🌀What is ventilation balancing?
Balancing a mechanical ventilation system is the technical process of measuring, adjusting and optimising airflow at every point in the ductwork so that it matches design or regulatory airflow rates. It is a fundamental stage of HVAC commissioning that directly determines indoor air quality, occupant comfort and the building's energy performance.
There are two main families of mechanical ventilation: extract ventilation (MEV), which simply extracts stale air from wet rooms (kitchen, bathroom, WC) while fresh air enters through facade trickle vents, and heat-recovery ventilation (MVHR), which both extracts air and supplies fresh air, recovering heat through a heat exchanger. Balancing differs significantly between the two: with MEV, only the extract ductwork is balanced, whereas with MVHR both networks (extract and supply) must be balanced and their mutual balance checked.
Air balancing is critical because a poorly balanced system can cause insufficient airflow in some rooms (health risk), excessive airflow in others (discomfort, energy waste), noise nuisance, damp and condensation problems, and premature equipment deterioration. With MVHR, an imbalance between supply and extract drastically reduces heat exchanger efficiency, cancelling out the system's energy benefit.
Balancing is carried out by a specialist, the HVAC commissioning engineer, who has the necessary measuring instruments (balometer, anemometer, manometer) and knowledge of current standards and regulations. The results are recorded in an air balancing report, which forms part of the as-built file (DOE) handed to the client at practical completion.
Why balance a ventilation system?
Indoor air quality
Correct balancing guarantees the air change rate each room needs. Insufficient airflow leads to a build-up of indoor pollutants (CO2, VOCs, fine particles), while excessive airflow causes uncomfortable draughts and dries out the indoor air.
Thermal comfort
A poorly balanced ventilation system can create cold zones in winter through uncontrolled air inlets or excessive extraction. Balancing maintains an even temperature throughout the building while ensuring effective ventilation.
Regulatory compliance (DTU 68.3)
DTU 68.3 (French standard) sets out the installation rules for mechanical ventilation systems. The French Order of 24 March 1982 sets the minimum extract airflow rates. A balancing report certifies that the installation complies and is a mandatory contractual document at handover.
Energy savings
A balanced ventilation system uses less energy. Excessive airflow overloads the fan motor and increases heat losses through air change. In an MVHR system, an imbalance between supply and extract drastically reduces heat exchanger efficiency.
Reduced noise nuisance
Whistling, vibration or rumbling in the ductwork is often a sign of excessive air velocities caused by poor balancing. Adjusting the airflow rates reduces duct velocities and limits acoustic nuisance for occupants.
Preventing damp and mould
Insufficient extract airflow in wet rooms (bathroom, kitchen, WC) encourages condensation and mould growth. Balancing ensures that each wet room receives the extract airflow needed to remove the moisture produced by occupants.
Types of ventilation system and how to balance them
Each type of ventilation system has its own balancing characteristics that call for a tailored approach. The commissioning engineer must know the technical features of each system to carry out balancing that is relevant and meets the expectations of the design consultant and the manufacturer.
Self-regulating extract ventilation (MEV)
Self-regulating extract ventilation maintains a constant extract airflow regardless of outdoor conditions (wind, temperature). Extract terminals are fitted with constant-flow modules. Balancing involves checking that each terminal delivers its design airflow and that total extract airflow matches the fan unit's capacity. Facade trickle vents must be sized to compensate exactly for the air extracted. An imbalance between inlets and extracts causes uncontrolled infiltration or uncomfortable pressurisation.
Humidity-sensitive extract ventilation (type A and B)
Humidity-sensitive ventilation modulates its airflow according to indoor humidity. In type A systems, only the extract terminals are humidity-sensitive (fixed air inlets). In type B systems, both air inlets and extract terminals are humidity-sensitive. Balancing is trickier because airflow varies. Airflow must be checked at minimum and maximum positions, humidity sensors must be confirmed to work correctly, and the modulation range must match the manufacturer's specifications. Measurements must be taken with a balometer suited to low airflow rates.
Heat-recovery ventilation (MVHR)
MVHR simultaneously extracts stale air and supplies fresh air pre-conditioned by a heat exchanger. Balancing is twofold: the extract and supply ductwork must each be balanced independently, then the overall balance between the two checked. An imbalance between supply and extract puts the building under negative or positive pressure, reducing heat exchanger efficiency (rated efficiency 80 to 95%). Supply airflow should be slightly lower than extract airflow (typical ratio 0.9 to 0.95) to maintain a slight negative pressure that helps remove moisture.
Thermodynamic MVHR (with heat pump)
Thermodynamic MVHR adds a heat pump to the heat exchanger to preheat or cool the supply air. Balancing must take into account not only airflow rates but also thermal performance. Airflow must match the manufacturer's requirements to achieve the stated COP (coefficient of performance). Too little airflow reduces heating/cooling capacity; too much degrades the COP. Measurements must be taken in both heating and cooling modes, as airflow may differ between operating modes.
