Legionella Control Measures: The Ongoing Maintenance Tasks Every Building Needs

Completing a legionella risk assessment is the essential first step -- but it is only the beginning. The assessment generates a control scheme: a programme of ongoing maintenance tasks and remedial works that must be implemented and recorded consistently. Without these controls, the assessment is simply a document gathering dust while the risks it identified continue to grow.

This guide covers the practical, day-to-day and periodic control measures that every building with a water system needs to have in place.

The Golden Rule: Record Everything

Before diving into individual tasks, one principle overrides everything else: every action must be recorded.

If the Health and Safety Executive (HSE) visits your premises, the first thing they will ask for -- after the risk assessment itself -- is the log book. They want to see evidence that the control measures are being carried out: who did what, when, and what the results were.

A log book with gaps is almost as damaging as having no controls at all. It suggests either that the work is not being done or that the management system has broken down. Either way, it represents a failure of the Duty Holder's obligations.

Ongoing Control Measures

Weekly: Flushing Little-Used Outlets

Any outlet (tap, shower, toilet) that is not used within a seven-day period is classified as "little used" and must be flushed weekly. Stagnant water in unused pipework provides ideal conditions for legionella bacteria to multiply, and that contaminated water can leach back into the wider system.

How to flush correctly:

- Run both hot and cold water through the outlet.
- For showers, remove the shower head and flush through the hose only, allowing the water to drop into the tray without creating an aerosol spray. The objective is to replace the stagnant water without dispersing potentially contaminated droplets into the air.
- Flush for approximately five times the volume of the supply pipework. For a tap connected by a short pipe run, a couple of minutes is typically sufficient.
- Record the date, the outlet flushed, who carried out the flushing, and any observations.

Practical tip: In buildings with many outlets -- offices with kitchenettes on every floor, for example -- create a flushing schedule that assigns specific outlets to specific days. This makes the task manageable and ensures nothing is missed.

Monthly: Sentinel Outlet Temperature Checks

Temperature is the primary control against legionella. Monthly temperature monitoring at sentinel outlets confirms that the system is maintaining safe temperatures throughout.

Sentinel outlets are the nearest and furthest outlets from each water source:

- For cold water: the nearest and furthest taps from the incoming mains (or cold water storage tank).
- For hot water: the nearest and furthest taps from the hot water cylinder, calorifier, or boiler.

Target temperatures:

If cold water consistently reads above 20 degrees or hot water consistently reads below 50 degrees at sentinel outlets, this is a control failure that requires investigation and corrective action. Persistent failures may also warrant legionella sampling to assess whether the bacteria has proliferated in the system.

Quarterly: Shower Head Cleaning and Descaling

Showers are a particular concern because they generate fine aerosols -- exactly the mechanism by which legionella bacteria is inhaled. Every shower head in the building must be dismantled, cleaned, and descaled on at least a quarterly basis.

Procedure:

1. Remove the shower head from the hose.
2. Disassemble the head (most unscrew into components).
3. Clean all components thoroughly, removing any scale, biofilm, or debris.
4. Disinfect the components (typically by soaking in a suitable disinfectant solution).
5. Reassemble and refit.
6. Record the date, shower location, who carried out the work, and the condition of the head.

In environments where showers see heavy use (e.g., construction sites, manufacturing facilities with wash-down areas), or where shower heads are found to be in poor condition, consider increasing the frequency to monthly.

When to replace rather than clean: If a shower head is heavily scaled, corroded, or damaged to the point where it cannot be properly cleaned, replace it. A shower head that cannot be effectively descaled is a reservoir for bacteria.

Six-Monthly or Annually: Cold Water Storage Tank Inspection

Cold water storage tanks should be inspected at least annually, ideally during the summer months when ambient temperatures are highest and tank temperatures are most likely to approach the danger zone.

Inspection checklist:

- Take a temperature reading from the tank (remote from the inlet).
- Visually inspect internal conditions: look for corrosion, sediment, sludge, biofilm, debris, or any foreign material.
- Confirm the lid is in place, close-fitting, and undamaged.
- Check insulation condition.
- Verify that the inlet and outlet are opposed for good cross-flow.
- Confirm the overflow and vent are screened to prevent ingress of insects or debris.

If the tank is in poor condition, recommend cleaning and disinfection as a remedial action.

Annually: Full Outlet Temperature Survey

In addition to monthly sentinel outlet checks, every outlet in the building should have its temperature tested at least once per year. This includes:

- All wash hand basins
- Kitchen taps
- Cleaner's sinks
- Showers
- Any other hot or cold water outlet

This annual survey confirms that temperature control is maintained across the entire system, not just at the sentinel points.

