How do you balance scalding risk against legionella?

Legionella control and scald prevention appear to conflict, and they do. Controlling legionella needs water kept hot, hot enough to burn skin. The two duties are reconciled not by cooling the system but by keeping it hot and fitting a thermostatic mixing valve at the point of use, which blends the water down to a safe temperature only at the tap or shower.

That single idea, hot in the pipe and safe at the outlet, is the whole answer. Everything else on this page explains why it is the right answer and why the tempting shortcut, simply running the water cooler, is the one to avoid.

Why the two risks pull in opposite directions

Legionella bacteria multiply in the range of roughly 20°C to 45°C. The standard temperature regime is built to keep water out of that band: hot water stored at 60°C, distributed so that it reaches 50°C (55°C in healthcare) within one minute at the outlet, and cold water held below 20°C. Above about 50°C legionella survival falls sharply, and at 60°C the bacteria are killed quickly. Keeping the hot system genuinely hot is therefore a core control measure, not a comfort setting. The legionella temperature chart sets out these figures in full.

The problem is that water hot enough to control legionella is also hot enough to scald. A skin burn depends on both temperature and contact time, and once water climbs past the mid-forties the safe contact time shrinks fast. So the same 50°C to 60°C water that protects the system from bacteria can injure the person using it. The tension is real, and pretending it away by lowering the storage temperature simply moves the danger somewhere less visible.

At what point hot water starts to scald

Scald severity is a function of temperature and exposure time, so the honest way to describe it is with both. Water at around 44°C is about the upper limit for comfortable, safe bathing. Above that, the risk climbs quickly:

• At about 50°C, a serious full-thickness burn can develop in roughly five minutes.
• At about 55°C, the same injury can occur in around ten seconds.
• At 60°C, the water in the calorifier, a serious burn can occur in roughly one second.

Those times are for adult skin. Children have thinner skin and burn faster and deeper at the same temperature, which is why a bath run straight from an unblended hot tap is one of the most dangerous domestic sources of scalding. The figures also explain why the answer is never to make the water "a bit cooler" as a compromise: the storage temperature has to stay high for legionella control, so the reduction has to happen at the tap instead.

Who is most at risk of scalding

Scald risk is not spread evenly. The people most likely to suffer a serious injury are those least able to sense hot water or move away from it quickly:

• Young children, whose skin is thinner and who cannot always reach or understand a tap.
• Older people, who may have thinner skin, slower reactions, and reduced mobility.
• People with reduced sensation, for example through diabetes, neuropathy, or spinal injury, who may not feel that the water is too hot until damage is done.
• People with limited mobility or cognitive impairment, who cannot withdraw from hot water promptly.

This is why scald control is treated most strictly in care homes, hospitals, schools, and any setting serving vulnerable users. In those buildings the healthcare distribution temperature is higher (55°C at the outlet), which makes the case for point-of-use blending stronger still, not weaker.

The right resolution: keep it hot, blend it down

A thermostatic mixing valve mixes hot and cold water to deliver a controlled, safe temperature at the outlet, and it is designed to fail safe, shutting off the flow if the cold supply is lost so that it does not suddenly run scalding hot. Crucially, the TMV sits close to the tap or shower it serves. The pipework up to that valve stays hot, so legionella control is preserved right up to the point of use, and only the final short length delivers blended, safe-to-touch water.

Recommended maximum blended temperatures are typically around 44°C for a bath, 41°C for a shower or washbasin, and lower again for a bidet. The valve is set and checked against these figures, the aim being that the person at the tap is not exposed to 60°C water, while the system behind the valve is kept out of the growth range. Because a TMV introduces a small volume of warm, mixed water, it becomes an asset to inspect and maintain, and it is one of the components that makes routine shower head and outlet checks worthwhile.

The mistake to avoid: turning down the calorifier

Faced with a scald complaint, the tempting fix is to turn the calorifier thermostat down so nothing comes out too hot. This is the wrong move. Dropping storage below 60°C, and distribution below 50°C, pulls the entire hot system into the 20°C to 45°C band where legionella multiplies. You would be swapping a scald risk, which is localised, foreseeable, and controllable at each outlet, for a legionella risk that seeds the whole building and reaches every tap and shower at once.

Scald prevention and legionella control are both duties under the same framework, and the recognised way to satisfy both is temperature control plus point-of-use blending, not a lowered set point. If a specific outlet is too hot, the answer is a correctly set and maintained TMV at that outlet, not a colder tank.

Recording temperatures and TMV checks

Because the system runs hot and the safety of each outlet depends on its blending valve, both sides need to be recorded. In practice that means logging hot water storage and distribution temperatures to confirm the system is staying hot, recording blended temperatures at TMV-served outlets to confirm they are staying safe, and scheduling the TMV servicing and failsafe checks the manufacturer specifies. A legionella log book is where these readings and checks are kept over time, and the risk assessment is where vulnerable-user outlets and their controls are first identified.

None of this makes a system safe by itself; recording a reading does not change the water. What the records do is show which control was in place, what it measured, and when, so that scald risk and legionella risk are each being managed rather than one being hidden behind the other. The underlying expectations sit in HSG274 and the ACOP L8 approved code of practice, which treat temperature control as the primary defence and expect scald risk to be managed at the point of use.