Does hot water kill legionella?

Yes. Water that is hot enough kills legionella, and the hotter the water, the faster it dies: die-off is rapid at 60°C, slow at 50°C, and below about 50°C the bacteria survive, multiplying freely between 20°C and 45°C. That relationship is the entire basis of legionella temperature control in the UK.

It is also why the guidance uses two different hot water figures rather than one. Storage is set at 60°C because that is hot enough to kill the bacteria in the vessel over a short period. Delivery is set at 50°C at the outlet (55°C in healthcare) because water loses heat as it travels through the pipework, and 50°C at the tap is the point below which the system starts drifting back toward the growth range. Our legionella temperature chart sets out the full set of figures in one place.

The distinction that matters: control versus disinfection

The point most pages miss is that 50°C at the outlet is a control temperature, not a kill temperature. At 50°C, legionella cannot multiply and the population dies back only slowly. That is fine for a system that is clean and well managed: holding every outlet above 50°C keeps the population suppressed indefinitely. But if a system has already been colonised, with biofilm established on the pipe walls, running at 50°C will not clear it. The bacteria inside the biofilm are sheltered, and the slow die-off at the outlet never reaches them.

Clearing a colonised system is a different exercise. Thermal disinfection raises the stored water temperature and circulates the hotter water right through the distribution system for a set period, so every branch and outlet sees a genuinely lethal temperature, not just the water heater. It is normally paired with cleaning and descaling, because scale and biofilm insulate the bacteria from the heat. Our guide to legionella treatment methods covers when thermal disinfection is used and how it compares with chemical disinfection.

Where hot water systems fail to stay hot

A cylinder thermostat set to 60°C does not guarantee the whole system is at 60°C. The base of the calorifier or cylinder is the recognised cold spot, where incoming cold water settles; if the vessel is oversized or the thermostat is low, the bottom can sit inside the growth range while the top feels scalding. Long pipe runs to distant outlets lose heat, so the last tap on a run can deliver water well below 50°C even when the near taps are fine. Blended outlets downstream of a thermostatic mixing valve deliberately mix hot down to a safe touch temperature, which puts that mixed water straight back into the growth range by design.

Checking therefore means measuring, not assuming. Take the temperature at sentinel outlets, the nearest and furthest on each loop, and at the calorifier flow and return, and let the water run long enough to reach its true temperature: up to one minute on hot outlets. The legionella temperature checker walks through how to take and interpret a reading, and legionella temperature control explains what each figure means.

Hot enough brings its own duty

Water stored at 60°C scalds quickly, so raising temperatures to control legionella increases the scald risk at baths, showers and basins. The two duties are reconciled with thermostatic mixing valves, which blend the water to a safe temperature at the point of use while the rest of the system stays hot. That trade-off, and who is most at risk from scalding, is covered in scalding versus legionella.

Finally, a temperature reading is only useful as a record. A weekly or monthly check that lives in someone's memory is no evidence at all. Write down the outlet, the reading, the date and who took it, in a log book, and keep the records for at least five years. The written scheme of control in your risk assessment should name which outlets are checked and how often, so the temperature regime is a defined control rather than a habit.