How often do you check a water system for legionella?

You do not check a water system for legionella once. You run a schedule of separate routine checks, each on its own frequency. For a typical hot and cold water system that means weekly flushing of unused outlets, monthly temperature checks, quarterly shower head cleaning, six-monthly tank inspection and an annual calorifier inspection.

The word people reach for is "monitoring", and it captures the idea better than "checking". Monitoring is the ongoing routine that keeps the controls set out in your written scheme of control working between risk assessments. This page is the calendar that pulls the other guidance together and points to the detail behind each task.

Monitoring is a schedule, not a single visit

The controls that keep legionella numbers down are simple: keep hot water hot, keep cold water cold, and keep water moving. Monitoring is how you confirm those controls are still holding. Because different parts of the system drift at different rates, the checks fall on different frequencies. Temperatures can slip in a month; a calorifier accumulates sludge over a year; a rarely used outlet can stagnate within a week.

So a monitoring schedule is a list of tasks, each with a point in the system, a frequency, and a place where the result is recorded. Miss the schedule and you have no evidence the controls ever worked, which an inspector can treat as a failure to manage the risk, regardless of whether anyone fell ill.

The typical monitoring calendar

The frequencies below are the starting points HSG274 Part 2 gives for a hot and cold water system. They are a sensible default, not a legal minimum, and your risk assessment can move any of them.

• Weekly. Flush any outlet that is not used regularly, running it long enough to draw through fresh water. "Little-used" generally means not used for a week or more. See flushing outlets and how long to run the taps.
• Monthly. Take temperatures at the sentinel outlets: the nearest and furthest points on each loop. On the hot side, check the flow and return at the calorifier, and confirm the sentinel outlets reach 50°C (55°C in healthcare) within one minute. On the cold side, confirm the sentinels stay below 20°C. See the temperature chart and temperature control.
• Quarterly. Dismantle, clean and descale shower heads and hoses, where fitted, because they generate the fine spray that carries the bacteria into the lungs. See cleaning shower heads.
• Six-monthly. Inspect the cold water storage tank, check its condition, its lid and screens, and its water temperature. See cold water storage tank inspection.
• Annually. Inspect the calorifier internally, or draw off from the base, to check for debris and corrosion. See calorifier inspection.

Source: HSE HSG274 Part 2 starting points, supporting ACOP L8. These are typical defaults; the risk assessment sets the actual frequencies for your building.

Thermostatic mixing valves sit slightly outside this neat list. They blend hot and cold to a safe delivery temperature, which creates warm water inside the valve itself, so they need servicing, cleaning and descaling at the interval the risk assessment sets. That is often six-monthly or annually, and sometimes more frequent in higher-risk settings. See TMV servicing for the detail.

Sentinel outlets and the rolling programme

You do not measure the temperature at every tap every month, and you are not expected to. Instead you pick sentinel outlets: the ones nearest and furthest from each calorifier on the hot loops, and nearest and furthest from the tank on the cold. If the extremes of the system are holding temperature, the points between them almost certainly are too.

The rest of the outlets are covered by a rolling programme. You check a representative share of them each month so that, over a set period, usually a year, every outlet has been measured at least once. Together the monthly sentinels and the rolling programme give you a picture of the whole system without a check on every single tap every single month.

Monitoring is not the same as testing

This is where a lot of schedules go wrong. Routine monitoring for most hot and cold water systems is about temperature and physical condition, not laboratory sampling. Temperature control is the primary means of controlling the risk, and a clear, complete temperature record is your evidence.

Sampling water for legionella is a separate activity. You do it where the risk assessment calls for it, where you cannot keep the system within the safe temperatures, or where you are relying on a different control such as a chemical treatment. So do not assume a monitoring schedule must include water samples, and do not assume a clear sample means the routine checks can lapse. See legionella testing for when sampling is appropriate.

Your risk assessment sets the real frequencies

Every frequency above is a typical starting point, not a fixed rule. The risk assessment is what turns them into your actual schedule. A quiet office with modern pipework and few outlets may sit comfortably on these defaults. A care home, a gym with busy showers, or a building with long pipe runs and stored water may need tighter intervals on some tasks and looser ones on others. If you do not have an assessment yet, start with do I need a legionella risk assessment and the risk assessment template.

A check you did not record did not happen

Every task on the calendar produces a reading or an observation, and every one of them needs to land in the same place: your log book. A temperature you took but did not write down is, for the purposes of an inspection, a temperature you never took. The record is what shows the schedule is real and that you acted when a reading fell out of range. See the log book for how to keep it, and what to do if legionella is detected for the actions a reading might trigger.

None of these frequencies is invented for this page. They come from HSG274 Part 2, the HSE guidance for hot and cold water systems, which supports the duties in ACOP L8. Following a monitoring schedule does not make a system safe on its own, and no schedule can promise that. What it does is give you the ongoing evidence that your controls are working, which is what the law asks of the person responsible.