Legionella thermometers

Legionella is a type of bacterium found in water systems. If conditions allow it to multiply, it can cause Legionnaires’ disease when contaminated water droplets are inhaled.

Legionella grows most effectively between 20 °C and 45 °C. This range is known as the risk or “danger zone”, where bacteria can multiply in hot and cold water systems.

Read our Legionella water temperature guide to learn more.

In the UK, duty holders such as employers, landlords and facilities managers are responsible for managing Legionella risk. This includes carrying out risk assessments, controlling water temperatures, and keeping accurate monitoring records in line with HSE guidance.

The frequency depends on the risk assessment and building type. Many sites carry out monthly checks, while higher-risk environments such as hospitals and care homes may require more frequent monitoring.

Water temperatures are measured using a calibrated digital thermometer with a probe. Checks are carried out at outlets, tanks and pipework to confirm hot and cold water remain within safe control limits.

UK guidance commonly uses both ‘first draw’ and flushed samples to assess risk:

Cold water:

• First draw: taken immediately (shows water sitting in outlet/pipework)
• After flushing: run ~2 minutes (or until stable temperature) to assess system water

Hot water:

First draw: taken immediately on opening
After flushing: run ~1 minute (or until stable, typically ≥50°C in higher-risk settings)

Why it matters:

• First draw shows exposure risk at the outlet
• Post-flush shows system-wide conditions

Key benchmarks:

• Cold water should be ≤20 °C after flushing
• Hot water should reach ≥50 °C within about 1 minute
  

Digital thermometers — Robust and calibrated industrial thermometers. Waterproof and Bluetooth-enabled options streamline checks.

Probes — Surface probes for pipes and tanks, immersion probes for running water, and wire probes for hard-to-reach areas.

Timer — Ensure cold running water reaches ≤20 °C within 2 minutes, and hot water reaches ≥50 °C within 1 minute.

Accurate temperature readings are essential for demonstrating compliance and reducing risk. Reliable thermometers help identify system issues early, such as poor circulation or failing hot water systems, before they become a hazard.