Legionella Risk in Winter 2025 – Preventing Legionnaires’ Disease in Domestic Water Systems

As winter approaches, many buildings begin switching heating and hot water systems back on after months of reduced use. This seasonal change can create ideal conditions for Legionella bacteria to grow in stagnant or lukewarm water. If not managed, it can lead to Legionnaires’ disease – a serious and sometimes fatal pneumonia caused by inhaling contaminated water droplets.

Why the risk increases in winter

• Stagnant water in pipework and tanks after periods of low use, such as during summer holidays or hybrid working patterns.
• Heating systems restarting can cause particular areas of the building to reach higher ambient temperatures and as a result increase water temperatures through thermal gain.
• Water Outlets – Hot outlets must be at a minimum of 50°C with cold outlets being below 20°C – key thresholds set out by the HSE’s Approved Code of Practice (L8) and HSG274 guidance.

Who should be most alert?

• Schools, colleges, and universities reopening after breaks.
• Leisure centres, gyms, and hotels with showers.
• Office buildings and workplaces with irregular water use.
• Communal Water Supplies within residential properties

Casa’s approach to prevention

At Casa Environmental, our UKAS-accredited Water Hygiene Department helps organisations across Bristol, the Southwest, and nationwide to stay safe and compliant through:
• Comprehensive Legionella risk assessments tailored to your property.
• Water sampling and testing in line with industry standards.
• Ongoing monitoring programmes to keep systems within safe limits.
• Clear, practical advice for duty holders to meet their legal obligations.

Why act now?

The Health and Safety Executive (HSE) continues to highlight Legionella control as a compliance priority. Regular monitoring and risk management are not only a legal duty but also essential for protecting staff, tenants, residents, and visitors.
👉 Don’t wait until problems arise. As the autumn and winter seasons begin, review your water hygiene plan today. Contact Casa Environmental to ensure your systems are safe, compliant, and ready for winter.

(For more information, see the HSE’s Legionella guidance).