The temperatures that matter: 20, 45, 50, 55 and 60 explained
Temperature control is the backbone of most written schemes. A short guide to what each number means, where it is measured and why the monthly readings are worth doing properly.
Most hot and cold water systems in the UK and Ireland rely on temperature, rather than chemicals, to control Legionella. The principle is simple: keep cold water cold and hot water hot, so the bacteria never get the warm, still water they need to multiply. The detail is in the numbers.
20°C - the cold water ceiling
Cold water should be stored and distributed below 20°C. In practice this is measured at the sentinel outlets after running the tap for up to two minutes. Cold water creeping above 20°C is common in summer, in plant rooms and on risers that share a duct with heating pipework, and it is one of the most frequent reasons we recommend insulation or re-routing.
45°C - the growth zone
Between roughly 20°C and 45°C Legionella can multiply, and it does so fastest around body temperature. This is the band we are trying to keep water out of. Every remedial action in a written scheme comes back to this range.
50°C - the hot water outlet minimum
Hot water should reach 50°C at the outlet within one minute of running. In healthcare premises the figure is 55°C, because the guidance recognises a more vulnerable population. Where scalding is a risk, thermostatic mixing valves are fitted close to the outlet, and the 50°C reading is taken upstream of the valve.
55°C - the return
On systems with a hot water circulation loop, the return to the calorifier should be at least 50°C, and many schemes set 55°C as their target to give a margin. A cool return is the classic sign of a loop that has been extended over the years without rebalancing.
60°C - the calorifier
Hot water should be stored at 60°C or above. Readings are taken at the flow from the calorifier. A calorifier that is stored below 60°C, or that stratifies with a cool layer at the base, needs attention.
Why the monthly readings matter
Monthly sentinel temperatures and annual readings at a representative sample of other outlets are the evidence that your scheme is working. Done badly, they are a box-ticking exercise. Done well, they give you an early warning: a cold outlet drifting from 17°C to 19°C to 21°C over a summer is a story, and a good responsible person reads it before the sampling results do.
If your team would like a short, practical session on what the readings mean and how to take them consistently, this is exactly what our Legionella awareness training covers.