Legionella Bacteria
The legionella species of bacteria prefer a temperature of 37°C. This is exactly the same temperature as humans and one of the reasons legionella can thrive inside our bodies.
Legionella bacteria grow fastest at 37°C. They will still grow when the water temperature is higher/lower than 37°C but the rate of growth slows down.
When temperatures are either ~15°C higher (or lower) than their favourite 37°C, they can no longer grow. In fact, they enter a dormant state and can remain in this condition for many months. If the water temperature returns somewhere in the range 20-45°C, the bacteria will become active once again.
What temperatures kill legionella?
- Legionella will die when the temperature drops below freezing point, but that is an unworkable solution.
- The only practical way to kill legionella is to raise the temperature to at least 60°C.
Looking at the chart below, it’s easy to see how legionella reacts to different temperatures.
In simple terms, Keep your hot water HOT and your cold water COLD
What are compliant water temperatures? 
The UK guidelines are based on the chart above.
- For hot water, store it at a temperature where legionella cannot survive and distribute at a temperature which is too high for it to grow.
- For cold water, maintain the supply cold enough so the bacteria are unable to grow.
Hot Water
(within 1 minute of running the outlet)
- Hot water should be stored at 60°C minimum
- If the hot water is recirculated, the return temperature as it enters back into the storage vessel must be 50°C minimum
- In hospital/healthcare premises the return temperature must be 55°C minimum
Cold Water
(within 2 minutes of running the outlet)
All cold water should never exceed 20°C.
Sentinel Outlets
The nearest and further outlets in the building are called “Sentinels”. Your risk assessment and Written Scheme should list the hot and cold water outlets which are deemed your Sentinel points. The temperatures at these outlets should be taken and recorded every month. The records have to be kept for at least 5 years.
If any of the temperatures don’t meet the requirements above (ie they are non-compliant), inform your manager so that they can investigate and rectify the problem.
Hot Water
If you have more than just a single hot water cylinder, and they serve different parts of the building; then there will be separate “Sentinels” for each unit.
Cold Water
If you have more than just a single cold water storage tank, and they serve different parts of the building; then there will be separate “Sentinels” for each tank.
In effect, checking the temperature at the Sentinel locations gives a quick “snapshot” of the whole building. If the water temperature is OK at the first outlet and still OK at the last outlet – there’s a good chance it’s OK everywhere.
Remember, you still need to check the temperature at every outlet, at least once each year.
What happens if a hot water sentinel outlet has a TMV fitted?
It is common for a Sentinel outlet to have a Thermostatic Mixer Valve installed. This will obviously alter the temperature at the outlet.
In these situations the temperature should be taken from the hot water supply pipe before the valve; this is best done using a surface probe thermometer.
Are Thermostatic Mixer Valves (TMVs) a legionella problem?
Taps near to the stored water vessel could easily have a temperature of 60°C. At this scalding temperature, it only takes 5 seconds to cause a 3rd degree burn.
This is a hazard to the young and elderly so TMVs are fitted close to the outlet to provide a safe temperature of 40°C.
If the property has a problem with their hot water so that the max temperatures were only 30°C for a couple of days, then small numbers of legionella would be able to get into the distribution system and colonise the pipework all the way to the outlets.
When the water temperatures return to normal, the legionella would be killed in the storage vessel and pipework. Unfortunately, legionella in the TMV and the pipework & outlet downstream won’t get killed because the temperature doesn’t rise above 40°C.
What’s more, since these temperatures are ideal for growth; the legionella bacteria can proliferate to levels that pose a threat to humans.
This is a hidden danger since all the Sentinel water temperatures can be perfect. This demonstrates the importance of regular legionella testing.
Stay Safe with Regular Legionella Testing
Because TMVs can mask hidden risks in your water system, compliant sentinel temperatures alone aren’t enough to guarantee safety. The only way to truly know if Legionella is present—and prevent potential outbreaks—is through routine, professional water testing.
At Aquacert, we provide UKAS-accredited legionella testing, risk assessments, and expert guidance tailored to your property type, whether residential, commercial, or healthcare.
Call us today on 0800 048 1737 Or contact our team online for a fast, no-obligation consultation.
Don’t wait for a problem to arise, protect your building, your tenants, and your peace of mind with proactive water safety management.