Understanding Storage Water and Preventing Bacterial Contamination

💧Understanding Storage Water and Preventing Bacterial Contamination Like Legionella

Stored water plays a vital role in many domestic and commercial heating and plumbing systems. However, when water is left to stagnate or sits at the wrong temperature, it can become a breeding ground for harmful bacteria — notably Legionella pneumophila, the cause of Legionnaires’ disease. Understanding how storage water systems work, and how to manage them safely, is essential for property owners, landlords, and engineers alike.

What Is Stored Water?

Stored water typically refers to hot or cold water held in a vessel — such as a cylinder, tank, or calorifier — for later use. This can be:

  • Domestic hot water cylinders (vented or unvented)
  • Cold water storage tanks (usually in lofts or plant rooms)
  • Thermal stores or buffer vessels
  • Commercial hot water systems

While these systems are designed to provide water on demand, they also introduce a risk: bacteria can multiply rapidly in stagnant or improperly heated water.

The Risk: Legionella Bacteria

Legionella is a naturally occurring bacterium found in freshwater environments. However, it becomes a danger when it proliferates in man-made water systems. Inhalation of aerosolised water droplets (e.g. from showers, taps, or cooling towers) contaminated with Legionella can cause Legionnaires’ disease — a potentially fatal form of pneumonia.

Conditions that Promote Legionella Growth:

  • Water temperatures between 20°C and 45°C
  • Stagnant or slow-moving water
  • Presence of biofilm, scale, or corrosion
  • Poorly insulated tanks or pipes
  • Dead legs or blind ends in pipework

Key Storage Temperatures

Controlling water temperature is the primary way to prevent Legionella growth:

  • Cold water should be stored below 20°C
  • Hot water should be stored at 60°C or above
  • Hot water must be delivered to outlets at a minimum of 50°C (or 55°C in healthcare settings)

Important: To prevent scalding, Thermostatic Mixing Valves (TMVs) can be used at outlets to blend water down to safe use temperatures (typically around 41°C).

Best Practices to Prevent Bacterial Contamination

✅ Design Out the Risk

  • Avoid dead legs or unused pipework
  • Ensure tanks and cylinders are well insulated
  • Use materials that discourage bacterial growth (e.g. WRAS-approved plastics or copper)

🔄 Ensure Regular Water Turnover

  • Design systems to promote frequent use of stored water
  • Minimise volume of stored cold water (e.g. 24-hour usage capacity)
  • Flush infrequently used outlets weekly (especially in commercial or rented properties)

🌡 Maintain Correct Temperatures

  • Fit thermostats and temperature gauges
  • Use immersion heaters, boilers, or calorifiers capable of reaching and maintaining 60°C
  • Periodically raise water temperature to perform thermal disinfection (for advanced users only — with caution)

🧪 Test and Monitor

  • Commercial and rental properties should conduct Legionella risk assessments
  • Periodic microbiological testing may be recommended
  • Temperature checks should be carried out regularly at sentinel outlets

Legal Responsibilities (UK Context)

Under the Health and Safety at Work Act 1974 and the Control of Substances Hazardous to Health Regulations 2002 (COSHH), duty holders (e.g. landlords, employers) must assess and manage the risk of Legionella.

Guidance is provided in ACOP L8 and HSG274 (Part 2) — these set out expectations for risk assessment, monitoring, and control in hot and cold water systems.

For domestic owner-occupiers, these duties do not apply, but good hygiene and proper maintenance are still strongly advised.

Signs of Trouble: What to Watch For

  • Tepid water from hot taps or showers
  • Cold water that feels warm to the touch
  • Long pipe runs with delayed water delivery
  • Scale build-up in kettles, taps, or showerheads
  • Unused bathrooms or outlet points

Final Thoughts

Water storage systems are perfectly safe when properly designed and maintained. The key is temperature control, system design, and routine maintenance. Whether you’re a homeowner, landlord, or facilities manager, understanding how to minimise the risk of Legionella and other bacterial contamination is an essential part of managing your water system.

If you're unsure about your setup or need a professional risk assessment, contact a qualified heating engineer or water hygiene specialist.

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