Heat-recovery ventilation (MVHR) has become essential under RE2020: it renews the air while recovering up to 90% of the heat from the extract air. But without rigorous air balancing, the advertised performance remains theoretical.
Prerequisites before balancing
- • Check the air-tightness of the ductwork (test to NF EN 12237, class A minimum, class C recommended).
- • Make sure all branch connections are made and all air terminals are installed.
- • Check that the MVHR unit is correctly configured (fan speeds, clean filters, bypass closed).
- • Have the design airflow rates specified by the design engineers for every terminal (supply and extract).
Tools required
- • Flow hood / balometer (e.g. Testo 420 or Swema): direct airflow measurement at terminals and diffusers.
- • Hot-wire anemometer: for in-duct measurements or linear grilles.
- • Micromanometer: measures pressure drops and static pressure in the ductwork.
- • Pitot tube: for velocity measurements in circular ducts (traverse method).
Balancing methodology
- 1. Measure the total supply and extract airflow at the unit → check the overall balance (deviation < 10%).
- 2. Measure each supply terminal and note the deviation from the design airflow.
- 3. Start by adjusting the index terminal (the one with the lowest airflow relative to its setpoint).
- 4. Adjust the dampers or adjustable plenums to reach the target airflow (tolerance ±10% according to DTU 68.3, French standard).
- 5. Repeat for all supply terminals, then for the extract side.
- 6. Final check: measure every terminal again and confirm that the supply/extract balance is maintained.
FIELD TIP
In multi-occupancy housing, remember to check airflow rates with the apartment doors both closed AND open. Air transfer between rooms can significantly change the airflow at the terminals.
DTU 68.3 points to watch
- • Total supply airflow must not exceed 110% of the design airflow.
- • Total extract airflow must not fall below 90% of the design airflow.
- • The difference between supply and extract must be controlled to avoid over- or under-pressurising the building.
- • Filters must be clean during balancing — a clogged filter distorts every measurement.
Properly executed air balancing guarantees healthy air, acoustic comfort and optimal energy performance. It also produces a balancing report that can be used for future maintenance.