A two-speed ventilation unit runs at low speed all the time and switches to high speed while you cook or shower. Yet the terms "low speed" and "high speed" appear nowhere in the French order of 24 March 1982. What the text requires is two distinct things: a permanent minimum total flow, set by article 4, and a kitchen peak flow, set by article 3, which must remain reachable at any time. A unit's two speeds are simply the most common way of meeting both, which is why a single-speed unit is not unlawful in itself: it is merely obliged to hold the peak continuously.
Understanding the ventilation unit and its operating speeds
The role of the ventilation unit in air renewal and moisture control
The mechanical ventilation unit is the engine of the ventilation system. It extracts stale air from wet rooms (kitchen, bathroom, WC) and allows fresh air in via vents in the living spaces. The result: healthier air, less condensation, and more stable humidity, so less risk of mould.
The difference between single-speed and two-speed ventilation: principle and settings
A single-speed ventilation unit runs at a near-constant flow rate. Tuning happens mainly at installation, through the vents and the ductwork. A two-speed unit provides a reduced flow rate for everyday use, then a boost mode (kitchen, shower) triggered by a switch, timer or sensor. The positions are set by the manufacturer, so improvised adjustments should be avoided.
What speed changes: airflow, noise and electricity consumption
The higher the speed, the greater the airflow. You clear moisture and odours faster, but noise from the vents and the unit can also increase. Electricity consumption rises, and in winter heat losses can increase too. The goal is a good balance between air quality and comfort.
What each speed actually has to deliver
Framing the question as "1 or 2" hides the real requirement, which is not a speed but a flow rate. The table below puts each mode against the article that commands it.
| What the control must allow | The requirement behind it | Where it is written |
|---|---|---|
| Low speed, continuous running | meet the dwelling's minimum total flow, from 35 m³/h with one main room to 135 m³/h with seven | article 4 |
| High speed, boost | reach the kitchen peak flow, from 75 m³/h with one main room to 135 m³/h from five | article 3 |
| Switching between them, at any time | the peak flow must remain reachable at all times | article 3 |
Hence the consequence a client never grasps on their own: leaving ventilation on low all year is not saving money, it is never reaching the flow required in the kitchen while cooking, and so returning moisture and pollutants into the dwelling.
And this is precisely where France's Agence Qualité Construction records a recurring defect: the boost cannot be engaged, for want of a pull cord, a corner pulley, a battery or a push button, or because the control is out of reach. The installation is compliant on paper, the peak is technically available, and nobody can trigger it. The handover check is one gesture: operate the control in front of the client and confirm the flow changes.
Two-speed ventilation day to day: speed 1 or 2?
The short answer: speed 1 all the time, speed 2 only during whatever produces steam or smells, then back to 1. Low speed provides the background air change, day and night; high speed holds the kitchen peak flow long enough to clear a spike.
| Situation | Speed | Indicative duration |
|---|---|---|
| Everyday life, night, absence, holidays | 1 | continuously |
| Cooking, especially boiling or frying | 2 | while cooking and 15 to 30 min after |
| Back-to-back showers, bath | 2 | until the condensation clears |
| Laundry drying indoors | 2 | while it dries |
| Full house, meal with guests | 2 | for the occasion |
A client who never switches to 2 finds their cooking steam on the windows. The fix is made on site: a timed switch or a push button near the kitchen that returns to speed 1 on its own.
Can you leave a ventilation unit on speed 2 permanently?
