High-Risk Systems: The Elevated Cold Water Temperature Threat

By Andrew Arnold

•

14/11/2025

In the UK, the primary method for controlling Legionella bacteria in cold water systems is maintaining water temperatures below 20°C. This temperature threshold is critical because Legionella bacteria thrive and multiply rapidly in the danger zone of 20°C to 45°C. During prolonged periods of hot weather or summer heatwaves, ambient temperatures can cause water in storage and distribution pipework to experience thermal gain, pushing it above the essential 20°C safe limit. For dutyholders, this temperature breach significantly elevates the Legionella risk, turning a controlled system into a potential breeding ground for the bacteria, and demanding an immediate review of their Legionella risk assessment, maintenance regime and control measures as mandated by HSG274 Part 2.

The Critical Risk: Oversized Storage and Low Turnover

The most significant risk factor in hot weather is the design and function of cold water storage tanks (CWST).

Thermal Gain: When a CWST is located in a warm roof space or plant room, it acts as a heat sink. Poor insulation, high ambient temperatures, and pipework running in close proximity to hot services all contribute to thermal transfer, raising the water temperature into the danger zone.
Oversized Storage: If a CWST is oversized relative to the building's demand, the water turnover rate is low. This results in stagnation, meaning the water remains in the tank for long periods, giving it maximum opportunity to absorb heat and allow any present Legionella bacteria to proliferate.

Reviewing water storage and turnover is the non-negotiable first step when external temperatures rise.

Solution 1: Reducing Cold Water Storage Volume

To effectively combat thermal gain and stagnation, dutyholders should proactively challenge the existing system design:

Size Review: Conduct a thorough review to determine if the existing cold water storage volume is necessary. The entire volume of stored water must turn over within a typical 24 hour period (12 hours in Healthcare premises). The volume of stored water should be reduced where necessary to achieve this.
Mains Reliance: Where possible remove cold water storage from the system and convert all facilities to a direct mains feed to improve water turnover and reduce the potential for thermal gain.
Relocation and Insulation: Where reduction is not possible, relocate the CWST to a cooler environment, ensure all pipework and the tank itself are protected with thick, well-maintained thermal insulation.

Solution 2: Increasing Flushing Regimes

Elevated temperatures can create localised risk areas, particularly in parts of the system where flow is naturally low. This necessitates immediate action to improve flow and purge warm, stagnant water:

Sentinel and Little-Used Outlets: Review the flushing regime for all sentinel outlets (those nearest and furthest from the tank) and all infrequently used outlets (e.g., staff showers, remote taps, hose points).
At least weekly flushing: Stagnant sections and dead legs must be flushed at least weekly to draw through fresh, cooler water and prevent the build-up of biofilm and nutrients that support bacterial growth. Increasing the frequency and/or duration of flushing can help to reduce system temperatures.
Aerosol Minimisation: Any flushing process must be carried out safely with minimal aerosol production to prevent the risk of inhalation and infection.

Solution 3: Additional Microbiological Sampling

When monitoring confirms that cold water temperatures are consistently rising above 20°C, the control strategy must be verified by targeted microbiological sampling.

Targeted Sampling: The Legionella risk assessment should be reviewed to identify high-risk areas where thermal gain is most likely. Samples should be taken at these locations, as well as at sentinel points, to prove the control measures (temperature and turnover) are working.
Verification: This additional sampling is a verification tool to confirm whether control has been lost and to trigger immediate remedial action, such as a system clean and disinfection, to restore safety.
Record Keeping: All temperature checks and sampling results must be accurately recorded, as this documentation demonstrates compliance with HSG274 Part 2 and is essential for defending any subsequent enforcement action.

Hot UK weather is no longer a seasonal anomaly; it is a critical threat that demands a fundamental shift in how dutyholders manage their cold water systems. Compliance with HSG274 Part 2 requires proactive management, starting with a robust Legionella risk assessment review. Implementing practical solutions—reducing storage volumes, enforcing stringent flushing, and backing up temperature control with microbiological sampling—is the only way to effectively reduce the risk of a potential Legionnaires' disease outbreak.

The Critical Risk: Oversized Storage and Low Turnover

The most significant risk factor in hot weather is the design and function of cold water storage tanks (CWST).

Thermal Gain: When a CWST is located in a warm roof space or plant room, it acts as a heat sink. Poor insulation, high ambient temperatures, and pipework running in close proximity to hot services all contribute to thermal transfer, raising the water temperature into the danger zone.

Oversized Storage: If a CWST is oversized relative to the building's demand, the water turnover rate is low. This results in stagnation, meaning the water remains in the tank for long periods, giving it maximum opportunity to absorb heat and allow any present Legionella bacteria to proliferate.

Reviewing water storage and turnover is the non-negotiable first step when external temperatures rise.

Solution 1: Reducing Cold Water Storage Volume

To effectively combat thermal gain and stagnation, dutyholders should proactively challenge the existing system design:

Size Review: Conduct a thorough review to determine if the existing cold water storage volume is necessary. The entire volume of stored water must turn over within a typical 24 hour period (12 hours in Healthcare premises). The volume of stored water should be reduced where necessary to achieve this.

Mains Reliance: Where possible remove cold water storage from the system and convert all facilities to a direct mains feed to improve water turnover and reduce the potential for thermal gain.

Relocation and Insulation: Where reduction is not possible, relocate the CWST to a cooler environment, ensure all pipework and the tank itself are protected with thick, well-maintained thermal insulation.

Solution 2: Increasing Flushing Regimes

Sentinel and Little-Used Outlets: Review the flushing regime for all sentinel outlets (those nearest and furthest from the tank) and all infrequently used outlets (e.g., staff showers, remote taps, hose points).

At least weekly flushing: Stagnant sections and dead legs must be flushed at least weekly to draw through fresh, cooler water and prevent the build-up of biofilm and nutrients that support bacterial growth. Increasing the frequency and/or duration of flushing can help to reduce system temperatures.

Aerosol Minimisation: Any flushing process must be carried out safely with minimal aerosol production to prevent the risk of inhalation and infection.

Solution 3: Additional Microbiological Sampling

When monitoring confirms that cold water temperatures are consistently rising above 20°C, the control strategy must be verified by targeted microbiological sampling.

Targeted Sampling: The Legionella risk assessment should be reviewed to identify high-risk areas where thermal gain is most likely. Samples should be taken at these locations, as well as at sentinel points, to prove the control measures (temperature and turnover) are working.

Verification: This additional sampling is a verification tool to confirm whether control has been lost and to trigger immediate remedial action, such as a system clean and disinfection, to restore safety.

Record Keeping: All temperature checks and sampling results must be accurately recorded, as this documentation demonstrates compliance with HSG274 Part 2 and is essential for defending any subsequent enforcement action.