Temperature control
Legionella temperatures: what the water should be
Legionella bacteria grow in water between 20°C and 45°C. HSE's control strategy is simple: store hot water at 60°C or above, deliver it at 50°C or above, and keep cold water below 20°C.
Last reviewed October 2026 against HSE guidance.
Get the free landlord legionella risk assessment template
Includes a temperature log sheet for hot and cold outlets. The download link is on the next page.
We do not sell your details. If you ask for a quote we pass your enquiry to a UK legionella assessor. Privacy.
The temperatures that matter
| Where | Target | How to check |
|---|---|---|
| Hot water cylinder (calorifier) | Stored at 60°C or above | Cylinder thermostat setting and a reading at the flow pipe |
| Hot water at outlets | At least 50°C within 1 minute of running | Thermometer under the running tap |
| Hot water at outlets in healthcare premises | At least 55°C within 1 minute | As above |
| Cold water at outlets | Below 20°C after running for 2 minutes | Thermometer under the running tap |
| Cold water storage tank | Below 20°C | Reading at the ball valve outlet |
Figures are HSE's control temperatures for hot and cold water systems.
HSE lists water between 20°C and 45°C as one of the conditions that increases the risk, because that range is suitable for growth. The aim is to keep water out of that range wherever it is stored or sits in pipework.
What temperature kills legionella?
HSE says hot water should be stored at 60°C at least, to kill legionella bacteria. Below 20°C the water is outside the range HSE gives as suitable for growth, which is why cold water is kept cold.
This is why a hot water cylinder turned down to save energy can become a risk. Water stored at 40°C or 45°C sits in the growth range. HSE's advice to landlords includes setting the cylinder so water is stored at 60°C, and telling tenants not to adjust it.
Combi boilers and electric showers
Combi boilers and instantaneous electric showers heat water as it is used, so there is no stored hot water. HSE says the risk is further lowered where they are installed. Electric showers are generally fed with cold water and heat only small volumes.
You still need to assess the property. Cold water can warm up in pipes that run through heated spaces, and outlets that are rarely used can hold stagnant water whatever the boiler type.
Balancing legionella control and scalding
Hot enough to control legionella is also hot enough to scald. HSE says there is a risk of scalding where water comes out of taps above 44°C, and that vulnerable people are most at risk.
For most people, HSE says the scalding risk is minimal where water is delivered at up to 50°C at hand-wash basins, and a warning notice may be enough. Where vulnerable people use baths or showers and the scalding risk is significant, a thermostatic mixing valve (TMV) is fitted to limit the outlet temperature to 44°C. The water feeding the valve should still reach at least 50°C.
How to check temperatures
- Use a digital probe thermometer. They cost a few pounds.
- Pick the outlets nearest to and furthest from the cylinder or tank. HSE calls these sentinel outlets.
- Run the hot tap for one minute and take the reading. It should be at least 50°C.
- Run the cold tap for two minutes and take the reading. It should be below 20°C.
- Write the readings down with the date.
In workplaces HSE's guidance is to check sentinel outlets monthly and to check stored cold water temperature every six months. For a simple rental, checks at the start of a tenancy and at routine visits are a proportionate approach. See legionella checks and flushing.
Questions people ask
What temperature kills legionella?
HSE says hot water should be stored at 60°C at least to kill legionella bacteria, and should reach at least 50°C at outlets within one minute.
What temperature does legionella grow at?
HSE identifies water between 20°C and 45°C as suitable for growth. Control measures aim to keep stored and standing water outside that range.
What should the cold water temperature be?
Below 20°C. HSE's guidance is that cold outlets should be below 20°C after running for two minutes.
Is 50 degrees hot enough to prevent legionella?
50°C is the minimum HSE gives for hot water reaching the outlets. The water should be stored hotter, at 60°C or above, so that it is still at least 50°C when it arrives.
Can legionella grow in a combi boiler system?
The risk is lower because no hot water is stored. Legionella can still grow in parts of any system where water stands still or warms up, such as unused outlets and dead legs.
Sources
- HSE: Hot and cold water systems
- HSE: Hot and cold water systems, things to consider
- HSE: What Legionnaires' disease is and typical symptoms
- HSE: Legionella and landlords' responsibilities
Checked against these sources in October 2026. This page is general information, not legal advice.
Start with the free template
An editable Word template for a simple rental property, based on the points HSE asks landlords to consider.
Get the free template →