Legionella Temperature Requirements: The Key Numbers Every Assessor Needs

Temperature control is the primary defence against legionella in hot and cold water systems. Legionella pneumophila -- the species responsible for the vast majority of legionnaires' disease cases -- thrives in water between 20 and 45 degrees C. Outside this range, the bacteria either cannot grow or are killed.

This guide provides a clear reference to every critical temperature threshold that assessors and duty holders need to know.

The danger zone: 20-45 degrees C

Legionella bacteria can survive in a wide range of temperatures, but they proliferate most rapidly between 20 and 45 degrees C. This range is often referred to as the "danger zone." The goal of temperature control is to keep water either below 20 degrees C (cold systems) or above 50 degrees C (hot systems), ensuring that water does not remain in the danger zone for prolonged periods.

Water temperature is not the only factor -- legionella also requires nutrients (biofilm, sediment, scale) and time. But temperature is the factor that assessors can most readily measure and control.

Cold water temperature requirements

Target: below 20 degrees C

HSG274 Part 2 requires cold water to be stored and distributed below 20 degrees C. This is a firm threshold, not a target to aim for -- water regularly reaching or exceeding 20 degrees C represents a risk condition.

Key measurements:

• Cold water storage tank: Water in the tank should be below 20 degrees C. Tanks in roof spaces, plant rooms, or other warm locations are at greatest risk.
• Sentinel cold water outlets: Cold water should reach below 20 degrees C within two minutes of running at the nearest and furthest outlets from the storage tank.
• Incoming mains temperature: The mains supply temperature should be checked as a baseline. In summer months, mains water temperatures can approach or exceed 20 degrees C in some regions.

When cold water temperatures exceed 20 degrees C, possible remedial actions include:

• Relocating or insulating the cold water tank
• Increasing cold water flow rates to reduce residence time
• Installing point-of-use cooling
• Reducing the tank size if it is oversized for the building's demand

Hot water storage temperature

Target: 60 degrees C or above

Hot water must be stored at 60 degrees C or above in calorifiers, cylinders, and other storage vessels. At this temperature, legionella bacteria are killed over time. Below 60 degrees C, bacteria can survive and potentially grow.

Key points:

• The thermostat should be set to at least 60 degrees C
• The temperature should be measured at the top and bottom of the calorifier
• If the bottom temperature is significantly lower than the top, this may indicate stratification or a faulty immersion heater
• After a period of non-use, the calorifier should be heated to at least 60 degrees C throughout before hot water is distributed

A common mistake is to set the thermostat at exactly 60 degrees C without checking the actual water temperature. Thermostat calibration drift is common, and the actual temperature may be lower than displayed.

Hot water distribution temperature

Target: 50 degrees C or above at outlets

Hot water must reach at least 50 degrees C at all outlets within one minute of running. This is the distribution standard set by HSG274 Part 2.

Key measurements:

• Sentinel hot outlets: Check the nearest and furthest outlets from each calorifier monthly
• Non-sentinel outlets: Rotate through all other outlets quarterly, so every outlet is checked at least once per year
• Return temperature: Where a hot water return circuit is fitted, the return temperature at the calorifier should be at least 50 degrees C

When hot water distribution temperatures are too low:

• Check the hot water return circuit for proper operation
• Insulate exposed pipework
• Check for long dead legs or excessive pipe runs
• Consider adding trace heating on long runs
• Check that all zones of multi-zone systems are balanced

TMV outlet temperatures

Target: typically 38-44 degrees C (application-dependent)

Thermostatic mixing valves blend hot and cold water to deliver safe temperatures at outlets used by vulnerable persons. The blended temperature is typically set between 38 and 44 degrees C, depending on the application:

• Healthcare settings: Usually 38-41 degrees C
• Care homes: Usually 41-43 degrees C
• Schools and nurseries: Usually 38-41 degrees C

The critical point for legionella assessors is that the pipework downstream of a TMV is in the danger zone. This means:

• Downstream pipework should be as short as possible (ideally less than 2 metres)
• TMVs should be fitted as close to the outlet as practical
• Monthly checks should confirm both the outlet temperature and the functionality of the fail-safe mechanism

Pasteurisation temperatures

Target: 70 degrees C for at least 2 minutes

When a system requires thermal disinfection (pasteurisation), HSG274 recommends raising the water temperature to at least 70 degrees C and maintaining it at that level for at least two minutes at each outlet. Some guidance recommends 60 degrees C for a longer period, but 70 degrees C is the most commonly applied standard.

Pasteurisation may be required:

• After a positive legionella sample
• After a period of extended building closure
• After significant system modifications
• As part of a recommissioning procedure

During pasteurisation, there is a serious scalding risk. Appropriate precautions must be taken, including warning signage, restricted access, and supervision at outlets.

Temperature recording and accuracy

Accurate temperature measurement is fundamental. Assessors and monitoring personnel should:

• Use a calibrated digital thermometer (calibration checked at least annually)
• Allow sufficient run time before taking readings (one minute for hot, two minutes for cold)
• Record the temperature to at least one decimal place
• Note the date, time, location, and name of the person taking the reading
• Flag any reading that falls outside the acceptable range for immediate review

Summary of key thresholds

• Below 20 degrees C: Cold water target -- legionella growth is inhibited
• 20-45 degrees C: Danger zone -- legionella can proliferate
• 50 degrees C: Minimum hot water distribution temperature at outlets
• 55 degrees C: Legionella dies slowly at this temperature
• 60 degrees C: Minimum hot water storage temperature -- legionella killed over time
• 70 degrees C: Pasteurisation temperature -- legionella killed within minutes

These numbers should be second nature to every assessor. Temperature control is the single most effective tool for managing legionella risk in water systems.