Technically, yes: the motor is designed to run at high speed, it is neither dangerous nor prohibited, since the order sets minimum flows and no ceiling. It is nonetheless almost always a bad bargain, and not mainly because of electricity.
| Item | What continuous speed 2 changes | Cautious order of magnitude |
|---|---|---|
| Motor electricity | power drawn rises sharply with flow | often 1.5 to 3 times the low-speed figure, i.e. one to a few hundred kWh more per year depending on the unit |
| Heating | every extra m³/h extracted is heated air sent outside | 700 to 1,200 kWh per heating season for an extra 50 to 60 m³/h, from the Mediterranean climate to the north-east of France (0.34 Wh/m³·K × flow × degree-hours at 19 °C from the 3CL method) |
| Comfort | noise at the vents, cold air at the inlets, drier air in winter | felt mostly at night, in bedrooms |
The motor costs little; it is the extracted heat that weighs (the French energy rating treats the two separately, see how it counts ventilation auxiliary consumption). Extended speed 2 is justified for a few days (drying out after works, water damage); beyond that it hides another fault: blocked air inlets, clogged vents, badly set low speed or a leaking network.
Never switch the ventilation off, even to save money
The order of 24 March 1982 requires general and permanent ventilation, at least during the period when the cold forces windows to stay shut. Switching the unit off stops the air change: condensation, mould and indoor pollutants set in. In a building fitted with gas-linked extraction (VMC-Gaz), where boiler flue gases pass through the same network, neither the unit nor the vents are to be touched: they belong to the building manager. On your installations, choose a control with two positions, low and high speed, and no off position: it is the simplest way to prevent the unit being switched off.
When to choose a single-speed ventilation unit on your jobs
Suitable cases: small dwellings, short ducting, standard use of wet rooms
A single-speed ventilation unit makes sense in small dwellings or compact houses. It works well when the ductwork is short, with few bends, and vents in the kitchen, bathroom and WC used "as usual". It's also a good choice if the occupants don't generate big daily humidity spikes.
Advantages: simplicity, purchase cost, quick installation and basic maintenance
You're going with a simple-to-install solution. A unit, ducts, vents, and you're done. The purchase cost stays low and commissioning is quick. Maintenance is limited to dusting the vents and air inlets, and checking the condition of the ducts and the unit.
Points of attention: peak comfort, air inlets, heat losses and noise nuisance
During showers or cooking, the flow rate doesn't increase, so comfort can be lower than with a humidity-controlled system. Plan for air inlets consistent with the window/door configuration. And keep in mind the heat losses linked to extraction, plus the risk of noise if the ductwork is too long, poorly hung, or lacking acoustic treatment.
When to favour a two-speed ventilation unit (and how to control it)
Relevant scenarios: large house, high humidity, large family, heavily used kitchen
A two-speed ventilation unit is useful when needs vary a lot. In a large house, or with a large family, humidity spikes follow one another (showers, laundry). In the kitchen, it helps clear moisture and odours quickly, while keeping a low speed the rest of the time to ventilate without excess.
Control and regulation: 2-speed switch, timer, hygrostat (depending on model)
The simplest control is via a 2-speed switch, often placed near the kitchen or a bathroom. To avoid it being left on, a timer automatically returns to low speed after 15 to 30 minutes depending on the setting. Some models add a hygrostat that switches to high speed when humidity rises, then back down once the air is healthy again.
Expected gains and limits: comfort, odour control, watch out for poor settings
In use, you gain comfort and odour control, with less steam and condensation. The limit is the setting. High speed activated too often increases noise, consumption and, on a single-flow system, heat losses. Plan for a clean setup, and regular maintenance of the vents and air inlets.
Practical selection criteria to decide between single-speed and two-speed
Sizing: regulatory airflow rates, pressure losses, duct length and vents
Start from the dwelling's ventilation tables. A ventilation system must meet the regulatory airflow rates room by room. If the ductwork is short, with few bends and 2 or 3 vents, single-speed is often enough. As soon as the ducts get longer, the connections multiply, or the vents are further away, pressure losses climb. Two-speed gives you margin at high speed while staying gentle at low speed, without oversizing the unit.
Job site compatibility: occupied renovation, loft access, electrical supply, acoustics
In an occupied renovation, two-speed is practical if you can add a kitchen or bathroom control. With limited loft access, favour a simple layout, fewer cables, fewer settings. On the comfort side, low speed helps keep night-time noise down. Also check the available power supply and the unit's location to avoid vibration. That location is settled during the survey: the position of the future unit, the service casing and its access, with the equipment taken from your catalogues.
