Avoid Recolonisation: 60°C Calorifier Pasteurisation (HSG274 Checks) for UK FMs
Calorifier pasteurisation means heating the calorifier to 60°C throughout, circulating that temperature for a sustained period, flushing every outlet and then verifying temperatures and samples to evidence control of Legionella. A competent technician should carry out the work under a facilities manager’s risk assessment, and anyone running hot outlets during the process must follow scald controls, since water above 50°C can cause serious burns within seconds.
TL;DR:
Pasteurisation involves heating the calorifier and all outlets to 60°C for at least one continuous hour, ensuring thorough temperature reach, including the base.
It is triggered by specific failures such as low temperature readings, positive Legionella samples, or recent system repairs, not as a routine measure.
Proper monitoring requires measuring flow, return, and base temperatures at multiple points, with regular testing at outlets using calibrated thermometers.
Biofilm formation remains unaddressed by pasteurisation, so physical cleaning and system design improvements are essential for long-term control.
Addressing underlying system faults, like dead legs or sediment buildup, is more effective than repetitive pasteurisation cycles for preventing recurrence.
Table of Contents
What calorifier pasteurisation is and when to use it
Pasteurisation is a thermal shock treatment. It raises every part of a calorifier, including the base where sediment and cooler water collect, to 60°C and holds that temperature long enough to kill Legionella bacteria present in the stored water and on surfaces the water reaches. It differs from routine temperature control, which simply maintains storage at 60°C day to day, and from chemical disinfection, which uses biocides such as chlorine to treat the system rather than heat.
Facilities teams typically turn to pasteurisation after a specific trigger rather than as a scheduled routine. Common reasons include:
A failed temperature check showing the calorifier or an outlet running below target.
A positive Legionella sample from routine monitoring.
Completion of works on the hot water system, such as a repair or a new section of pipework.
Prolonged stagnation, for example after a building has been unused for an extended period.
Pasteurisation has a real limitation: it does not remove biofilm. Biofilm acts as a protective shield around Legionella bacteria, and systems with established biofilm are likely to be rapidly recolonised after heat treatment unless the deposits are physically cleaned out and any underlying design fault is fixed. Treating the heat problem without treating the biofilm problem buys time, not a solution.
Regulatory standards and temperature targets to follow
The benchmark figures come from HSE guidance: calorifiers should store hot water at a minimum of 60°C and deliver it at 50°C within one minute of the outlet opening, rising to 55°C in healthcare premises. These are the numbers a pasteurisation cycle has to beat, not just approach.
Hitting those figures depends on where you measure, not only what the calorifier’s main gauge reads. HSG274 part 2 recommends thermometer pockets at the flow, the return and the base of the vessel, because a calorifier can show 60°C at the top while the base, where cold feed water enters and sediment settles, stays considerably cooler. Return temperature matters particularly, since a return of at least 50°C indicates the whole stored volume is being adequately heated.
Recording what you measure is not optional paperwork, it is the only evidence that control was actually achieved. A log that shows:
The flow and return temperatures at the point of treatment.
The base temperature of the calorifier, taken separately from the gauge reading.
The date, duration and outlets flushed during the pasteurisation cycle.
Sentinel outlet checks for the weeks that follow.
Gauge-only checks miss stratification entirely. A calorifier can pass a glance at the control panel and still have a cold layer sitting at the base where Legionella can survive, which is exactly why internal inspections or boroscope checks are recommended alongside routine thermometer readings. Our internal reference on HSG274 temperature limits sets out how these targets apply across different building types.
When pasteurisation is appropriate and when to choose other controls
Pasteurisation is a response to a specific failure, not a default fix for every hot water concern. Before scheduling a cycle, work through this checklist:
Check the risk assessment for the actual trigger: a failed temperature reading, a positive sample or a known period of stagnation each point to a different response.
Confirm the fault is thermal rather than microbiological contamination from a dirty system; heavy sediment or visible fouling usually means draining and chemical disinfection should come first.
Rule out a design fault, such as a dead leg or missing anti-stratification pump, that will simply cause the same failure again within weeks.
Where temperature control genuinely cannot be achieved in part of the system, consider whether an alternative control, such as chlorine dioxide dosing or copper-silver ionisation agreed within the water safety plan, is more appropriate than repeated heat treatment.
Treat pasteurisation as a one-off correction, never as a substitute for the remedial works a recurring fault actually needs.
Step-by-step calorifier pasteurisation procedure
A pasteurisation cycle runs in a fixed sequence. Skipping a step, particularly the drain and descale stage, is the most common reason cycles fail to hold.
Review the risk assessment and plan the shutdown. Confirm which outlets, wards or tenants will lose hot water, and schedule the work for low-occupancy hours where possible.
Isolate the calorifier and post warning signage. Anyone who might run a hot tap during the cycle needs to know water could exceed safe handling temperatures.
