What CIBSE Commissioning Code A requires for air systems — commissionable design, pre-commissioning checks, proportional balancing, tolerances and the records that get witnessed at handover.
Commissioning Code A is the technical code for air distribution systems. Where Code M governs how commissioning is managed, Code A sets out how air systems are actually set to work, regulated and proven: fans, air handling units, ductwork, terminal units, diffusers and grilles. It is the reference a commissioning specialist works to when balancing air on site.
Code A expects the system to be balanceable before anyone arrives with an anemometer. That means regulating dampers at branch and terminal positions, test holes at accessible straight lengths, adequate straight duct upstream and downstream of measurement points, index runs identified, and design airflows recorded on the drawings and schedules the commissioning team will use.
Static completion first: ductwork complete and leakage tested to the required class, plant installed and safe to run, filters fitted and clean, dampers accessible and set to a nominal open position, fan rotation and drive checked, and controls able to hold the system in the operating mode being tested. Balancing an incomplete system is the single most common cause of rework.
Fans are set to work and total system airflow is established, then flows are regulated proportionally — branch by branch, terminal by terminal — working from the index run back towards the plant. Proportional balancing keeps the ratios stable as the fan is finally trimmed to design total flow, rather than chasing each outlet in isolation.
Code A works to stated tolerances on measured against design airflow, typically within around ±10% at terminals with total system flow held close to design. Instruments must be in calibration, the measurement method suited to the location, and every reading recorded with the method, instrument and date so the result is auditable rather than asserted.
The deliverable is a complete set of air balance sheets showing design, measured and percentage variation for every terminal, plus fan and AHU performance readings and damper positions. Those results are then witnessed by the commissioning manager or an independent agent, and they feed directly into O&M manuals, Green Star and NABERS evidence, and the handover documentation.
Balancing started before ceilings and partitions are closed, so the system changes after the results are signed; no regulating dampers installed at branches; test holes that cannot be reached; and no allowance in the programme for re-balance after fit-out changes. Each is foreseeable, and each turns into a late defect list against the occupancy certificate.
Code A tells the specialist how to balance the air. Code M is what makes sure it happens at the right point in the programme, by an independent party, with the results witnessed and recorded. On a live PCM project both run side by side — the air balance sheets are tracked against the commissioning programme, not collected at the end.
Read CIBSE Code M Explained