---
title: "Commissioning new water systems: preventing Legionella from day one"
source_url: https://legionella.io/articles/commissioning-new-water-systems-preventing-legionella-from-day-one/
canonical_url: https://legionella.io/articles/commissioning-new-water-systems-preventing-legionella-from-day-one/
pillar: "Building Types & Use Cases"
summary: "A new water system isn't a safe one. The commissioning mistakes that let Legionella colonise fresh pipework before anyone moves in, and how to avoid each."
primary_keyword: "new build Legionella"
date_published: 2026-01-05
date_reviewed: 2026-06-26
author: "Legionella.io editorial team (REMOTE TECH LTD)"
reviewed_against: "HSE L8 and HSG274 guidance"
region: "United Kingdom"
license: "(c) REMOTE TECH LTD. Quote freely with attribution and a link to source_url."
---

# Commissioning new water systems: preventing Legionella from day one

A brand-new water system feels like the one place Legionella could never take hold. Fresh pipework, never used, signed off by the installer. That instinct is the trap. The riskiest stretch in a water system's life is often the weeks or months between the day it is first filled and the day people actually start using it, and on a construction programme, that gap can run to months.

Picture the usual sequence. Pipework is charged with water early so it can be pressure-tested for leaks. Then the programme slips: fit-out runs late, sign-off moves, occupancy lands a quarter behind plan. That test water now sits in warm pipes inside a heated building, going nowhere. By handover, the system everyone still calls "new" has spent longer as a tepid, stagnant reservoir than plenty of tired old buildings manage. New build Legionella is rarely about anything exotic. It comes from ordinary commissioning choices made in the wrong order.

## The mistakes that turn a clean install into a colonised one

### Treating "new" as proof that it's clean

**What it looks like.** The building changes hands with no Legionella risk assessment, on the logic that an unused system has nothing to assess yet.

**Why it happens.** The duty doesn't feel real until people are in, and a fresh system genuinely has had no chance to grow a problem so far.

**The fix.** Assess before the system goes into service, not after the first complaint. The whole value of assessing at commissioning is catching design and standing-water risks while they are still cheap to correct: a spur that can be cut, a tank that can be swapped, a flushing plan that can start straight away. The duty to assess and control sits with whoever runs the building, and it applies from the point water is in the system, not from opening day [1][4]. For the failure modes in detail, [Risk assessment errors that lead to Legionella growth](https://legionella.io/articles/risk-assessment-errors-that-lead-to-legionella-growth/) covers the assessment errors that let growth start.

### Filling the system early, then walking away

**What it looks like.** Pipework is filled for pressure testing months ahead of use, then left full and untouched while the rest of the build finishes around it.

**Why it happens.** Leak testing happens to the mechanical programme's schedule; nobody schedules what the water does afterwards. Draining down feels wasteful, so it stays full.

**The fix.** Commission wet services as close to occupancy as the programme allows, so water goes in and gets used in roughly the same window. Where an early fill is unavoidable, either drain the system down until it is needed, or put it on a documented flushing regime that keeps every outlet turned over while it stands. Standing water is the root cause behind most of these failures, new or old; [Neglected water systems: the danger of stagnation](https://legionella.io/articles/neglected-water-systems-the-danger-of-stagnation/) is worth reading on why stagnation does the damage.

### Letting the water become nobody's job

**What it looks like.** Between the commissioning engineer leaving and the duty holder taking over, no one owns the live, water-filled system. Flushing lapses because it was never anyone's task.

**Why it happens.** Responsibility transfers on paper at handover, but water is present long before that. The orphan period falls down the gap between the principal contractor and the incoming operator.

**The fix.** Name a responsible person for the water from the moment it enters the system, and write the flushing and monitoring tasks into the construction phase, not just the occupied-building regime. It should be plain, in writing, who turns the taps over each week before anyone moves in.

### Building in dead legs and oversized storage

**What it looks like.** Generous cold-water tanks "for future expansion", capped spurs left long for a phase two that may never come, plant rooms plumbed for flexibility no one will use.

**Why it happens.** Oversizing feels prudent at design stage and costs little to install. The consequence, water dwelling too long to stay cold or sitting dead in a capped branch, only shows up later.

**The fix.** Size storage to realistic demand, not to a hypothetical future, and keep any genuine future connections as short and as close to live pipework as possible. Dead legs and infrequently used outlets are exactly the features HSE flags as creating foreseeable risk [3]. The cheapest time to design them out is before the walls close up, and a water safety plan approach from the design stage exists precisely for this [5].