Air balancing method, step by step
Air balancing a ventilation system follows a rigorous 8-step methodology. Each step is essential to ensure reliable, compliant balancing. Skipping a step can distort all the results and lead to unsuitable settings.
Checking the installation (ductwork, connections)
Before taking any measurements, the entire system must be visually inspected: condition of flexible and rigid ductwork, no crushed sections or right-angle bends, quality of connections to terminals and fan unit, presence and condition of fire dampers. Flexible ducts must not have excessive lengths or loops that increase pressure losses. Every connection must be airtight and secured with a suitable clip.
Checking the fan unit (filters, belt, motor)
The fan unit is the heart of the installation. Check the condition of the filters (G4 or F7 depending on type), belt tension (where applicable), the condition of the motor and bearings, and the fan's direction of rotation. Clogged filters increase pressure losses and reduce airflow. The fan curve must be known in order to interpret airflow and pressure readings. Record the current operating point (airflow/pressure) and compare it with the design duty point.
Measuring airflow at extract terminals
Using a calibrated balometer, measure the airflow at each extract terminal. Place the hood so it seals against the terminal or ceiling. Allow the reading to stabilise for 10 to 15 seconds before recording it. Note the measured airflow, terminal number, room and expected airflow (design or regulatory). Repeat the measurement three times if necessary to confirm the value. If a balometer cannot be used (inaccessible or non-standard terminal), use a vane anemometer with a reducing cone and apply the appropriate correction factor.
Measuring airflow at supply terminals (MVHR)
For MVHR installations, also measure the airflow at each supply terminal. The method is the same as for extract terminals. Pay particular attention to the supply air temperature to check that the heat exchanger is working properly. Total supply airflow should be slightly lower than total extract airflow. Compare measured airflow with the design values shown on the consultant's ductwork drawings.
Adjusting terminals and dampers
Based on the differences between measured and design airflow, adjust the terminals and balancing dampers. On self-regulating terminals, adjustment is made by changing the free area (turning the adjustment ring). On terminals with dampers, adjust the damper opening. Always start with the index terminal (the one furthest below its set point) and finish with the most favoured terminal. After each adjustment, re-measure airflow across the whole system, as changing one terminal affects the airflow at the others.
Checking pressures in the system
Measure static pressures at various points in the system with a differential pressure gauge. Check the available pressure at the fan unit, the pressure losses in main and branch ducts, and the pressure at terminals. These measurements help locate any obstructions or leaks and confirm that the fan is operating within its optimum range. Available pressure must be sufficient to serve the index terminal (the one at the end of the run with the highest pressure losses).
Checking ductwork airtightness
Ductwork airtightness is a key factor in ventilation system performance. Duct leaks reduce airflow at terminals and increase fan energy consumption. Testing can be carried out by pressurising the system with a calibrated fan and measuring the leakage rate, or by visual inspection and smoke testing. The target airtightness class depends on the type of installation: class A minimum for dwellings, class B or C for commercial buildings (EN 1507 and EN 12237). The most common leakage points are connections, wall penetrations and duct joints.
Writing the air balancing report
The balancing report is the contractual document certifying that the installation complies. It must include: identification of the installation and building, date of work, references of the measuring instruments used (with calibration date), the airflow measured at each terminal alongside the corresponding design values, measured pressures, and any observations or reservations. The report must be signed by the engineer and handed to the client. It is a key part of the as-built file (DOE).
Airflow measuring instruments
Ventilation balancing relies on accurate measurements taken with professional airflow instruments. The choice of instrument depends on the type of measurement (airflow, velocity, pressure), the installation layout and the required accuracy. All instruments must be calibrated and hold a valid calibration certificate.
Balometer (airflow at terminals)
The reference instrument for measuring airflow at ventilation terminals. The balometer (flow hood) uses a capture hood that completely covers the terminal and reads volumetric airflow directly in m³/h. Typical accuracy: +/- 3% of reading. Measuring range: 25 to 3,500 m³/h depending on model. Requires annual calibration.
Hot-wire anemometer
Measures air velocity from the cooling of an electrically heated wire. Very sensitive at low velocities (0.1 to 20 m/s). Ideal for measurements in small ducts or for detecting air leaks. Requires temperature compensation. Airflow measurement requires a methodical traverse of the duct cross-section (cross-pattern or log-Tchebycheff measuring points).
Vane anemometer
Measures air velocity from the rotation of a vane. Typical measuring range: 0.4 to 40 m/s. Less accurate than a hot-wire at low velocities but more robust. Available as handheld models with vane diameters from 16 to 100 mm. To measure airflow in a duct, multiply the mean velocity by the free area.