Brief the team and confirm PPE. Technicians handling drain valves and hot pipework need heat-resistant gloves and eye protection as standard.
Drain the calorifier fully. Open the lowest drain valve and run until the water is visibly clear; cloudy or discoloured water at this stage usually means sediment or biofilm has built up.
Inspect and descale if needed. Where sludge or scale is visible, this is the point to remove it, since running an anti-stratification pump before descaling can disturb sediment and worsen distribution rather than improve it.
Refill and begin heating. Close the drain, refill the vessel and bring the boiler or immersion heater up to temperature.
Heat to 60°C throughout, not just at the gauge. Monitor flow, return and base readings until all three confirm 60°C, which can take considerably longer than the control panel suggests.
Run anti-stratification circulation. Where the calorifier has a shunt pump or coil mixing arrangement, operate it to move warm water from the top to the base; HSG274 recommends achieving 60°C throughout for at least one continuous hour to be confident the whole volume, not just the upper section, has reached a lethal temperature for Legionella.
Flush every outlet in sequence. Starting with the outlet closest to the calorifier and working outward, run each tap, shower and sentinel point until the discharge reads at or above target temperature for that location.
Manage TMV-protected outlets separately. Thermostatic mixing valves will blend the hot feed down to a safe output temperature by design, so flushing at the valve, not just downstream of it, is necessary to confirm the hot side actually reached temperature.
Allow the system to cool and resume normal operation. Reopen isolation points once flushing is complete and remove warning signage only when outlets are back to normal safe temperatures.
Wait before sampling. Microbiological samples taken within 48 hours of treatment can give false negatives, so hold off confirmation sampling until that window has passed.
Update the risk assessment and file the evidence. Log the dates, temperatures, outlets flushed and sample results, and flag any sediment or scale found for remedial follow-up.
Pro Tip: Photograph the drained water before and after descaling. A clear before-and-after record is often the fastest way to justify remedial works to a client or senior manager who was not on site.
Before closing the job, check for signs that heat alone will not solve:
Heavy sediment or scale that reappears shortly after cleaning.
A base temperature that consistently lags the flow temperature by several degrees.
Any dead leg or disused branch that never gets flushed during normal use.
Where any of those turn up, flag the calorifier for remedial works rather than simply repeating the pasteurisation next quarter.
Managing scald risk and thermostatic mixing valves during pasteurisation
Heating a calorifier to 60°C and flushing outlets at that temperature creates a genuine scald hazard, so the cycle needs the same discipline as any other high-risk maintenance task. Before starting, restrict access to affected washrooms and kitchens, post clear warning signage at every outlet involved, and confirm that only trained staff operate taps during the active flush.
Keep PPE on throughout the flushing stage, not just during draining.
Schedule the hot flush for periods when vulnerable occupants, such as children, patients or elderly residents, are not using the affected outlets.
Never leave a flushed outlet running unattended.
Reconfirm TMVs are functioning correctly once the cycle ends, before reopening outlets to general use.
TMVs need particular care. Isolating or bypassing a valve during pasteurisation lets the hot side reach the temperature it needs, but the valve must be tested afterwards to confirm it is blending correctly before anyone uses that outlet normally again. In healthcare or care settings, where outlet targets rise to 55°C, the margin between an effective flush and a scald risk is narrower, which makes trained supervision of every outlet non-negotiable.
Pro Tip: Test TMV output temperature with a calibrated thermometer, not by hand, immediately after reinstating normal flow. A valve that drifted during isolation will not always feel obviously wrong to the touch.
Monitoring, verification and sampling after pasteurisation
A pasteurisation cycle is only as good as the evidence behind it. Measurement needs to happen at four points: the flow, the return, the base of the calorifier and every sentinel outlet, using a calibrated thermometer or a data logger rather than relying on the boiler’s own display.
Routine sentinel outlet checks are typically expected monthly, alongside calorifier flow, return and base readings, and that monthly rhythm is what catches a slipping system before it needs emergency treatment again.
A practical monitoring timetable after a pasteurisation cycle looks like this:
Immediately after the cycle: confirm flow, return and base temperatures all reached target, and record outlet temperatures as each was flushed.
At 48 hours: take confirmation samples, since sampling earlier than this can produce a false negative that masks a problem the heat treatment did not fully resolve.
Monthly thereafter: resume standard sentinel outlet and calorifier checks, watching specifically for any return to the pattern that triggered the original pasteurisation.
A single clean sample at 48 hours is encouraging, not conclusive. Because biofilm can shelter surviving organisms and recolonise a system within weeks, a sustained run of passing monthly checks is a far stronger indicator of genuine control than one early result. Our guide to completing temperature monitoring checks sets out how to record these readings consistently across a site, and our temperature monitoring checklist is built specifically for the weeks following a pasteurisation cycle.
