Every commercial building we work in has at least one of them. The tap in the store room. The shower in the staff changing area that nobody has used since the last person who cycled to work left. The washroom on the floor that has been half-empty since the office reorganised. They look harmless. They are, in fact, the exact conditions legionella bacteria like: water sitting still, at room temperature, going nowhere.
This is not a scare piece. For most small premises the risk is genuinely low and the controls are genuinely simple. But it is one of the few plumbing topics that carries a legal duty, and it is very commonly misunderstood — so it is worth understanding properly.
What actually creates the risk
Legionella bacteria live in water systems at low levels almost everywhere. They become a problem when they multiply, and they multiply in warm, still water — roughly between 20°C and 45°C. They cause illness when that water is breathed in as a fine spray or aerosol, which is why showers, spray taps and similar outlets get the attention rather than, say, a toilet cistern.
So the risk is not “old pipework”. The risk is stagnation plus the wrong temperature. A brand-new system with an outlet nobody uses is more of a concern than an old system that gets used constantly.
The temperatures that matter
HSE’s guidance is built around keeping water out of that 20–45°C window, and the numbers are worth knowing:
- Hot water should be stored at 60°C or above.
- It should reach 50°C at the outlet within a minute of running (55°C in healthcare premises).
- Cold water should be stored and distributed below 20°C.
Those three figures explain a lot of otherwise odd-looking plumbing decisions — why cylinder thermostats are set where they are, why cold pipework should not run alongside hot or through warm voids, and why a long run to a distant outlet is worth thinking about.
They also come with a safety trade-off worth naming: water stored at 60°C will scald. That is why thermostatic mixing valves exist on baths and basins in premises where scalding is a real concern — you store hot enough to be safe from bacteria and mix it down to safe at the point of use, rather than compromising on the storage temperature.
The single most useful control: use the outlets
The general principle in HSE’s guidance is that outlets should be used at least once a week, to keep water moving and stop it stagnating. For a building with a handful of quiet outlets, that is the whole control measure — a named person, a weekly walk round, run every little-used tap and shower for a few minutes, write down that you did it.
That last part matters more than people expect. If you have a duty to manage the risk, a simple dated record of a weekly flush is what demonstrates you are managing it. A routine nobody wrote down is very hard to evidence later.
The other half of the job is getting rid of the outlets you do not need at all. A capped-off spur left behind by an old layout — a dead leg — is a permanent pocket of still water that no flushing routine reaches. When we alter pipework or fit out a commercial washroom, removing redundant legs back to the live pipe is part of doing it properly, not an extra.
If you are a landlord
Landlords do have a duty to assess and control the risk of legionella exposure for tenants. The reassuring part, which HSE says explicitly, is that for a typical domestic property this does not require an in-depth, detailed assessment — for most domestic hot and cold water systems, temperature control is the reliable measure.
In practice, for a normal rental, that means: set the cylinder so water is stored at 60°C, keep cold water cold, avoid debris getting into the system, and flush the system through before a new tenant moves in — which is the single highest-value moment, because a void period is exactly when water has been sitting still. Then tell your tenant the sensible basics: do not adjust the cylinder thermostat, and let you know if the hot water is not getting properly hot.
Void periods and empty holiday lets are the cases worth taking seriously, and they are the ones most often missed. There is more for landlords in our autumn plumbing checklist, and we work with landlords and agents across the county through our landlord services.
If you run a business
The same logic scales up. Identify the outlets that get little use, put a weekly flush on somebody’s list, keep a record, check your stored hot water is actually reaching temperature rather than assuming it, and get redundant pipework removed rather than capped. Premises that run seasonally — a function room, a summer-only facility, a building that empties out over Christmas — deserve particular attention, because a shutdown is a stagnation event by definition.
Where it goes beyond that, it goes beyond a blog post: larger or higher-risk systems need a proper written risk assessment by someone competent to do it, and that is the right answer rather than a checklist.
Where we come in
The plumbing half of this is our day job: getting stored hot water to the right temperature, sorting out cold pipework that runs somewhere warm, removing dead legs, fitting mixing valves where scalding is a concern, and putting planned maintenance on a footing where somebody is actually responsible for it. We cover offices, pubs, salons, and rental property across Cheltenham, Gloucester and the wider county.
If you are not sure whether your building’s quiet corners are a problem, ask us to look. Call 07386 705571 or request a free site survey.
The temperature and flushing guidance above is drawn from HSE’s published guidance on controlling legionella in hot and cold water systems. It is general information, not a risk assessment for your premises — check the current HSE guidance for your own situation.