Ventilation Commissioning Calculator — Flow Balance Check

Ventilation commissioning is the process of testing and adjusting a ventilation system to ensure it delivers the design airflow rates at each terminal. This involves measuring airflow at every supply and extract terminal with a calibrated flow hood or anemometer, comparing against the design values, and adjusting dampers until each measured rate meets its design value. Under Approved Document F (Vol 1, 2021, Appendix C) the pass test is that the measured rate is equal to or greater than the design value.

For Part F compliance in new dwellings, Approved Document F Volume 1 (2021), Appendix C, sets the pass test as: measured air flow equal to or greater than the design value at each fan/terminal. ADF does not specify a symmetric ±10% band — any terminal reading below 100% of design must be adjusted until it matches design. The ±10% figure widely quoted for air balancing comes from BSRIA BG 49 (Commissioning Air Systems), which is a best-practice balancing tolerance, not a Building Regulations requirement. The flow-measuring instrument itself must have an accuracy of ±5% and be UKAS-calibrated within the last 12 months (ADF Vol 1, Section 4).

Under Part F, ventilation commissioning must be carried out by a competent person. For domestic installations, the installer is usually responsible. The results must be recorded on a commissioning sheet and a copy given to the building control body. For commercial systems, a specialist commissioning engineer is typically engaged.

Approved Document F (Vol 1, 2021), Appendix C, sets out the minimum commissioning sheet: the ventilation system type, design flow rates for each terminal, measured flow rates, the air-flow instrument used with its accuracy and UKAS calibration date, fan speed settings, and the name, signature and date of the competent person who installed and commissioned the system. Specific fan power (SFP) is not part of the Approved Document F record — it is a Part L energy check, with limits set by the Domestic Building Services Compliance Guide (continuous MEV ≤ 0.7, continuous supply ≤ 0.5, MVHR with heat recovery ≤ 1.5 W/(l/s)).

Use a calibrated balometer (flow hood) placed over the diffuser or grille to directly measure the airflow in L/s. For small terminals, a rotating vane anemometer can be used to measure velocity, then multiply by the duct cross-sectional area. The instrument should be calibrated within the last 12 months per BSRIA guidance.