A flow measurement only means something against a requirement: that of the French order of 24 March 1982, valve by valve, between 15 and 45 m³/h continuous depending on the room, and up to 135 m³/h at kitchen boost. Before concluding that the fan unit is at fault, two installation checks explain most missing flow: the valve must sit at 1.80 m minimum above the floor and 20 cm minimum from its adjacent surfaces, failing which it will never deliver its nominal figure.
Why measure ventilation flow rate on site in 2026
Ensuring air quality and avoiding mould: the points to check
A ventilation system that delivers as intended is your best ally against moisture. On site, check the actual flow rate at the vents, the condition of the ductwork, and the continuity of the air inlets.
- Room-by-room flow rates, depending on the type of dwelling and system.
- No obstruction, ducts properly connected, limited leaks.
- Correct operation of the speed settings and correct positioning of the vents.
Spotting an undersized or poorly adjusted ventilation system: signs and consequences
Windows running with condensation, stale odours lingering, weak suction. These clear signals often point to ductwork that's too long, a tired fan, or poor balancing. The result: discomfort, higher heating consumption, and the return of mould.
Securing your jobs: diagnostic traceability and client expectations
In 2026, clients want proof, not promises. Keep a measurement sheet (date, instrument, values, settings). That's your evidence at handover and a simple aid for explaining mechanical ventilation maintenance.
What you measure, and what you compare it to
The most common error is not in the instrument, it is in the reference. A flow recorded without the dwelling's size means nothing, since the requirement depends on the number of main rooms.
| Measurement point | Comparison figure | Where it comes from |
|---|---|---|
| Kitchen valve, continuous | 20 to 45 m³/h by dwelling size | article 4 |
| Kitchen valve, boost | 75 to 135 m³/h by dwelling size | article 3 |
| Bathroom valve | 15 or 30 m³/h by dwelling size | article 3 |
| WC valve | 15 or 30 m³/h by dwelling size | article 3 |
| Sum of all valves, continuous | 35 to 135 m³/h by dwelling size | article 4 |
Note the last row: it is the only one that checks unambiguously, because it does not depend on how flow is shared between valves. If the sum is there but one valve is weak, the problem is balancing. If the sum is not there, look at the ductwork or the unit.
The order of checks, before blaming the unit
Start with what costs nothing. Is the valve at its height and clearances, or has a unit been pushed in front of it since handover? Are the air inlets in the main rooms present, correctly sized, unobstructed, and above all absent from the service rooms? Does air transfer actually happen, door undercut or grille, or has a solid door been fitted afterwards?
Those three points cover most of the defects the Agence Qualité Construction records on single-flow installations, and they are visible without an instrument. Only then does measurement become informative, because it bears on a network whose geometry is correct.
Two record-keeping habits to finish. Write the dwelling's size at the top of the sheet, or your figures will be unreadable in six months. And measure at boost as well as continuous: boost is where the gaps show, and boost is where the client will complain.
What you're actually measuring: flow rate, pressure, and ventilation balancing
Extracted and supplied air flow rate: orders of magnitude and common mistakes
Flow rate is the quantity of air actually moved. In a dwelling, you're often talking about a few tens of m³/h per vent, with higher peaks in the kitchen. The classic mistake is measuring too quickly. Without a measuring cone, with a clogged grille, or with a closed door, your flow reading can be thrown off.
Available pressure, pressure losses, and the impact of the ductwork (vents, ducts, bends)
A ventilation system can look "good on paper" and fall short on site if the available pressure is eaten up by pressure losses. Ducts that are too long, tight bends, a reduced diameter, overly restrictive vents. The result: flow rate drops, noise rises. Aim for a network that's as simple as possible, airtight and correctly sized, particularly regarding insulating ventilation ducts in unheated lofts.
Room-by-room balancing: kitchen, bathrooms, WC, bedrooms, and living rooms
Balancing is about distribution. You extract more in the kitchen, bathrooms, and WC. You bring in fresh air via air inlets or supply into living rooms and bedrooms, then let it circulate under the doors. Adjust, then re-measure. Good room-by-room balancing avoids moisture, odours, and discomfort. In a block of flats the same work is redone floor by floor, because the pressure varies from the base to the head of a shared extract riser.
Diagnostic tools for measuring ventilation flow rate
Anemometer and measuring cone: choosing the right equipment for the vents
For a reliable ventilation diagnosis, an anemometer alone is rarely enough at a vent. It's more accurate with a measuring cone, especially on circular vents or air inlets where the flow is turbulent. Check the cone's seal against the fitting, let the flow stabilise for a few seconds, then take several readings if the vent is wide.
Flow hood: when it saves time and when it can mislead you
A flow hood is a genuine time-saver on site when the vents are accessible and repetitive. It can mislead you if the hood leaks, if the grille is too close to an obstruction, or if the speed is low, with errors from recirculation inside the hood. A cross-check on one vent with a cone helps calibrate the gap.
Indirect measurement: manometer, smoke pencils, and simple tests to complete the diagnosis
Complement the flow measurement with a manometer to read the useful pressure (clogging, a crushed duct, imbalance). Smoke pencils, or a simple thin sheet of paper, confirm the direction of airflow and any parasitic inlets. These tests don't replace a measured flow rate, but they quickly explain why the ventilation isn't hitting its values.
Field method: getting your flow measurements right without skewing the diagnosis
Preparing the visit: condition of vents, filters, doors, windows, and the ventilation regime
Before any measurement, put the installation back into a stable configuration. Vents accessible and clean, filters dusted off, no cover or warped grille. Close the windows. Set the interior doors to their normal-use position and note it. Set the mechanical ventilation to its normal regime, no boost mode. Wait a few minutes for the ventilation to stabilise.
Taking the readings: instrument positioning, stabilisation time, and useful repetitions
Use a cone or a measuring hood suited to the vent, well centred on it, with no leak around the edges. Keep the instrument still. Let the reading stabilise, then note it down. Take 3 passes and keep the average. A single reading is often a falsely good figure.
Recording and interpreting: measurement table, gaps, corrective actions (adjustment, cleaning, replacement)
Record room by room. Measured flow rate, vent setting, observations. If the gap is genuinely significant, start simple. Clean the vents and filters, check for crushed ducts, then adjust. If the fan or a vent is worn out, replace it. Re-measure after each action. Argile attaches these readings and photos to the job file, room by room.
Practical cases and sound decisions after a ventilation diagnosis
Single-flow mechanical ventilation: adjusting vents, detecting leaks, and repair priorities
After taking flow measurements, start with the basics. Clean the vents, check the flaps and air inlets. Locate crushed, disconnected, or punctured ducts in the loft. A leak can "pull" air from the wrong place. Repair and seal as a priority, then adjust the vents.
Balanced mechanical ventilation: checking flow rates, filters, and network imbalances
Check the condition of the filters and the heat exchanger. A clean filter avoids flow-rate loss and protects the unit. Measure supply and extraction room by room. If it drifts, look for a closed damper, a pinched duct, poorly insulated ductwork, or poor balancing. Fix these before touching the speed settings.
After insulation or window replacement: checking the ventilation flow rate and adapting the air inlets
The tighter you make a building, the more ventilation becomes the item that comes back to you. Confirm the required flow rates at survey, check that door undercuts survive the follow-on trades, and replace unsuitable grilles before handover rather than on a callback. The target is a sufficient flow rate without draughts, with air inlets correctly sized and their break-off sections left in place. Those undercuts and inlets belong on the survey rather than on a callback: the visit pins them room by room, with the unit and the equipment for the works.
