A wet sprinkler system holds water at all times. That is the point of it. The pipework sits charged, under pressure, waiting for a fire that may never come — which means the water inside it can sit for years. Most Legionella risk assessments skip past it, and most of the time that is the right call. The trap is knowing exactly where “most of the time” stops.

The deciding question is not whether the system holds stagnant water. It almost certainly does. It is whether that stagnant water can ever mix back into the water people drink, wash in, or breathe as a fine spray. If a system is genuinely sealed off from the wholesome supply and never produces an aerosol anyone is exposed to, it is a fire-safety asset, not a Legionella one. If it shares water with the mains, or someone routinely operates it and stands in the discharge, it belongs in your assessment.

Here is the core principle to carry through every situation below: follow the water, not the label. A pipe is in scope because of what it is connected to and who is exposed, not because of what colour it is painted or which contractor maintains it.

Where the real risk hides

Fire suppression is rarely one system. It is a family of them, and they sit at very different points on the risk scale.

A traditional wet sprinkler installation, fed through a dedicated tank and backflow protection, is largely a closed loop. The water is old and undisturbed, but it does not return to the supply and nobody is exposed to a spray from it in normal use. Static water in a sealed system is not, by itself, the hazard — exposure to an aerosol containing the bacteria is what matters [1].

The components that change the picture are the connections and the operable outlets:

  • Mains-fed hose reels. Many older buildings still have rubber hose reels plumbed directly to the wholesome cold supply. Each reel is a permanent dead leg holding warmed, static water — and when tested or used, it throws a jet that breaks into spray.
  • Sprinkler fill and make-up points. Where a jockey pump or an automatic fill valve tops up a sprinkler tank from the mains, the boundary between fire water and drinking water is exactly where backflow protection has to be right.
  • Test-and-drain valves and inspector’s test points. These are operated deliberately, often in plant rooms, and can release a fine mist.
  • Idle wet risers and landing valves in stairwells that are charged but almost never run.

Dead legs and rarely-used parts of a system are repeatedly flagged as features that let bacteria multiply, because the water in them stays warm and undisturbed long enough for colonies to establish [2]. A charged hose reel is close to the purest example of that on most sites.

Applying it across four situations

A mid-rise office with hose reels on every floor. The sprinkler system is tank-fed and isolated, so it scopes out with a note explaining why. The hose reels do not — they tap the mains. In my view these are the single most overlooked stagnation point in commercial property, because facilities teams treat them as fire kit and the Legionella assessor treats them as someone else’s pipework. They are a shared dead leg. Either bring them into the flushing regime or, better, have them assessed for permanent isolation or replacement with a non-water type, in agreement with the fire risk assessment.

A warehouse with a fully wet sprinkler grid. No mains cross-connection beyond a protected fill point, no occupied space below the pipework that gets sprayed in normal operation. This scopes out — but the scoping decision itself should be written down, with the backflow arrangement at the fill point named as the boundary you are relying on. Stagnation that cannot reach people or the supply is a recorded judgement, not an ignored one.

A healthcare or care setting. The bar rises here, and so does the scrutiny of every cross-connection. Treat any mains-fed fire outlet, fill point or test valve as in scope until proven otherwise, and align the decision with the building’s wider water safety plan rather than assessing the fire system in isolation.

A landlord’s small commercial unit with a single hose reel. Low complexity, but the same logic. One mains-fed reel, rarely touched, sitting full of tepid water. It either gets flushed on a defined schedule or gets isolated. Doing neither is the gap an enforcing officer or an incident investigation will find.

Field checklist: scoping fire suppression in a Legionella risk assessment

Work through this on site, recording an answer and a reason for each line.

  • Identify every wet fire system and outlet — sprinklers, wet risers, landing valves, hose reels, foam/deluge fill points. Walk the building; do not rely on the fire-system drawings alone.
  • Trace each one’s water source. Mains/wholesome supply, dedicated fire tank, or other. The source decides almost everything.
  • Find and verify backflow protection at every point where a fire system draws from the mains. Record the device type and whether it has been checked.
  • List the mains-fed dead legs — hose reels and charged branches are the priority. Note pipe run length and last-use evidence.
  • Identify operable outlets that aerosolise — test-and-drain valves, inspector’s tests, hose reel nozzles. Note who operates them and where they discharge.
  • Decide scope for each system — in or out — and write the reason. “Isolated, no cross-connection, no exposure” is a valid out; an unexplained out is not.
  • Set a control for everything in scope — scheduled flushing, temperature check, isolation, or removal — and name who owns it.
  • Cross-check against the fire risk assessment before changing or isolating anything, so a Legionella control does not weaken fire cover.
  • Record temperatures where water is accessible and relevant, remembering bacteria multiply most readily in roughly the 20–45°C band [2].

The items people skip are the last three. Scoping a system out is easy; writing the defensible reason, and reconciling the decision with the fire side, is the work that actually holds up.

A caveat worth stating plainly

Fire suppression exists to save lives, and nothing in a Legionella assessment should quietly disable it. Isolating a hose reel, capping a dead leg or altering a fill arrangement is a decision for a competent person taken jointly with whoever owns the fire risk assessment — never a unilateral call by the water-hygiene side. This article is general guidance to help you ask better questions on site; the actual scope, controls and frequencies come from a site-specific assessment carried out under BS 8580-1, where the assessment itself defines which systems and components are surveyed [3].

The concrete next step today: walk one building and physically locate every hose reel and sprinkler fill point, then check whether each appears anywhere in your current risk assessment. If you are tracking that on paper or a spreadsheet that nobody updates after the survey, move those assets into a digital logbook where each one carries its own scope decision, control task and flushing record — so the reason a system was scoped out is still legible the day someone asks.

FAQ

Do mains-fed hose reels need to be on a flushing schedule?

If a hose reel is plumbed to the wholesome supply, it is a dead leg holding static water and creates the conditions bacteria favour. The usual options are a defined flushing regime, permanent isolation, or replacement with a non-water reel — your risk assessment sets which, and the frequency, in agreement with the fire risk assessment [2].

Whose responsibility is the sprinkler system, fire or water hygiene?

Both, at different points. The fire risk assessment owns its function and reliability; the Legionella assessment owns any point where it touches the wholesome supply or exposes people to an aerosol. The two should be reconciled, not run in separate silos, ideally under a single water safety plan.

Is backflow prevention enough to scope a sprinkler fill point out?

Verified, correctly-specified backflow protection is the boundary that lets you treat the fire side as separate from the drinking supply. Record the device, confirm it has been checked, and only then rely on it. An assumed device you have not seen is not a control [1].

Sources

[1] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - ACoP and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [2] HSE, “Systems most likely to create legionella risk”. https://www.hse.gov.uk/legionnaires/risk-systems.htm [3] BSI, “BS 8580-1:2019 - Risk assessments for Legionella control. Code of practice”. https://knowledge.bsigroup.com/products/water-quality-risk-assessments-for-legionella-control-code-of-practice-1