Pitot tube + micromanometer
The Pitot tube measures the velocity pressure in a duct, from which air velocity is calculated with the formula V = √(2.Pd/ρ). Combined with a precision differential pressure gauge (minimum resolution 1 Pa), it is the standardised reference method (NF EN 12599). Required for high-pressure systems or for reference measurements when calibrating other instruments.
Smoke testing (airflow visualisation)
Smoke pellets or smoke machines make air movement visible, reveal leaks in the system, confirm airflow direction and identify dead zones with no air change. A qualitative tool that complements quantitative measurements. Useful for revealing short-circuiting between supply and extract air.
Sound level meter (acoustic check)
Checking noise levels is an integral part of balancing. A class 2 (minimum) sound level meter measures the sound pressure level (in dB(A)) at terminals and in rooms. The French regulatory limits are 30 dB(A) in bedrooms and 35 dB(A) in living areas (Order of 30 June 1999). Excessive levels often indicate air velocities that are too high or turbulence at terminals.
Regulatory ventilation airflow rates
Minimum extract airflow rates are set by the French Order of 24 March 1982 (as amended) on ventilation of dwellings. These values are the regulatory minimum and must be met during balancing. The table below shows the airflow rates for a dwelling with 3 main rooms (F3). Airflow rates vary with the number of main rooms in the dwelling.
Kitchen
45 - 135 m³/hMaximum airflow at boost setting (cooking). Minimum airflow for continuous air change.
Bathroom (with WC)
30 m³/hConstant airflow for self-regulating installations. Variable from 15 to 30 m³/h for humidity-sensitive systems.
Bathroom (without WC)
15 m³/hLower airflow, as there is no need to remove WC odours.
Separate WC
15 m³/hConstant airflow. Can be increased to 30 m³/h for heavily used WCs (commercial buildings).
Utility room / laundry
15 m³/hExtraction required to remove moisture from drying laundry.
Source: French Order of 24 March 1982 (as amended) on ventilation of dwellings. Airflow rates are given for an F3 dwelling. For commercial buildings, refer to the French Labour Code (articles R. 4222-1 to R. 4222-26) and standards NF EN 16798 and NF EN 13779.
Frequently asked questions about ventilation balancing
Q. How often should a ventilation system be balanced?
A ventilation system should be balanced at initial commissioning, then checked every 3 to 5 years as part of preventive maintenance. If the ductwork is modified (terminals added or removed, ducts changed, fan unit or motor replaced), a complete rebalance is essential. In commercial buildings subject to periodic regulatory inspections, an annual airflow check is strongly recommended to maintain compliance and energy performance.
Q. How can I tell if my ventilation system is poorly balanced?
Several telltale signs point to an unbalanced ventilation system: heavy condensation on windows (especially in winter), lingering odours in certain rooms (kitchen, WC), cold draughts near supply terminals, excessive noise in ducts or at terminals (whistling, humming), stuffy air or a lack of fresh air. In dwellings, mould in corners or behind furniture is a clear sign of inadequate ventilation. Only a balometer measurement by a professional can quantify the deviations precisely and confirm the diagnosis.
Q. Can I balance a ventilation system myself?
Fully balancing a ventilation system requires professional measuring instruments (balometer, anemometer, manometer) that are expensive to buy and need regular calibration. While a homeowner can adjust the opening of extract terminals with a screwdriver, a complete balance with quantitative measurements and a written report must be carried out by a qualified professional, ideally a certified HVAC commissioning engineer. Rough adjustments made without instruments can worsen existing imbalances and create new problems (condensation, discomfort, energy waste).
Q. How much does professional ventilation balancing cost?
The cost of professional ventilation balancing depends on the size and complexity of the installation. For a single dwelling with extract ventilation and 4 to 6 terminals, allow €150 to €400 incl. VAT. For residential blocks or commercial buildings, the price depends on the number of terminals, the complexity of the ductwork and the type of system: €500 to €2,000 excl. VAT for a small commercial building (fewer than 20 terminals), and considerably more for large developments or industrial installations. These prices generally include measurements, iterative adjustments, the balancing report and travel.
Q. What is the difference between ventilation balancing and maintenance?
Ventilation maintenance is routine work that includes cleaning filters, extract terminals and the fan unit, checking the mechanical condition of components (belt, bearings, motor) and replacing wear parts. Balancing is a separate, complementary technical operation that involves precisely measuring the airflow at each terminal with calibrated instruments, then adjusting settings so that airflow matches regulatory or design values. Maintenance is in fact an essential prerequisite for balancing: clogged filters or blocked terminals completely distort airflow measurements and make balancing ineffective.
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