Temperature is the primary control for legionella in a hot and cold water system, and it works because the bacteria multiply in a middle band and stop at either end. Keep hot water hot enough to kill it, keep cold water cold enough to stop it growing, and keep everything moving so nothing sits in the middle. Everything else in a written scheme of control exists to support those three sentences.
The ranges, and what HSE names
Legionella multiplies in the warm middle of a water system and is killed at high temperature. HSE names the control parameters directly: setting the temperature of the hot water cylinder, the calorifier, so that water is stored at 60°C, and describes a low risk case where hot water is fed from instantaneous heaters or low volume water heaters supplying outlets at 50°C. Cold water is kept cold, which in practice means below the growth range and away from heat sources.
What the monthly checks measure
Hot water flow and return at the calorifier. Hot water at sentinel outlets, meaning the nearest and the furthest from the source, checked for temperature after running for a defined period. Cold water at sentinel outlets and at the tank, checked for the temperature it reaches after running. A rotating sample of other outlets so every one is covered over a year. And the readings are recorded, because an unrecorded check is indistinguishable from one nobody made.
Stagnation, which beats temperature every time
A system held at the right temperatures still fails if water does not move. HSE's general principle for properties left vacant is that outlets on hot and cold water systems should be used at least once a week to maintain a degree of water flow and minimise the chances of stagnation. That is why flushing regimes exist in student accommodation over the summer, in partly let offices and in hotel rooms nobody books.
Scalding, and why the controls fight each other
Storing water at 60°C is hot enough to scald, which is why thermostatic mixing valves are fitted where vulnerable people use the outlet. The valve solves the scalding problem and creates a legionella one, because the mixed water downstream of it sits in the growth range. That is the reason TMVs are on the servicing schedule rather than fitted and forgotten.
Showers, and the practical question
A shower head atomises water, which is exactly the exposure route that matters. Where a shower has not been used for a while the recognised precaution is to run it before use in a way that does not create spray, then to clean and descale the head and hose on the schedule the written scheme sets. Running a shower for a few minutes is a mitigation, not a control, and it does not replace the temperature regime.
Questions people ask about legionella temperature
What temperature kills legionella?
High temperature kills it and the middle of the range lets it multiply. HSE names storage at 60°C in the hot water cylinder as a control parameter, with outlets supplied at 50°C in the low volume heater case it describes.
What is the legionella growth temperature range?
The bacteria multiply in the warm middle of a water system and are controlled by keeping hot water hot and cold water cold. The written scheme sets the values monitored for your system.
How long should you run a shower that has not been used?
Long enough to displace the standing water, and in a way that avoids creating spray. It is a mitigation for a specific outlet, not a substitute for the temperature and flushing regime.
How often are water temperatures checked?
Monthly at sentinel outlets in most schemes, with other outlets rotated so all are covered over a year, and the readings recorded. The exact frequency comes from the written scheme of control.