What CIBSE Commissioning Code W requires for water systems — commissionable design, flushing and cleaning to BSRIA BG 29, water quality sampling, proportional balancing and the witnessed records that go into handover.
Commissioning Code W is the technical code for water distribution systems. It covers everything that carries water through a building — chilled water, heating hot water, condenser water and domestic services — and sets out how those systems are filled, flushed, cleaned, chemically treated, set to work, regulated and proven. Where Code M governs the management of commissioning, Code W governs what actually happens at the pipe.
A system can only be balanced if it was drawn to be balanced: regulating valves at every branch and terminal, fixed-orifice or pressure-independent valves selected for the design flow rather than the pipe size, test points either side of each measuring device, air vents at high points, drains at low points, and an index circuit identified on the drawings. Flushing bypasses, temporary strainers and fill points also need to be designed in, not improvised on site.
Pre-commission cleaning follows the sequence set out in Code W and BSRIA BG 29: static flush to remove debris, dynamic flush at velocities high enough to lift settled material (commonly around 1 m/s minimum in every branch), chemical clean where required, neutralise and re-flush, then dose with inhibitor and biocide. Terminals are isolated during the aggressive stages so debris is not driven into coils and control valves.
Cleanliness is proven with samples, not opinions. Typical acceptance evidence includes suspended solids, turbidity, iron and total bacteria counts taken from representative points, with results compared against the specified criteria and the make-up water baseline. Samples are dated, located and traceable — an undated sample from an unnamed point is worthless when a coil blocks two years later.
Pumps are set to work and total system flow established, then flow is regulated proportionally: the index terminal is set first and each branch brought into ratio, working back to the plant before the pump is finally trimmed. Typical acceptance is measured flow within roughly ±10% of design at terminals, with total system flow held at design. Variable-flow systems additionally need control valve authority and differential pressure setpoints verified across the operating range.
The deliverable is a complete water balance record — design, measured and percentage variation for every valve — plus flushing certificates, chemical treatment dosing records, water sample results and pump performance readings. These are witnessed by the commissioning manager or an independent agent and feed the O&M manuals, warranty position and Green Star / NABERS evidence.
On Australian projects Code W sits alongside the local duty-holder obligations for warm and cooling water systems — cooling tower registration, risk management plans, monthly sampling and AS/NZS 3666 maintenance requirements. Commissioning is where those regimes start: a system handed over with stagnant dead legs, unproven temperatures or no baseline water quality result hands the operator a compliance problem on day one.
Flushing started before the system is complete so half the branches are never cleaned; no regulating valves on terminal branches; balancing carried out before the fit-out changes the loads; inhibitor dosed into a system that was never properly cleaned; and no allowance in the programme for the re-flush after late pipework alterations. Each is foreseeable and each becomes a defect against the occupancy certificate.
Code W is not only a new-build code. On an operating asset the same logic drives retro-commissioning: poor water quality, low ΔT, blocked coils and failed control valves almost always trace back to a system that was never properly cleaned or balanced. PCM runs that investigation and re-verifies the result.
Asset investigations & retro-commissioning