Heat kills Legionella. That much is settled: the bacteria stop multiplying above roughly 50°C and die off faster the hotter the water gets. Thermal disinfection turns that fact into a procedure. You raise the temperature of a hot water system and drive that heat right out to every tap and shower, so the bacteria are killed where they actually live, in the pipework and fittings, not just in the cylinder.
The catch is that a heat purge only counts if the heat reaches the far end. A calorifier gauge reading 70°C tells you nothing about the dead-legged shower on the top floor that never climbs above lukewarm. Below are the questions site teams ask most often, answered for a UK water system run under L8 and HSG274 [1][2].
What is thermal disinfection, exactly?
It is a deliberate, time-limited operation: heat the stored hot water above its normal running temperature, then work through the system outlet by outlet, running each one hot for long enough to kill what is growing inside it. You will hear it called a heat purge, heat disinfection, or pasteurisation. Unlike chemical dosing it leaves no residual, so once the water cools the protection ends. It treats the system as it is on the day, and nothing more.
Isn’t this just keeping the hot water at 60°C?
No, and conflating the two is one of the more common mistakes. Routine temperature control is the day-job: hot water stored at or above 60°C, delivered hot enough to hit the benchmark figure at the tap within about a minute, and cold water kept genuinely cold below 20°C [3]. That regime suppresses growth continuously. A thermal disinfection is a periodic, higher-intensity intervention, usually a remedial step after a problem or a scheduled purge of the parts of a system that routine control struggles to keep hot, such as long branches and seldom-used outlets. One is the baseline. The other is a reset.
What temperature does each outlet need to reach, and for how long?
The principle is that the water arriving at the outlet, not the water leaving the calorifier, has to get hot enough for long enough to be lethal. UK guidance describes raising the stored temperature and then running each tap and shower in turn until the water at that outlet reaches a high temperature, commonly cited as at least 60°C, and holding it there for a set number of minutes [2]. The exact figure and dwell time belong in your written scheme; healthcare premises work to tighter rules again under HTM 04-01 [5]. Whatever the target, measure at the outlet and write down what you actually got. A shower that only reached 48°C has not been disinfected, however hot the cylinder was.
When is a heat purge the right tool?
Reach for it when something has gone wrong, or when temperature is the lever you can actually pull. Common triggers: a positive Legionella sample, bringing a building back into use after a long shutdown, recommissioning after works, or a stubborn failure to hold temperature on one particular leg. Heat is also the natural choice on systems where chemical disinfection is awkward, or where you want to treat the existing pipework without bolting on dosing equipment. It is a poor standing substitute for fixing a design fault, though. If an outlet needs purging every month because it sits on a dead leg, the better answer is to remove the dead leg.
How do you run one without scalding anyone?
This is the part that bites. Water hot enough to kill Legionella is hot enough to cause a full-thickness scald in seconds, and during a disinfection you are deliberately sending that water to outlets people normally use in safety. So plan it as a safety operation, not just a plumbing task: restrict access to the area, sign and isolate outlets, bypass or remove anything that would normally deliver tempered water, and put a competent person in charge of the sequence. In occupied buildings that usually means working out of hours. The scald risk to staff and occupants deserves the same weight as the microbiology you are there to manage.
Why do TMVs and dead legs trip this up?
Because they sit between you and the heat. A thermostatic mixing valve (TMV) exists to prevent scalding by blending hot with cold, which means full-temperature water never reaches the tap downstream of it unless you bypass or remove the valve for the purge. Dead legs and rarely-used branches cool fastest and take longest to heat through, so they are both the highest risk and the hardest to disinfect. Scale compounds it: a furred-up shower head or a limescaled outlet insulates the bacteria and slows the heat-up, which is why descaling is a partner task rather than an afterthought (see Showerhead cleaning and descaling schedules). Map the route around your sentinel outlets and the known awkward branches before you start.
How often should it be repeated?
There is no universal interval. Repetition is driven by why you did it in the first place and by what your monitoring shows afterwards. A purge done as a one-off remedial measure may not need repeating if the underlying cause is fixed and temperatures then hold. A leg that only stays safe because you keep purging it is telling you to redesign, not to diarise more purges. Your risk assessment and written control scheme set the frequency, and both should be revisited whenever the system, its pattern of use, or its results change.
What proof do you need afterwards, and does it replace sampling?
A thermal disinfection is only as good as its record. For each outlet you want the temperature reached, the time held, who did the work, and any outlet that fell short of target plus the follow-up action. That per-outlet log is what shows the purge did something, rather than the cylinder simply running hot for an afternoon. It does not replace sampling: a clean sample and a completed disinfection answer different questions, and HSE is clear that testing frequency follows the system and the risk assessment rather than a fixed calendar [4]. Use sampling to verify, not to declare victory.
If you only do one thing
Before the next purge, walk the system and list every TMV, dead leg and low-use outlet between the calorifier and the furthest tap. Those are the points where heat dies on the way out, and they decide whether the exercise works at all. Build the sequence around them, put a thermometer at each outlet, and leave a line on the record for the temperature you actually measured.
One closing caution worth keeping in view: none of the figures above are a substitute for your own written scheme. The lethal temperature you aim for, the dwell time at each tap, and how often you repeat the exercise are calls for a competent person working from your risk assessment, not numbers to lift from a web page. And because the water involved can scald in seconds, the safe-working plan deserves as much care as the temperatures themselves.
Sources
[1] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [2] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [3] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [4] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm [5] NHS England, “Health Technical Memorandum 04-01: Safe water in healthcare premises”. https://www.england.nhs.uk/publication/safe-water-in-healthcare-premises-htm-04-01/