How to Monitor Water Temperature for Legionella Control

Why temperature matters

Legionella pneumophila, the bacterium that causes Legionnaires' disease, is temperature-sensitive. Below 20°C it is effectively dormant. Between roughly 20°C and 45°C is its growth range, with multiplication fastest in the middle of that band. At 50°C, HSE data indicates around 90% of the bacteria are killed within about two hours. At 60°C, Legionella is killed within minutes.

This is why L8 and HSG274, the guidance supporting the COSHH 2002 duty to control biological agents, use 60°C storage and 50°C at outlets (55°C for healthcare) as the practical targets rather than a single kill-point figure. They are guidance figures, not a number written into the regulations themselves, but they are what a competent risk assessment is measured against.

Hot water systems: the calorifier and sentinel outlets

The vessel that stores and heats water for distribution is usually called a calorifier or hot water cylinder in UK systems, even where a boiler supplies the heat to it. Checking "the boiler" temperature alone tells you what is being produced, not what is arriving at outlets.

HSG274 Part 2 identifies sentinel outlets, typically the outlet closest to the calorifier and the one furthest from it, as the points you check routinely because they represent the best and worst case for that system. Water needs to reach 50°C within one minute of running at a sentinel hot outlet (55°C in healthcare and care settings). Other outlets not designated as sentinel are checked less often, on a rotational basis, so that every outlet in the system is covered over the year rather than checked individually every month.

Cold water systems

Cold water should stay below 20°C, reaching that temperature within two minutes of running at an outlet. Cold water systems are often overlooked because the incoming mains supply is genuinely cold, but pipework routed through heated areas such as lofts or airing cupboards, tanks positioned in warm spaces, poor insulation, and stagnant dead legs can all let cold water warm into the growth range before it reaches a tap.

Checking points typically include the incoming mains, the storage tank, and outlets furthest from the supply or closest to a heat source. Where a system has a known history of warm cold water, more frequent checks than the routine schedule are justified until the cause is fixed.

TMVs and mixed outlets

Thermostatic mixing valves (TMVs) blend hot and cold water to produce a safe temperature at the tap or shower, typically in the 38 to 44°C range, to prevent scalding. The check that matters for Legionella control is not the mixed output temperature: it is the hot water supply reaching the valve upstream, which still needs to meet the normal hot water targets before it is blended down. A TMV set correctly at the outlet tells you nothing about whether the supply feeding it is hot enough to control Legionella.

TMVs also need routine servicing in their own right, because a valve that has drifted or failed can deliver water that is too hot (a scald risk) or, less obviously, can mask a supply problem if only the blended output is ever checked.

Warm water systems

Some systems run intentionally warm rather than hot, and these need particular care because they sit inside Legionella's growth range. This is a different thing from a closed heating circuit: a wet central heating system that circulates water at 45 to 55°C is a sealed loop that people do not drink from or shower in, so it is not a Legionella exposure route in the way a domestic hot or cold water system is.

Genuine warm water risks are things like TMV-blended outlets, low-temperature hot water returns, or any domestic water system that cannot reach a proper hot or cold target. Emergency eyewash and safety showers are a separate case again: they typically run from the mains cold supply or a dedicated tepid supply, not by diluting the hot water system, and should be assessed and flushed like any other infrequently used outlet.

The monitoring schedule

HSG274 Part 2's Table 2.1 sets out the routine schedule most systems should follow, though the written scheme of control for your specific system is what actually sets the frequency, and can vary it based on risk:

• Sentinel outlets (hot and cold): checked monthly, confirming hot reaches the outlet target within one minute and cold reaches below 20°C within two minutes.
• Calorifier flow and return: checked monthly, confirming the flow leaving storage and the return both remain within target.
• Cold water storage tank temperature: checked six-monthly, with a full physical inspection of the tank annually.
• Non-sentinel (representative) outlets: checked on a rotational basis so that all outlets are covered over the course of a year.
• TMVs and showers: serviced and checked according to the manufacturer's instructions and the written scheme, alongside routine checks of the hot supply feeding them.

A higher-risk system, one with a history of temperature problems, complex pipework, or vulnerable occupants, may need more frequent checks than this baseline. The written scheme should say so explicitly rather than leaving frequency to be worked out on the day.

Recording the data

Write down the temperature, the time, the location, who took the measurement and the date, for every check. A temperature log is what demonstrates the risk is actually being managed, and it is the first thing HSE or an environmental health officer would ask to see if a case of Legionnaires' disease occurred on the premises.

What to do when a reading is wrong

If hot water is not reaching target: check whether the calorifier or water heater thermostat is set correctly, whether the appliance itself is faulty or overdue a service, and whether heat is being lost in the distribution pipework through poor insulation. If cold water is running warm: trace the pipework for routes through heated spaces, check tank location and insulation, and address any stagnant dead legs. In either case, the fault and the remedial action taken should be recorded alongside the reading that flagged it.

Full technical detail on the monitoring regime, including Table 2.1, is in HSG274 Part 2 on hse.gov.uk. A legionella risk assessment sets the monitoring points and frequency for your specific system rather than a generic schedule.