Hot and Cold Water Temperatures for Legionella Control

Temperature is the main control for legionella in most buildings. Keep cold water cold and hot water hot, and the bacteria cannot multiply. This guide explains the numbers and how to check them.

The temperatures that matter

• Below 20°C: legionella does not multiply.
• 20°C to 45°C: the range where it grows, fastest at around 35°C to 40°C.
• Above 50°C: it cannot multiply.
• Above 60°C: it is killed within minutes.

These figures come from the Health and Safety Authority and the national guidelines published by the Health Protection Surveillance Centre.

What that means for your system

Most control schemes aim to store hot water at 60°C or above, deliver hot water to outlets at 50°C or above, and keep cold water below 20°C. Your legionella risk assessment sets the exact targets and check points for your building.

How to check

1. Pick the outlets nearest to and furthest from the water heater and the cold water tank.
2. Run the hot tap and measure the temperature after about a minute.
3. Run the cold tap and measure after about two minutes.
4. Check the stored hot water temperature at the heater.
5. Record every reading with the date, the outlet and your name.

Common reasons temperatures fail

• The water heater thermostat is set too low to save energy.
• Cold water pipes run beside hot pipes or through warm ceiling voids.
• Storage tanks sit in hot roof spaces without insulation.
• Outlets are rarely used, so water stands and drifts to room temperature.

Hot water and scalding

Water hot enough to control legionella can scald. Where children, older people or vulnerable adults use the outlets, thermostatic mixing valves bring the temperature down at the point of use. The valves need their own servicing.

When temperature control is not enough

If a system cannot hold these temperatures, your risk assessment should set out other controls, and sampling becomes more important.

Safer Solutions provides legionella risk assessments and temperature monitoring across Ireland. Call 086 275 9728, email info@safersolutions.ie or request a quote.