How do hot surfaces fit into legionella control?

Legionella control requires hot water and heating systems to run hot, but hot outlets, pipes and radiators can burn. The recognised answer is to keep the system hot for bacterial control and make the point of contact safe: thermostatic mixing valves at outlets, and insulation or guards on hot surfaces, rather than turning the temperature down.

Two different risks, one source of heat

When people talk about hot water safety they usually mean scalding: skin injury from contact with hot water at a tap, shower or bath. There is a second, quieter risk in the same buildings: contact burns from hot surfaces. Radiators, exposed heating pipework and hot supply pipes can all reach temperatures that injure skin on touch, and HSE guidance for health and social care records serious injuries and fatalities caused by exactly this kind of contact.

Both risks matter most for the same people: young children, older people, and anyone with reduced sensation, limited mobility or slower reactions, who may not feel the heat or cannot move away from it. That is why the detailed rules live in care and healthcare guidance, and why any building serving vulnerable users needs to think about water temperature and surface temperature together.

Why the system has to stay hot

Legionella multiplies in water between roughly 20°C and 45°C, so the standard regime keeps water outside that band: hot water stored at 60°C, distributed to reach 50°C at the outlet within a minute, or 55°C in healthcare premises, and cold water kept below 20°C. The full set of figures is on our legionella temperature chart, and temperature control explains the regime. These are not comfort settings. They are the primary control measure in HSG274 and ACOP L8, and water hot enough to meet them is also hot enough to scald or burn.

HSE HSG274 Part 2 and ACOP L8; HSE scalding and burning guidance for health and social care; HSE HSG220 for care homes.

What the guidance says about hot surfaces

The HSE's scalding and burning guidance for health and social care is explicit on surfaces: where there is a risk of a vulnerable person sustaining a burn from a hot surface, the surface should not exceed 43°C when the system is running at its maximum design output. The suggested precautions are insulation or suitable covers, and low-surface-temperature radiators exist for exactly this purpose. Notice what the guidance does not say: it does not tell you to run the heating cooler. The surface is made safe at the point of contact, not at the boiler.

The same logic runs through HSG220, the HSE's health and safety in care homes guidance, which expects hot water to be distributed at 50°C or higher to control legionella, with thermostatic mixing valves fitted where a scald risk is identified. Heat is kept in the system; safety is engineered at the outlet and at the surface.

The right fix: protect the point of contact

For water, the point of contact is protected by a thermostatic mixing valve fitted close to the outlet. The TMV blends hot and cold to a set maximum, typically 44°C for a bath and 41°C for a shower or basin in healthcare, and it fails safe if the cold supply is lost. The pipework behind the valve stays hot, so legionella control is preserved right up to the point of use. Our page on balancing scalding and legionella covers this in depth.

For surfaces, the equivalent measures are physical: insulation around exposed hot pipework, covers or guards on radiators and valves, or low-surface-temperature emitters designed so their outer surface stays below the 43°C threshold. Again, the system behind the protection runs at full temperature.

Neither protection is fit-and-forget. A TMV creates a short length of blended warm water that can itself sit in the growth range, so valves need regular servicing and the known TMV3 failure modes need checking. Insulation and guards need to survive cleaning, maintenance and daily knocks. Both belong on the inspection schedule.

The wrong fix: turning everything down

Faced with a scald complaint or a burn from a radiator, the tempting response is to turn the plant down so nothing gets dangerously hot. That solves the visible risk by creating an invisible one. Dropping storage below 60°C and distribution below 50°C pulls the hot system into the 20°C to 45°C band where legionella multiplies, and the bacteria reach every tap and shower in the building at once. A burn risk is local, foreseeable and controllable at each point of contact; a colonised water system is neither.

The two duties are not in competition once they are understood. Both are satisfied by the same design principle: heat in the system, safety at the point of contact. If one outlet or one surface is too hot, the fix belongs at that outlet or that surface, not at the calorifier.

Recording both sides of the duty

Because the system runs hot by design, the evidence has to show that each protection is in place and working. In practice that means logging hot water storage and distribution temperatures, recording blended outlet temperatures at TMV-served fittings, scheduling TMV servicing and failsafe checks, and noting where surface protections such as insulation and guards are fitted and inspected. The risk assessment is where vulnerable users and their outlets are first identified, and the log book is where the ongoing readings live. In care settings, the scald and burn side of that record-keeping sits alongside the legionella side, and an inspector can ask for either. Our page on legionella in care homes covers how the duties combine in that setting.