Thermal disinfection — pasteurisation — kills Legionella by driving genuinely hot water out to every outlet in a planned order, then recording the temperature each one reached. The sequence is: clean first, raise the calorifier, purge outlet by outlet from nearest to furthest, verify the awkward ends, then restore the system. This is the heat version of a chemical shock dose.

It is also the procedure with the sharpest safety edge. Water hot enough to pasteurise pipework will cause a deep scald in seconds, and during the purge you are deliberately sending that water to taps people use in safety the rest of the week. Treat the scald risk during disinfection as the main hazard of the day, not a footnote.

The concept and the reasoning behind heat as a control sit in Thermal disinfection: using heat to control Legionella. What follows is the ordered, recordable how-to: what to do, in what order, and how to know each step is actually finished.

What you are aiming for, and what to have ready

The target is easy to state and hard to hit: hot water at a lethal temperature, held long enough, at every outlet — including the ones routine running never gets properly hot. Get these in place before you start.

  • A genuine trigger. A positive sample, commissioning a new or altered system, a return from a long shutdown, or a leg that will not hold temperature. If a positive result brought you here, the purge belongs inside the wider response, not instead of it.
  • A clean system. Scale and biofilm insulate the bacteria beneath them and slow the heat-up, so a furred shower head can sit lukewarm while the gauge reads hot. Clean and descale first, then disinfect — the order set out in Cleaning and disinfection after remedial work: thermal and chemical approaches.
  • A competent person and a method statement. Under ACoP L8 and HSG274 this is planned, supervised, recorded work, not an improvised afternoon with the immersion turned up [1][2]. If you contract it out, use a provider working to the Legionella Control Association code of conduct [6].
  • A calibrated thermometer used at the outlet. A surface or insertion probe you trust. The calorifier gauge is not your evidence; the reading at the tap is.
  • Scald-risk controls, planned in advance. Out of hours where the building is occupied, outlets signed and access restricted, occupants told, and every thermostatic mixing valve identified for bypass or removal — because a TMV exists to block the very heat you are trying to deliver.
  • A clear sense of what “hot enough” means. The underlying lethality is covered in What temperature kills Legionella?.

One limit to fix in your head before you light it off: heat disinfection only treats the hot water system. It does nothing for cold storage, cold distribution or the cold feed to a mixed outlet. If the cold side is implicated, you need the chemical route in How to shock-chlorinate (disinfect) a water system: a step-by-step procedure, not more heat.

The pasteurisation sequence, in order

Work through this as written. Each step has a point at which it is genuinely finished and a way it tends to go wrong.

