Legionella temperature chart

Updated 16 August 2026

This chart is the single most important reference for anyone managing a water system. Legionella growth is almost entirely controlled by temperature in most domestic hot and cold water systems. Get the numbers right and the risk is low. Get them wrong and no amount of other control will compensate.

Temperature ranges

Monitoring targets from HSG274

Monitoring frequency

For a typical domestic hot and cold water system, HSG274 Part 2 recommends:

• Monthly: hot water flow and return at the calorifier
• Monthly: hot and cold temperatures at sentinel outlets (nearest and furthest from the calorifier)
• Annually: temperature at every outlet on a rotating schedule
• Six-monthly: temperature in every cold water storage tank
• Weekly: flushing of any outlet used less than once a week

Why 20 and 50 are not arbitrary

The two numbers that matter most are 20 and 50. Below 20°C, legionella growth is so slow that it is effectively dormant over the timescales relevant to a water system. Above 50°C, legionella dies at a rate fast enough that any organisms reaching the outlet will be killed. Between those numbers is the growth range.

The 20 and 50 targets are not the fastest growth or the fastest kill temperatures, which are around 37°C and 70°C respectively. They are the conservative boundaries where you can be confident that the system is either safely cold or safely hot.

When you cannot maintain the targets

Some buildings cannot consistently hit 50°C at every outlet or keep cold water below 20°C in summer. In these cases, temperature control stops being the primary measure and a chemical control (chlorine dioxide dosing, silver-copper ionisation) is used instead. The risk assessment will specify the alternative control and the sampling regime to verify it is working.

See our treatment guide for the methods and when each applies.

How to check water temperatures for legionella

Checking water temperatures is a monthly job for most systems, done with a surface-contact or immersion thermometer. On the hot side, measure the flow and return at the calorifier and confirm the sentinel outlets reach 50°C within one minute of running the tap. On the cold side, confirm the sentinel outlets stay below 20°C after running for up to two minutes. Record every reading, in range or not, in the log book, because an unrecorded check cannot be evidenced later. See temperature control for the full monitoring pattern and how long to run the taps when you take a reading.

Common questions about legionella temperatures

What temperature should hot water be at the tap?

Hot water should reach 50°C at the outlet within one minute of opening the tap, or 55°C in healthcare premises. It is stored hotter, at 60°C, and blended down at a thermostatic mixing valve where scalding is a risk. So the hot water tap temperature you feel is deliberately lower than the storage temperature, but the water arriving at the outlet should still test at 50°C or above.

What temperature should cold water be?

Cold water should stay below 20°C, both in storage and at the tap, reached within two minutes of running the outlet. Cold tap water in the UK is often well below this, but it can drift up in summer, in warm plant rooms, or where cold pipes run alongside hot ones, which is why the 20°C cold water target is checked rather than assumed.

What temperature kills legionella?

Legionella starts to die above 50°C, is killed within a few minutes at 60°C, and dies almost instantly at 70°C. Boiling water (100°C) destroys it immediately. This is why hot water is stored at 60°C and thermal disinfection is carried out at 60°C or above at every outlet.

What is the legionella temperature range?

Legionella multiplies between roughly 20°C and 45°C, fastest at around 37°C. Below 20°C it is dormant and above 50°C it dies, so control keeps water out of the 20°C to 45°C danger zone.