Prevention: design and maintenance to reduce recurrence
Heat treatment fixes a symptom. The measures that stop it recurring sit in the calorifier’s design and its maintenance schedule, and they are considerably cheaper than repeat emergency callouts.
Fit a timer-controlled shunt pump or anti-stratification coil, and set the timer to run during the period when boiler output and vessel temperature are highest, often the early morning heating cycle, so warm water is actively driven to the base rather than left to settle.
Schedule internal inspections and drain-off temperature checks on a recurring basis rather than only when a problem is suspected, since a gauge reading alone will not reveal a cold base layer.
Set a descaling interval appropriate to local water hardness; calorifiers in hard-water areas silt up faster and need more frequent attention.
Remove dead legs and disused branches wherever a system survey identifies them, since stagnant water in an unused pipe run defeats even a well-run pasteurisation cycle elsewhere in the system.
Insulate pipework and balance circuits so that short branch lengths genuinely deliver 50°C, or 55°C in healthcare settings, within one minute at every outlet, as HSG274’s design guidance sets out.
Consider water softening on hard-water sites, since reduced scale build-up directly extends the interval between descaling jobs.
A platform such as Curcle’s water hygiene software can help larger estates keep these recurring tasks, certificates and asset registers in one auditable record rather than scattered across spreadsheets and paper logs.
Practical protocol notes and our site checks
On site, the detail that separates a pasteurisation cycle that holds from one that fails again within a month is almost always in the checks around the heat treatment, not the heat itself. Thermometers are calibrated before every cycle and pockets are placed at flow, return and base rather than relying on a single reading, since that is the only way to catch stratification the main gauge will not show.
An on-site checklist typically covers:
Drain water clarity and visible sediment before refilling.
Base temperature against flow temperature, flagged wherever the gap is wide.
TMV function once outlets are reinstated to normal use.
Any sign of biofilm or scale that points to a maintenance issue rather than a one-off thermal fault.
Where a client needs the wider picture rather than a single pasteurisation visit, our Legionella risk assessments, water sampling and automated temperature monitoring services sit alongside this work to confirm the fix has actually held.
Practitioner viewpoint: maintenance first, heat treatment second
Pasteurisation gets the attention because it is dramatic: a full shutdown, hot water running through every outlet, a result you can log the same day. Maintenance gets neglected because it is boring: a shunt pump timer checked quarterly, a drain valve opened for thirty seconds to see what colour comes out. The second one is what actually stops the first one being necessary.
We have seen sites repeat pasteurisation every few months because nobody addressed the dead leg or the undersized anti-stratification pump causing the failure in the first place. Once the remedial works went in, the cycle stopped. The heat treatment was never the fix, it was the warning sign that something upstream needed attention.
If your site has pasteurised more than once for the same reason, that pattern is worth a proper risk assessment, not another repeat treatment.
— Sammi
How we can help with pasteurisation and long-term control
We work on exactly the problems this guide covers, from the risk assessment that tells you whether pasteurisation is the right call to the remedial works that stop you needing it again. Our services include:
[
Water tank cleaning and system disinfection, including calorifier drain, descale and chemical disinfection where contamination is suspected.
TMV servicing and general remedial works, covering the anti-stratification pumps, dead legs and insulation fixes that reduce recurrence.
Legionella water testing and analysis from £55 one-off per sample, for confirmation sampling once the 48-hour window has passed.
Before you contact us, have your recent monitoring logs, calorifier make and age, and a note of any previous pasteurisation cycles to hand. Get in touch for a quote on a risk assessment or testing, or ask about a Legionella awareness training session for your maintenance team.
This article is general information, not a substitute for advice from a qualified doctor. Consult a qualified healthcare professional about your own circumstances before acting on anything here.
FAQ
What temperature should a calorifier be?
A calorifier should store hot water at a minimum of 60°C, with outlets delivering at least 50°C within one minute of opening. In healthcare premises, outlet targets rise to 55°C to account for vulnerable occupants.
What temperature kills Legionella?
Legionella bacteria are killed by sustained exposure to 60°C, which is why HSG274 recommends holding that temperature throughout the calorifier for at least one continuous hour during pasteurisation. Lower temperatures slow bacterial growth but do not reliably kill it, which is why routine storage at 60°C and a dedicated pasteurisation cycle serve different purposes.
Can Legionella be killed by boiling water?
Boiling water will kill Legionella, but it is not a practical control for a calorifier or a hot water system, since the standard regime relies on sustained 60°C storage rather than boiling point. Pasteurisation achieves the same bacterial kill at a temperature the system is designed to reach and hold safely.
How long should I run the taps to flush out Legionella?
During pasteurisation, each outlet should be run until the discharge itself reads at or above the target temperature for that location, which confirms hot water from the treated calorifier has actually reached the tap. Outside a pasteurisation cycle, routine sentinel outlet checks are typically carried out monthly rather than as a fixed flushing duration.
Sources
Recommended
Comments