Water Temperature Control

Temperature is the primary control for Legionella in most water systems, because the bacterium is so sensitive to it. The principle is simple to state and harder to deliver across a real building: keep hot water hot enough to suppress growth, keep cold water cold enough to do the same, and keep both moving so the temperatures hold all the way to the outlet.

The figures behind the principle are well established in HSE guidance. Legionella thrives in roughly the 20-45C range. Cold water is therefore generally kept below 20C, and hot water systems are typically run so that water is stored at around 60C and distributed so it reaches at least 50C (commonly 55C in healthcare premises) at the tap within about a minute of running. At higher temperatures the bacteria are killed progressively faster. These are widely cited control values rather than universal legal limits. Your risk assessment, and standards such as HSG274 Part 2 and HTM 04-01 in healthcare, set what applies to your system, and the duty is to control risk, not simply to hit a number.

The catch is that hot water hot enough to control Legionella is also hot enough to scald. This is the central tension of temperature control, and it is usually resolved with thermostatic mixing valves (TMVs) fitted close to vulnerable outlets, which blend the water down to a safe delivery temperature at the point of use while the stored and distributed water stays hot. The valves themselves then become assets that need servicing, because a failed or poorly maintained TMV can create the very tepid conditions you are trying to avoid.

Delivering these temperatures consistently is where most schemes are won or lost. Long pipe runs, oversized or poorly insulated storage, dead legs, blended water sitting in pipework, and calorifiers that stratify or accumulate sediment all undermine control. So monitoring is part of the job: representative temperatures are checked and recorded on a defined schedule, sentinel outlets (the nearest and furthest from each system) confirm the extremes, and an out-of-range reading triggers investigation rather than a tick in a box.

This section covers how to set, deliver and prove temperature control across hot and cold systems: storage and distribution targets, TMVs and the scalding trade-off, calorifier management, insulation and dead legs, and what to do when readings drift. As always, treat the numbers as guidance to be confirmed through a competent, site-specific risk assessment and current HSE and NHS publications, not as fixed rules that fit every building.