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 legionella temperature range, 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 legionella water temperature 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.
The legionella cold water temperature, and the one people forget
The control temperatures are three numbers and the cold one is the one that gets missed. Cold water should be below twenty degrees, hot water should be stored at sixty and reach the taps at fifty or above, and in healthcare the distribution figure is higher. What people monitor is the hot side, because a cylinder has a thermostat and a hot tap is easy to check. The cold side has no thermostat and drifts: a storage tank in a summer loft, a cold main run alongside an uninsulated hot pipe or through a warm plant room, a shower mixer that keeps the cold dead ended at room temperature, a rarely used outlet in a hot corridor. Any of those sits the cold water in the twenty to forty five degree band where the organism multiplies fastest. So the monitoring regime checks the cold at the tap and at the tank, records it, and treats a warm cold tap as a finding rather than a quirk of the building.
What is a TMV valve, and where one is required
A thermostatic mixing valve blends stored hot water with cold at the point of use so that the water reaching the tap or the shower is at a safe temperature while the system behind it stays hot enough to control legionella. That is the whole reason it exists: the two requirements pull in opposite directions, since the organism is controlled by storing at sixty degrees and distributing at fifty or above, and water at those temperatures scalds. A valve is expected wherever the people using the outlet cannot be relied on to react to hot water, which in practice means care settings, children's facilities, hospitals and, under the building regulations for new work, baths in dwellings. Two maintenance points follow. The valve is a control on a safety critical system, so it is checked and serviced at the interval the assessment sets, typically annually, including a failsafe check that the hot supply shuts off if the cold fails. And a valve fitted too far from the outlet leaves a warm dead leg between the two, which is the exact condition the temperature regime exists to prevent.
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.
Is there a legionella temperature chart to work to?
There is a short set of control parameters rather than a chart, and they are the same few numbers every scheme is built on: water stored in the calorifier hot enough to kill the organism, hot water reaching the sentinel outlets hot enough to control it within a defined running time, and cold water held below the range in which it multiplies. HSE publishes those parameters; a chart pinned to a plant room wall is a copy of them. What the chart cannot carry is the part that fails most often, which is stagnation, because water at the right temperature that never moves is still a problem.
How to test for legionella in water, and who should do it
Sampling is a specific procedure rather than filling a bottle: samples are taken at defined points, usually a pre flush and a post flush from the same outlet, into sterile containers with a neutraliser for any biocide, kept cool and delivered to an accredited laboratory inside its stated holding time, and the result is a count of legionella organisms per litre against the action levels in the guidance. The sampling itself is what invalidates most results, so it belongs to somebody trained in it. And it is a check on the control scheme rather than a substitute: a clear result from a system running at the wrong temperatures means nothing about next month.