Total cost: unit purchase, accessories, installation time, maintenance and callouts
Look at the total cost. Two-speed can cost more to buy and requires a switch, a relay or a timer, so more installation time. Maintenance is similar, but there's one more point to diagnose. If high speed is only used at peak times, consumption can stay contained. That total cost is set out line by line in the costing: Argile pulls equipment, labour and accessories from the work plan and applies VAT line by line.
Humidity-controlled ventilation stuck on high speed: causes to check
A humidity-controlled unit is not meant to stay on high speed. In this system, flow varies mainly at the vents, which open with the room's humidity, and the kitchen peak only engages on command, with a timer (the principle is detailed in the article on the humidity-controlled extract vent). If the unit or the kitchen stays at maximum, the cause is almost always the control, the wiring or the kitchen vent.
| Symptom | Likely cause | Check |
|---|---|---|
| High speed as soon as power is on, without touching anything | high-speed wire permanently live, or both speeds powered together | unit terminal block and manufacturer's diagram, power off |
| Control has no effect, continuous high speed | faulty two-speed switch, timer or relay stuck | operate the control, check the voltage on the high-speed terminal |
| Strong flow in the kitchen only | timed kitchen vent stuck open: flat battery, jammed pull cord, clogged mechanism | clean, change the battery, time the return to base flow |
| Strong airflow everywhere, though the unit is on low speed | humidity-controlled vents wide open from real humidity (laundry, overcrowding) or clogged | measure the humidity, clean the vents |
| High speed triggering at random | hygrostat or sensor out of adjustment, electronic control fault | setpoint and manufacturer's procedure |
Never disconnect high speed to quieten the unit: the kitchen peak becomes unreachable again, the very defect the Agence Qualité Construction records above.
Choosing a quiet two-speed single-flow ventilation unit
What counts is the noise heard in the room at low speed, which is how the regulation measures it (the thresholds and the diagnosis of a noisy installation are in the article on ventilation noise). Five criteria separate two models:
- Sound level under equal conditions: sound pressure at a given distance or sound power, at the same flow. A dB(A) figure without a measuring point cannot be compared.
- The low-energy motor, often electronically commutated: it turns more slowly for the same flow, runs cooler and is quieter. Its conventional consumption, in W-Th-C, appears in the French technical assessment; the order of magnitude is one to a few dozen W-Th-C depending on the type and size of dwelling, higher on older motors.
- A real gap between the two speeds, ideally with an adjustable low speed, so as not to ventilate harder than needed at night.
- Decoupling: anti-vibration hangers or mounts, flexible connections, no rigid contact with the roof structure or a bedroom partition.
- The network: ducts of the right diameter, wide bends, a silencer on the kitchen side if the unit is close to living spaces.
Installation and commissioning in 2026: best practices for effective ventilation
Installation checklist: hangers, joint sealing, duct insulation in cold zones
Effective ventilation is decided first at installation. Fix the unit on anti-vibration hangers, plan for maintenance access, and avoid crushing the ducts. Take care with thorough sealing at every joint (clips, suitable tape) to limit air leaks and noise. In unheated lofts, insulate the ducts and address cold spots, especially on the extraction side, to reduce condensation.
Settings and balancing: airflow checks, smoke tests, simple on-site measurements
At commissioning, check the actual airflow rates vent by vent. An anemometer or measuring cone is enough on site to spot an unbalanced network. Complement this with a smoke test to visualise flow reversals, overly airtight doors, or missing air inlets. Note the settings and leave a simple sheet for the client.
Maintenance to plan: vents, any filters, unit cleaning and spotting common faults
Plan for simple maintenance. Regular dusting of the vents, checking and replacing filters if present, cleaning the unit and checking condensate. Common faults: a clogged vent, a disconnected duct, noise linked to a leak or too high a flow rate.