  1. Raise and stabilise the calorifier store. Bring the stored hot water above its normal running temperature and let it recover after the first heavy draw-off. UK guidance commonly describes raising the calorifier to around 70°C for a thermal purge, though the figure for your system belongs in your written scheme [2]. Done when: the store holds the raised temperature and has reheated after the first outlets are run. Common failure: starting the outlet purge before the cylinder has recovered, so the temperature collapses three taps in.
  2. Check for a cold base. Calorifiers stratify, and the coolest, sludgiest layer sits at the bottom near the return. Measure low on the vessel, not just at the flow. Done when: the base and return read hot, not only the top. Common failure: a cool sediment layer that never reaches temperature and quietly reseeds the system.
  3. Purge outlet by outlet, nearest to furthest. Open each outlet to full flow, measure the temperature at that outlet, and hold it at target. A regime commonly cited in guidance is running each outlet until the water reaches at least 60°C and holding it there for a set period — often quoted as around five minutes at that temperature — but your scheme sets the actual numbers, and healthcare premises work to their own figures under HTM 04-01 [2][5]. Move methodically so nothing is skipped. Done when: the outlet has reached and held target on the thermometer, and you have written the reading down. Common failure: starting the clock when you opened the tap rather than when it actually reached temperature.
  4. Bypass the TMVs and chase the dead legs. Any outlet downstream of a thermostatic mixing valve will never see full heat unless that valve is bypassed or removed for the purge — that is the whole point of the valve. 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 treat. Done when: every TMV-served outlet and every dead-leg branch has been individually heated to target and logged. Common failure: skipping the disabled WC, the cleaner’s tap, the spare room “nobody uses” — exactly the outlets a heat purge exists to reach.
  5. Re-check the extremities. Go back to the furthest and least-used outlets and confirm they still hit target at the end of the run, not just on the first pass. Done when: the far points still reach temperature after the rest of the system has been worked. Common failure: a comfortable reading in the plant room masking a lukewarm one at the top of the riser.
  6. Restore temperatures, reinstate TMVs, return to service. Bring the calorifier back to its normal running set point, re-commission every TMV you bypassed, and confirm tempered outlets deliver a safe temperature again before anyone uses them. Done when: the system is back to its routine regime — stored at or above the standard figure, hot at the tap within about a minute, cold below 20°C — and scald protection is fully restored [3]. Common failure: handing the building back with the calorifier still at 70°C and a TMV still bypassed, which is a live scald hazard.
  7. Verify later, not on the spot. Pasteurisation leaves no chemical residual, so the protection ends as the water cools — the record is your evidence that it worked. Where your assessment calls for clearance sampling, follow the method and timing in your scheme rather than grabbing a sample the same hour [4]. Done when: the per-outlet record is complete and any verification sampling is booked to the right method and interval. Common failure: declaring success on a hot thermometer alone.

Record each outlet as you go, not from memory at the van

The purge is only as good as its log. For every outlet you want the temperature reached, the dwell held, who did it, and — for any tap that fell short — the figure it actually hit and the follow-up. That per-outlet record is what distinguishes a real outlet-by-outlet hot flush from the cylinder simply running hot for an afternoon. Paper sheets in a warm plant room get smudged or back-filled later; logging each temperature and timestamp straight into a digital logbook at the tap gives you a trail an assessor can follow and flags the outlets that need a re-purge before you leave site.

Competence, limits and the safety line

This is general procedure, not a site-specific method statement. The exact store temperature, the outlet target, the dwell time and the sampling that follows are calls for a competent person working from your own risk assessment and written scheme — not numbers to lift from a web page. And because the water you are moving can cause a full-thickness scald in seconds, the safe-working plan deserves at least as much care as the microbiology. Nothing here is legal, medical or design advice.

FAQ

Do I have to bypass every TMV, or can I leave some in service?

If you leave a thermostatic mixing valve in service, the outlet downstream of it does not get disinfected — the valve blends the heat away to protect the user, exactly as designed. So every TMV serving an outlet you intend to purge has to be bypassed or removed, then reinstated and proven afterwards. The trade-off is that the bypass removes scald protection during the purge, which is why access control and out-of-hours working matter so much.

Can I heat-disinfect the cold water system too?

No. A heat purge only reaches the hot water system. Cold storage, cold distribution and the cold feeds to mixed outlets stay cold, so heat does nothing for them. If your problem is on the cold side, or spans both, the right tool is chemical disinfection — the sequence in How to shock-chlorinate (disinfect) a water system: a step-by-step procedure.

How soon can the building go back into use?

Only once temperatures are back to the normal running regime, every bypassed TMV is reinstated and proven to deliver a safe tempered temperature, and the system is no longer carrying purge-level heat to outlets people can reach. Re-occupation before scald protection is restored is the most dangerous shortcut in the whole job; the microbiological result can wait for sampling, but the scald hazard cannot wait for anyone.

What to do before you start

Walk the system and build the outlet list now, on paper or in your logbook: every tap and shower from the calorifier to the furthest point, with the TMVs, dead legs and low-use outlets marked. That list is your running order and your record sheet in one. Get it written, confirm who holds the area during the purge, and the procedure itself becomes the straightforward part.

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/ [6] Legionella Control Association, “Code of Conduct for Service Providers”. https://www.legionellacontrol.org.uk/