracted air finds its way back inside. Sizing the network follows the same rules as on a mechanical system.
Managing on-site constraints: humidity, odours, noise, back-draught and wind
In unheated spaces, insulate the duct and check the falls to limit condensation. Secure outlets with an anti-backflow device suited to the terminal. On acoustics, isolate the mechanical assistance from the structure and limit transmission through partitions. In a multi-unit building, the first risk to investigate is back-draught between dwellings on a shared duct, which calls for a draught study rather than an on-site judgement.
Checking performance and air quality after the works
Useful measurements: flow rate, smoke test, anemometer and check points
First confirm the direction of flow, dry to wet, with a smoke test or a sheet of paper. Then measure the rate with a flow hood at each terminal and compare against article 3, not only against the manufacturer's data. On a draught-driven system, take the measurement under the least favourable conditions you can reproduce, small temperature gap and no wind, because that case decides real compliance. The measuring instruments and their use are chosen before the visit.
Adjustments and balancing: air inlets, extract points, dampers and consistency across wet rooms
Adjustments are made room by room. Set inlets and extract points to avoid an over-extracted kitchen and bedrooms with no supply, work the dampers, then re-measure to validate the balance. Service rooms stay the priority, without creating excessive negative pressure that would draw in odours or air from a garage. Record every damper position in the report; it will be the starting point for any later intervention.
What you record, and why it matters beyond handover
The handover file for an assisted ventilation system earns its keep twice. It establishes compliance with the extract rates of the order, which covers you on the workmanship warranty if a damp defect appears. And it lets the assessor pick the right category among the six in the table above, instead of defaulting to the least favourable one for want of evidence. A photo of the terminal, a photo of the inlets, a dated flow record and the data sheet for the assistance device: those four items are enough, and they are worth the gain the client bought from you. The full nomenclature is covered in our article on the 38 ventilation categories in the French energy assessment.
Regulatory framework, safety and aid related to ventilation in 2026
What mechanical assistance costs in running power
The method does not calculate hybrid systems directly: it takes the auxiliary consumption of a self-regulating mechanical system from 2001 to 2012 and applies a ratio for the time spent in mechanical mode. In a detached house that ratio is 0.083, for a conventional boost running time of 14 hours per week. In a multi-unit building it is 0.167, for 28 hours per week. That is the quantified argument for hybrid over a conventional mechanical system: the fan is there, but it is only credited with a fraction of the running time.
Fire safety, combustion appliances and clearances
Article 8 requires the ventilation system to deliver the rates needed for any combustion appliances in the dwelling to work properly, and that is the first thing to investigate on a reused duct. Article 14 prohibits connecting an individual mechanical device to a communal air outlet installation, passive ones included, which rules out certain assistance solutions in multi-unit buildings from the outset. At wall penetrations, provide the required sleeves and sealing, and keep safety clearances from flue ducts. The comparison between passive and mechanical sets out the workable configurations.
Aid and requirements depending on 2026 projects: certification, energy audit and consistency with insulation
On the aid side, ventilation can be included in energy saving certificate and retrofit grant applications depending on the pathway and the scope of works, with the company holding the relevant certification. For a major renovation, an energy audit and support may be required. The point to hold on to is that the tighter the envelope, the less stack draught alone can cope, which makes revisiting the air inlets inseparable from the insulation package. Those inlets to create are surveyed with the rest of the works, wet rooms, vents and the unit to be installed included. On whether keeping an unpowered system still makes sense, see our analysis of natural ventilation against the energy assessment.