Annually: Calorifier Inspection

If the hot water system includes calorifiers (indirect hot water cylinders):

- Where the calorifier has an inspection hatch, open it and inspect the internal conditions. Look for scale, corrosion, sludge, or debris.
- Where internal inspection is not possible, drain a sample from the drain valve and inspect it for clarity, sediment, or discolouration.
- Confirm the stored water temperature is at or above 60 degrees.
- If a secondary circulation (flow and return) system is present, confirm the return temperature is at or above 50 degrees.

Annually: TMV Servicing

Thermostatic mixing valves (TMVs) must be serviced at least annually. This is a requirement under HSG274. The service includes:

Fail-safe check:

1. Open the blended water outlet (run the tap).
2. Isolate the cold water supply to the TMV.
3. Monitor the outlet temperature. As the hot water component increases, the TMV's internal wax element should respond and shut off the flow before the water reaches a dangerous temperature.
4. For TMV3 valves, shut-off should be virtually instantaneous once the cold supply is removed.

Internal cleaning:

1. Isolate both hot and cold supplies to the TMV.
2. Remove the valve body and disassemble.
3. Extract the internal strainers (small mesh filters on the hot and cold inlets).
4. Clean and disinfect all components.
5. Reassemble, reconnect, and test for correct operation and no leaks.

Remedial Works: One-Off Corrective Actions

The risk assessment will also identify remedial works -- one-off corrective actions required to bring the system into compliance. These are typically prioritised by urgency:

Immediate Priority

- Remove dead legs. Sections of redundant pipework with no active outlet must be physically removed and capped back to the live system. Dead legs are a direct source of stagnation and potential bacterial contamination.
- Fit or replace tank lids. An uncovered cold water storage tank is an immediate risk. A close-fitting, compliant lid must be fitted without delay.
- Address grossly non-compliant temperatures. If hot water storage is significantly below 60 degrees or cold water is significantly above 20 degrees, corrective action is urgent.

High Priority

- Install or repair insulation on cold water storage tanks and pipework to prevent heat gain.
- Reposition or replace inlet/outlet connections on tanks to ensure opposed arrangement and good cross-flow.
- Clean and disinfect tanks found to have significant internal contamination.
- Install shunt pumps on large calorifiers to prevent thermal stratification.
- Redirect calorifier vent pipes to a separate tundish rather than discharging into a cold water storage tank.

Medium Priority

- Reduce tank capacity or replace with a smaller tank where turnover exceeds 24 hours.
- Install temperature gauges on calorifiers to facilitate routine monitoring.
- Establish the management structure if not already in place: appoint and document the Duty Holder, Nominated Responsible Person, and deputies in writing.

Legionella Sampling: When Is It Necessary?

There is no blanket legal requirement to carry out routine legionella sampling. Sampling is warranted when:

- Control measures are consistently failing (e.g., temperatures repeatedly outside compliant ranges).
- A case of legionnaires' disease is associated with the building.
- The risk assessment identifies a system or area of particular concern (e.g., heavily contaminated tank, long dead leg discovered).
- As a precautionary measure when commissioning a system or returning it to use after a prolonged shutdown.

When samples are taken, they must be analysed by a UKAS-accredited laboratory to ensure the results are legally defensible and scientifically valid.

Training and Competence

The people carrying out these control measures must be competent -- they need to understand what they are doing and why. Key training considerations:

- The Nominated Responsible Person and deputies should receive formal legionella awareness training.
- Refresher training is recommended every five years to keep knowledge current.
- Anyone taking temperature readings must understand how to use the equipment correctly and how to recognise anomalous results.
- The appointment of the Responsible Person (and their acceptance of the role) must be documented in writing. This removes any ambiguity about who is responsible for what.

Keeping the System Under Review

The control scheme is not static. It must evolve as circumstances change:

- When building use changes, reassess the water system demands.
- When new outlets are added or removed, update the asset register and monitoring schedules.
- When control measures consistently fail, investigate root causes rather than simply recording the failures.
- When new regulations or guidance are published, review the control scheme for compliance.

A well-managed legionella control programme is a cycle: assess, implement, monitor, review, and adjust. The buildings that get into trouble are invariably the ones where the cycle has been broken -- where the assessment was done but the controls were not implemented, or where the monitoring was started but gradually abandoned.

Conclusion

Legionella control is not a one-time activity -- it is an ongoing commitment. The control measures described here are not optional extras; they are the practical implementation of your legal duty to manage the risk. Weekly flushing, monthly temperature checks, quarterly shower head maintenance, annual inspections, and TMV servicing all work together to keep water systems safe.

The key to success is consistency: carry out the tasks on schedule, record everything, investigate anomalies promptly, and review the programme regularly. Combined with a suitable and sufficient risk assessment, these measures form a robust defence against one of the most persistent biological hazards in the built environment.