Does boiling water kill legionella?

Yes, boiling water kills legionella. At 100°C the bacteria die quickly, because boiling is far above the temperatures they can tolerate: die-off is already rapid at 60°C, and every degree above that shortens the time further. For a kettle, a pan, or any small volume of water you can bring to the boil, boiling is a perfectly effective treatment.

What boiling cannot do is protect a building. A water system is not a volume of water you can lift onto a stove; it is tanks, cylinders, pumps and metres of pipework, much of it holding water at room temperature or in the warm 20°C to 45°C band where legionella multiplies. You cannot boil a plumbing system, and even if you could heat all of it, the water would pass back through the growth range as it cooled, ready to be seeded again by any biofilm left on the pipe walls.

The recolonisation problem

This is the point that catches people out. Boiling is a moment in time, not a state. Water that has been boiled is free of live legionella at that moment, but as soon as it cools below about 50°C it becomes habitable again. Pour that water into a colonised container, or let it stand in pipework with an established biofilm, and it can be recolonised. The same logic applies in reverse to the building: a kettle fed from a contaminated tank produces safe boiled water every time, while the tank itself stays exactly as contaminated as it was. Treating the water in front of you never fixes the system behind it.

The system-level equivalent: thermal disinfection

When a building's system does need heat treatment, the method is thermal disinfection, not boiling. The stored water temperature is raised and the hot water is circulated through the whole distribution system, so every branch and outlet sees a temperature well above 60°C for a set period. It is normally combined with cleaning and descaling, because scale and biofilm shelter bacteria from heat, and it is carried out under a defined procedure with the scald risk managed while the system runs hot. Legionella treatment methods compares thermal and chemical disinfection and when each is used.

Where boiling genuinely fits

Boiling has a legitimate role for small, self-contained volumes: kettles, pans, and the water reservoirs of small appliances that you empty, boil-fill or clean regularly. What it should never become is a substitute for system control. If the routine in a building has drifted to "we just boil the water", that is a sign the underlying controls, temperature management, flushing, cleaning, have failed or were never set up.

The routine that actually controls legionella is unglamorous: hot water stored at 60°C and reaching at least 50°C at outlets within a minute, cold water below 20°C, little-used outlets flushed weekly, and every check written into a log book and kept for at least five years. The legionella temperature checker helps you take the readings, and the written scheme in your risk assessment is where the schedule belongs. Our overview of legionella temperature control explains why those numbers, and not the kettle, do the work.