Legionella pasteurisation cycles

Legionella bacteria can grow where water is stored or moves slowly at favourable temperatures, then reach people in fine droplets from showers, taps or other outlets. A heat-pump hot-water system therefore needs a defined bacterial-growth control, not simply a cylinder temperature chosen for low energy use.

A periodic pasteurisation cycle is one possible part of that control. It heats stored water above the normal operating target for a set period. It is not a universal programme that can safely be copied from another house, cylinder or controller.

Start with the system risk

Risk depends on more than the controller setpoint. Relevant factors include:

• storage volume and everyday water turnover
• sections of pipe with little or no flow
• cylinder stratification and sensor position
• water temperature throughout the system, not at one sensor alone
• showers or other outlets that create aerosols
• periods when the property is empty
• scale, sediment and biofilm
• the age, health and vulnerability of users

A well-used domestic system is different from a large managed building, healthcare setting or little-used property. The responsible person should apply the guidance and legal duties that fit the premises.

What the current MCS design standard requires

As at 22 July 2026, MIS 3005-D Issue 3.0 requires an MCS-designed domestic hot-water system to include a means of preventing bacterial growth, including Legionella. The standard does not prescribe one pasteurisation temperature or one frequency for every installation.

Where periodic pasteurisation is the selected method, MIS 3005-D says the frequency should follow a bacterial risk assessment. That distinction matters. A weekly programme found in one manufacturer’s menu is not evidence that weekly operation is suitable for every system.

The design and handover record should identify:

• the chosen bacterial-control method
• the temperature sensor used to confirm it
• target, hold and completion logic where a thermal cycle is used
• any immersion or supplementary heater involved
• the required schedule and what happens after a failed cycle
• user and maintenance actions

HSE temperature guidance has a defined scope

As at 22 July 2026, HSE guidance for hot and cold water systems managed under its Legionella framework describes a common temperature-control approach in which hot water is stored at no less than 60°C and reaches outlets at at least 50°C within one minute, or 55°C in healthcare premises. It also expects cold water to remain below 20°C where practicable.

Those are HSE control values for the systems and dutyholders covered by that guidance. They should not be converted into an invented universal heat-pump cycle such as “run once a week” without considering the premises, system design and risk assessment. Landlords and employers have explicit duties to assess and control the risk. A private owner still needs a safe system, but the legal framework and management responsibilities are not identical in every setting.

A successful cycle must heat the relevant water

The temperature shown on a controller represents its sensor location. Water in a cylinder stratifies, so the top, middle and base can be at different temperatures. A sensor near the top may reach its target while cooler water remains below it.

The cylinder, sensor pocket, heat source and control sequence must be designed together. A genuine thermal pasteurisation process needs to achieve its specified temperature and hold condition across the volume and circulation path that the risk-control design covers.

Check whether the controller records successful completion or merely that the programme started. A cycle that times out below target is a failed control measure, even if the water became warmer.

The heat pump and immersion have different roles

Normal domestic hot-water production is usually kept as low as the service and hygiene design permits because heat-pump efficiency falls as required water temperature rises. Some systems can complete the full high-temperature cycle with the compressor. Others use the heat pump for the first part and an immersion heater for the final lift.

Direct-electric energy used by the immersion should be visible where practical. It is useful for spotting repeated or failed cycles, but the monitoring must not encourage an owner to disable a hygiene function simply to improve an efficiency graph.

A pasteurisation cycle and an ordinary hot-water boost are different. A boost may provide more usable hot water without reaching or holding the conditions required by the bacterial-control method.

Scald protection is separate

Water hot enough for thermal control can scald. A thermostatic mixing valve can reduce the temperature delivered at an outlet or into the distribution pipework. It does not prove that the cylinder has been pasteurised, and it does not remove the need to control low-flow pipework.

Mixing valves need correct installation, accessible strainers and servicing in accordance with their instructions. If their cold supply fails or the valve sticks, outlet temperature can become unsafe.

Scheduling without weakening the control

Where the risk assessment and product allow a periodic cycle to be scheduled, it can be placed when hot-water demand is low and the required heat source is available. Tariffs, solar generation and battery state may influence timing, but not the required thermal outcome.

Account for:

• the full time needed to heat and hold the cylinder
• space-heating interruption under hot-water priority
• noise and electrical demand from an immersion
• hot-water draws that can interrupt the cycle
• the controller’s retry behaviour

Do not move the cycle into a short low-price window if that prevents completion.

After absence or a change in use

Prolonged vacancy can create stagnation. A holiday mode may protect the heating system from frost while suspending normal hot-water turnover, so it is not automatically a Legionella control.

The appropriate recommissioning process depends on the building and risk. In a managed property, follow its written control scheme and competent-person advice. Avoid improvising by running showers and creating aerosols before the system has been assessed and brought under control.

Changes such as adding a bedroom, shower or larger cylinder can alter water use and risk. Review the hot-water design rather than assuming the old programme remains suitable.

When a cycle fails

Repeated failure needs investigation. Possible causes include:

• immersion heater or electrical-supply fault
• heat pump reaching an operating limit
• sensor or sensor-pocket problem
• target or hold time set incorrectly
• hot-water draws during the cycle
• excessive heat loss or a stuck diverter valve
• controller schedule, clock or firmware error
• scale restricting heat transfer

Record the displayed temperatures, start and stop times, energy source and fault message. Do not repeatedly reset a safety cut-out or raise hidden installer limits.

If the premises has vulnerable occupants, a formal written control scheme, persistent temperature failure or suspected contamination, use competent water-hygiene advice. The control principle does not replace a site-specific risk assessment.

Related entries

• Domestic hot-water cylinders
• Backup immersion heater
• Hot water priority control
• Handover settings and user controls
• Seasonal heat pump maintenance