Understanding Legionella Bacteria
Legionella pneumophila is a waterborne bacterium responsible for Legionnaires' disease — a severe, sometimes fatal form of pneumonia. The bacteria occur naturally in water sources but become dangerous when they colonise building water systems and multiply to high concentrations.
The critical factor is temperature. Legionella is dormant below 20°C and killed above 60°C, but between these temperatures it multiplies rapidly. The highest growth rate occurs between 35°C and 40°C — precisely the range found in poorly managed hot water systems, mixing valves, and tepid water outlets.
Temperature Warning Signs
Temperature is the single most reliable indicator of legionella risk. If your water system is operating within the danger zone, bacterial colonisation is not just possible — it's likely. The table below shows the key temperature thresholds every building manager should know.
| Temperature | What It Means | Action Required |
|---|---|---|
| Below 20°C (cold water) | Safe — legionella dormant | No action, maintain this level |
| 20°C – 35°C | Risk zone — bacteria can multiply | Investigate and correct immediately |
| 35°C – 40°C | Maximum danger — rapid multiplication | Urgent remediation required |
| 40°C – 45°C | Bacteria survive but growth slows | Still dangerous, correct system |
| 50°C+ at outlets | Target for hot water distribution | Acceptable — must reach within 1 minute |
| 60°C+ at calorifier | Target for hot water storage | Maintain consistently |
Physical Warning Signs
Beyond temperature, several visible signs can indicate legionella contamination. Discoloured or rusty water at taps or showers suggests corrosion and sediment — both of which provide nutrients for bacterial growth. A musty or earthy smell from hot water is another red flag that warrants immediate investigation.
Visible biofilm — a slimy coating found on the inside of tanks, pipes, or shower heads — is a direct indicator of microbial colonisation. If you can see biofilm at accessible points like shower heads or overflow pipes, it is almost certainly present throughout the system. Biofilm provides a protected environment where legionella can thrive even when water temperatures are periodically raised.
- •Lukewarm water from taps that should be hot
- •Cold water that feels warm or tepid
- •Discoloured, rusty, or cloudy water
- •Musty or unusual smell from hot water outlets
- •Visible slime on shower heads, taps, or tank surfaces
- •Low water pressure suggesting partial blockages or scale build-up
High-Risk System Components
Certain parts of a water system are inherently more susceptible to legionella growth. Dead legs — sections of pipework that are capped off or no longer serve an active outlet — trap stagnant water at ambient temperature. In London buildings that have been modified over decades, dead legs are extremely common and often undocumented.
Water storage tanks, particularly older header tanks in loft spaces, are another critical risk point. Tanks without properly fitted lids, insect screens, or overflow protection can accumulate debris and organic matter that feeds bacterial growth. Calorifiers (hot water cylinders) that aren't maintaining 60°C throughout are equally dangerous. Thermostatic mixing valves (TMVs) can create ideal conditions for legionella at the blending point if not properly maintained.
What To Do If You Suspect a Problem
If you recognise any of these warning signs, act immediately. Legionella contamination does not resolve itself — it worsens over time. As a first step, check water temperatures: hot water should reach 50°C at outlets within one minute, and cold water should be below 20°C. If either is outside these limits, your system needs professional attention.
Avoid using showers or other spray-producing outlets until the system has been assessed, as the aerosol spray is the primary transmission route for legionella. Run all taps for several minutes to flush through stagnant water. Then contact a qualified water treatment company for professional testing and risk assessment . Do not attempt to self-treat the system — incorrect chlorination can be ineffective or dangerous.
Prevention: The Most Cost-Effective Approach
A professional legionella risk assessment typically costs a fraction of emergency remediation. Regular monitoring — monthly temperature checks and quarterly water sampling — catches problems before they become crises. For London landlords, this isn't optional: it's a legal requirement under ACOP L8 with potentially unlimited fines for non-compliance.