Short answer. A written scheme of control is the document that turns your legionella risk assessment into a day-to-day management plan: who does what, what gets checked, how often, and what happens when a result falls outside the acceptable range. If your risk assessment identifies an ongoing legionella risk, which most commercial water systems do, you need one.
If you have had a legionella risk assessment, the report probably recommended that you implement a written scheme of control. If you are like most building managers, your next thought was: what exactly is a written scheme of control?
It sounds bureaucratic, but it is one of the more practical documents in your water safety paperwork, and you almost certainly need one.
What is a written scheme of control?
A written scheme of control sets out how you manage legionella risk in your water system on an ongoing basis. It takes the findings and recommendations from your legionella risk assessment and turns them into a practical, day-to-day management plan. If the risk assessment is the diagnosis, the written scheme is the treatment plan.
The legal basis
The underlying legal duty comes from the Control of Substances Hazardous to Health Regulations 2002 (COSHH), which treats legionella bacteria as a biological agent employers must assess and control, alongside the general risk assessment duty in the Management of Health and Safety at Work Regulations 1999. HSE's Approved Code of Practice L8, Legionnaires' disease: The control of legionella bacteria in water systems, sets out what compliance looks like in practice, including the requirement that where an assessment identifies a risk, the duty holder must prepare a written scheme for preventing or controlling it. The detailed technical guidance behind the monitoring figures used in most schemes, including temperature checks and frequencies, comes from HSG274 (Legionnaires' disease: Technical guidance), published in three parts covering evaporative cooling systems, hot and cold water systems, and other risk systems.
An Approved Code of Practice is not itself law, but failing to follow it is likely to be treated as evidence of non-compliance in any enforcement action. In practice, treat it as a mandatory requirement.
What should it contain?
A good written scheme of control covers several key areas:
Roles and responsibilities. Who is the duty holder? Who is the appointed responsible person for water safety? Who carries out the routine monitoring tasks? Who reviews the scheme? Every role should be named, with contact details and a clear description of responsibilities.
Description of the water system. A schematic or written description of the system, including cold water storage tanks, hot water generation (calorifiers, boilers), distribution pipework, outlets, and any ancillary systems such as cooling towers, spa pools or decorative fountains. This should be kept up to date as the system changes.
Risk assessment summary. The key findings from the risk assessment, including the identified risks and their priority ratings, which give context for everything else in the scheme.
Control measures. The specific actions required to manage the identified risks. These typically include temperature control parameters (hot water stored at 60°C minimum and reaching 50°C at the outlet within a minute of running, or 55°C in healthcare premises under HTM 04-01; cold water below 20°C), flushing regimes for infrequently used outlets, cleaning and disinfection schedules, treatment programmes such as chemical dosing where used, and elimination of dead legs.
Monitoring schedule. A detailed timetable of routine monitoring: what needs to be checked, how often, what is being measured, and the acceptable range. Under HSG274, this typically means monthly temperature checks at sentinel outlets, with other, representative outlets checked on a rotation so that every outlet in the system is tested at least once a year, alongside an annual inspection of cold water storage tanks and calorifiers.
Recording requirements. What records need to be kept, in what format, and for how long. Temperature logs, flushing records, maintenance records, inspection reports, water sampling results and corrective action records all need documenting.
Corrective actions. What happens when a monitoring result falls outside the acceptable range. If a cold water temperature comes back at 24°C, what is the procedure, who is notified, what investigation is triggered, and what remedial action follows?
Review schedule. When and how the scheme is reviewed. L8 and HSG274 expect the scheme to be reviewed regularly, and immediately whenever there is reason to believe it is no longer valid, such as after a significant change to the water system, building use or occupancy, rather than on a fixed annual cycle alone.
Who prepares it?
The written scheme should be prepared by someone competent in legionella risk management, usually as part of, or immediately following, the legionella risk assessment. The assessor who understands your water system and has identified the risks is best placed to specify the control measures and monitoring regime.
The duty holder does not need to write the scheme personally, but does need to understand it, implement it and confirm it is being followed.
Where written schemes go wrong
The most frequent problem is not the absence of a written scheme, it is one that exists on paper but is not implemented in practice: temperature checks that should happen monthly but do not, flushing regimes that are specified but nobody carries out, corrective actions that are documented but never followed up.
The second common problem is a scheme that has not been updated. The building has been refurbished, outlets have been added or removed, but the scheme still describes the original installation. A written scheme is only useful if it is current and actively followed.
Do you need one?
If your legionella risk assessment identifies any risk that requires ongoing management, which is the case for most commercial buildings, you need a written scheme of control. That covers offices, hotels, care homes, schools, hospitals, leisure centres, residential landlords with communal water systems, and essentially any building with a water system serving multiple outlets.
The exception is where the risk assessment itself concludes the risk is negligible and no ongoing controls are needed. For most buildings, that is not the case. Full guidance is available from HSE on legionnaires' disease and the control of legionella bacteria in water systems, and York Green's legionella risk assessment service can carry out the assessment and prepare the written scheme together.