Mechanical ventilation: sizing fresh-air flow rate room by room for effective ventilation

Understanding the basics of ventilation and the flow rates to target by room

Fresh air and extraction: what mechanical ventilation must actually deliver day to day

Good ventilation does two things. It brings fresh air into living spaces, then extracts air laden with humidity, odours and pollutants. The mechanical ventilation system must keep up this air renewal even with the windows closed. Otherwise you lose comfort and the building fabric takes the hit.

Wet rooms vs dry rooms: flow-rate and air-transfer logic

Air is supplied through dry rooms (living room, bedrooms) via air inlets. It's extracted from wet rooms (kitchen, bathroom, WC). Common target flow rates, depending on settings and home size, are often around 45 m³/h continuous in the kitchen, with a higher boost setting, 15 to 30 m³/h in the bathroom, and 15 m³/h in the WC. Air transfer happens through door undercuts. Keep these passages clear.

Regulatory references and best practices to know in 2026

For both houses and apartments, base yourself on the order of 24 March 1982 (and its amendments) for the minimum flow rates. On the installation side, rely on NF DTU 68.3 and the manufacturers' instructions. In 2026, simple maintenance makes the difference. Cleaning the vents, checking the air inlets, and inspecting the ductwork. That's the right reflex before talking about damp or mould.

Calculating fresh-air flow rate room by room: a simple, repeatable method

Choosing the approach: by volume, by number of occupants, or by use

For each room, pick a single rule, then stick to it. By volume, you target an air-change rate n and calculate Q = V x n. By number of occupants, you start from a flow rate per person for bedrooms and the living room. By use, you increase extraction in wet rooms — kitchen, bathroom, WC. This method makes the ventilation readable and comparable from one room to another.

Accounting for floor area, ceiling height and the home's airtightness

Calculate the actual volume V in m3. Floor area in m2 multiplied by ceiling height. In a very airtight home, the planned flow rate matches the need more closely. In older housing, air leaks can sometimes mask undersizing. Check the air inlets, the grilles, and the condition of the joinery before finalising the flow rates.

Avoiding sizing mistakes that degrade comfort and air quality

A poor setting shows up fast. Too low, and the air goes stagnant. Too high, and you create noise, draughts and heat losses. Keep these good markers in mind.

• Check the units and the velocities in the ducts.
• Ensure consistency between supply and extraction, and leave an air passage under the doors.
• Set, measure, then adjust after a few days of use.

Adapting sizing to the type of mechanical ventilation and the ductwork

Simple-flow ventilation: self-regulating, humidity-controlled, and the effect on flow rate

With self-regulating units, the flow rate stays nearly constant. Size on the regulatory flow rates and the available pressure of the unit, otherwise some vents won't reach their target flow rate and the ventilation becomes uneven. With humidity-controlled units, flow rates vary with humidity. Plan for peak operation (kitchen, wet rooms) to avoid a network that's too "tight".

Heat-recovery ventilation: balancing, heat recovery and flow rate per vent

With heat-recovery ventilation, supply air and extract air must stay close in balance. Good balancing improves heat recovery and limits noise. Size vent by vent according to the rooms, then set the flow rates with a flow meter or anemometer, not "by ear".

Ductwork: diameters, pressure losses and settings to hold the flow rates

The ductwork makes the difference. Diameters that are too small, cumulative lengths, tight bends and fittings increase pressure losses and force the fan to work harder. Keep runs short, use gentle bend radii, and provide balancing dampers. The final commissioning — unit speed, then vent adjustment — is often what "locks in" the correct flow rates.

Checking and adjusting flow rates on site: essential checks

Measuring flow rate at the vents: equipment, points to watch and tolerances

On a ventilation installation, measure the flow rate at each vent with a balometer (flow hood) or a suitable anemometer. Take the readings under real conditions. Clean ductwork, filters in place, doors in their intended position. Compare against the design flow rates and the regulatory values. A tolerance of around 10% is often used on site. To go further on metrology and methods, see also measuring ventilation flow rate.

Adjusting air inlets and undercutting doors: ensuring proper transfer

Without functional air inlets, extraction chokes. Check the type of air inlets, that they open properly, that none are blocked, and their consistency with the main rooms. On the transfer side, a proper door undercut avoids flow-rate loss and whistling.

Signs of a poorly calibrated flow rate: condensation, odours, noise, discomfort

A flow rate that's too low shows up quickly. Persistent condensation, mould, lingering odours. A flow rate that's too high can create noise, draughts and negative pressure that slams doors shut. The final room-by-room adjustment gets the house "circulating" again.

Common special cases in renovation: adjusting ventilation without oversizing

Very airtight homes after insulation work: guaranteeing air renewal

After careful insulation work, air no longer "passes through" on its own. Ventilation must therefore ensure a constant supply of fresh air, without noise or unnecessary losses. Aim for an adjustable installation (measured flow rates, suitable vents, humidity-controlled if relevant) and take care with the air inlets. A check at the end of the job avoids compensating for airtightness with flow rates that are too high.

Open-plan kitchen, windowless bathroom, distant WC: balancing the flow rates

When wet rooms are atypical, prioritise the sources of humidity and odours. Set useful flow rates for the kitchen and bathroom, and secure the air transfer (door undercuts, grilles). In an open-plan kitchen, a timed "boost" control is often more effective than permanent oversizing.

Homes with a wood stove or fireplace: safety, negative pressure and compatibility

With a combustion appliance, ventilation must not create unfavourable negative pressure. Provide a dedicated air supply, and check the compatibility of the stove, the flue and the mechanical ventilation system. If in doubt, have the setting validated by a professional and add a CO detector. Flue-gas safety remains the priority.