Why Commissioning Matters
New pipework is not clean. During construction, pipes are exposed to dust, debris, flux residues, and sometimes contaminated water used for pressure testing. Without proper commissioning, these contaminants enter the building's water supply from day one. For refurbishments, the situation can be worse: cutting into existing pipework introduces bacteria from the environment into a system that may already have biofilm present.
BS 8558 (Guide to the design, installation, testing, and maintenance of services supplying water for domestic use within buildings) sets out the commissioning requirements. Compliance is expected by building control and, for commercial properties, by the HSE.
The Commissioning Process
Commissioning a new or modified water system follows a defined sequence. Each step must be completed before moving to the next, and records should be kept throughout.
- •System flushing to remove construction debris and loose material
- •Chlorination at the appropriate concentration and contact time per BS 8558
- •Thorough flushing to remove chlorinated water until residual levels drop below 0.5 mg/l
- •Microbiological sampling from representative points across the system
- •Laboratory analysis to confirm acceptable bacterial levels
- •Temperature commissioning to verify hot and cold targets are met at all outlets
- •Documentation and handover of commissioning certificates
Common Commissioning Failures
The most frequent problem is rushing. Construction programme pressures often push for early occupation before commissioning results are confirmed. This creates a situation where building occupants use water that has not been verified as safe. Another common issue is inadequate flushing before chlorination: if debris is not removed first, the chlorine treatment is less effective.
Partial commissioning is also problematic. If only part of the system is treated (for example, the cold water supply but not the hot), untreated sections can harbour bacteria that quickly recolonise the entire system. Every section of new or modified pipework must be included in the commissioning scope.
Documentation and Handover
On completion, the commissioning engineer should provide a comprehensive handover pack including chlorination certificates, laboratory results, temperature records, and a schematic of the system as installed. This documentation forms the baseline for the building's ongoing water management programme and is essential evidence for the first legionella risk assessment.