### Rushing or skipping the pre-commission clean

**What it looks like.** The system is brought into use without a proper clean and disinfection, or it is done but the certificate and readings never make it into the records.

**Why it happens.** It lands at the tail end of a squeezed programme, when everyone wants the building open.

**The fix.** Clean and disinfect the system before it goes into service, following the method and contact arrangements in HSG274 and the manufacturer's instructions, and keep the paperwork [2]. Bring the system into use promptly afterwards; a disinfected system left to stand again simply restarts the clock. From that point, run it at the temperatures your scheme sets, keeping stored hot water hot and cold water genuinely cold [2].

### Treating the commissioning certificate as the finish line

**What it looks like.** Sign-off is filed as proof the water is safe, the logbook is never started, and the flushing regime that should run from day one quietly never begins.

**Why it happens.** A certificate looks like a conclusion. It is actually a starting gun.

**The fix.** Handover should pass the duty holder a usable package: the risk assessment, the written control scheme, the operating and maintenance information, and a live logbook with the first tasks already in it. A digital logbook makes that transfer cleaner than a box of folders; see [Are digital records legally acceptable in the UK?](https://legionella.io/articles/are-digital-records-legally-acceptable-in-the-uk/) on whether digital records hold up.

## If you change one thing, close the gap

Most of these mistakes share a single root: the time between water-in and water-used. Shrink that gap and the rest gets easier. Push to commission wet services late, fill late, use early, and treat any unavoidable standing period as a managed risk with flushing and records rather than dead time on the programme. A new building is not a safe building. It is an empty one, and empty is where Legionella likes to start.

If your building is mid-commission right now, the most useful thing you can do this week is find the date the system was first filled and set it against the planned occupancy date. That gap is your risk, and it stays unmanaged until someone is named to own the water sitting in those pipes.

This is general guidance, not a commissioning specification or a substitute for a competent water-safety assessment. The exact disinfection method, contact times, temperatures and flushing frequencies for your system come from HSG274, the manufacturer's instructions, and a competent person who has actually seen your drawings.

## Common questions

### Does a brand-new building need a Legionella risk assessment before anyone uses it?
Yes. The duty to assess and control the risk attaches to the water system, not to the occupancy. A system that has held water, even just test water, through the build is exactly the kind that benefits from an assessment before first use, while design and stagnation problems can still be fixed cheaply [1][4].

### How long can a newly filled system sit before it becomes a risk?
There is no universal safe period, and any figure you have been quoted should be treated cautiously. Risk builds whenever water sits warm and still, so a system standing in a heated building can develop a problem within weeks. Your risk assessment and HSG274 set what is acceptable; if commissioning slips, the practical answer is to flush regularly or drain down [2].

### Who is responsible for the water system before the building is handed over?
During construction the responsibility sits with whoever controls the site and the system, usually the principal contractor, until it transfers to the operator at handover. The cleanest approach is to write that ownership, and the flushing tasks that go with it, into the contract so the live system is never left unattended [4].

## Related reading

- [Neglected water systems: the danger of stagnation](https://legionella.io/articles/neglected-water-systems-the-danger-of-stagnation/)
- [Risk assessment errors that lead to Legionella growth](https://legionella.io/articles/risk-assessment-errors-that-lead-to-legionella-growth/)
- [Are digital records legally acceptable in the UK?](https://legionella.io/articles/are-digital-records-legally-acceptable-in-the-uk/)
- [Legionella in mixed-use residential and commercial complexes](https://legionella.io/articles/legionella-in-mixed-use-residential-and-commercial-complexes/)

## Sources

[1] HSE, "Legionnaires' disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)". https://www.hse.gov.uk/pubns/books/l8.htm
[2] HSE, "Legionnaires' disease: Technical guidance (HSG274)". https://www.hse.gov.uk/pubns/books/hsg274.htm
[3] HSE, "Systems most likely to create legionella risk". https://www.hse.gov.uk/legionnaires/risk-systems.htm
[4] HSE, "Legionnaires' disease - what you must do". https://www.hse.gov.uk/legionnaires/what-you-must-do/index.htm
[5] BSI, "BS 8680:2020 - Water quality. Water safety plans. Code of practice". https://knowledge.bsigroup.com/products/water-quality-water-safety-plans-code-of-practice