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How the Nest thermostat helps prevent Legionella bacteria

Legionnaires’ disease is a potentially fatal type of pneumonia that is contracted by inhaling water droplets containing Legionella bacteria. The 3rd-gen Google Nest Thermostat’s Bacteria Prevention feature works with both On/Off hot water systems and OpenTherm hot water system boilers. To help prevent bacterial growth, it will heat your hot water tank even if your home is in Away mode.

Legionella bacteria can develop in water that has been exposed to impurities like rust, algae and limescale, in temperatures between 20°C and 45°C. The bacteria are dormant below 20°C and do not survive above 60°C (Source: HSE UK).

Bacteria Prevention is an exclusive feature of 3rd-gen Nest Learning Thermostats. Bacteria Prevention mode will activate if your domestic water hasn’t been heated for at least two consecutive hours in the last 48 hours. This can happen if there is a gap of more than 48 hours in your hot water heating schedule or if you are away from home for longer than 48 hours. For instance, if you have a second home or holiday home and only need hot water for weekends, you can create a hot water schedule for Saturday and Sunday, and the Bacteria Prevention mode will automatically heat the hot water tank during the week.

Your Nest thermostat will let you know that Bacteria Prevention mode is running by showing a water droplet inside a circle Thermostat bacteria prevention icon on the temperature screen.

Note: The Bacteria Prevention icon Thermostat bacteria prevention icon (water drop with circle) is different from the hot water boost icon Hot water icon (droplet) (no circle). To learn about these or any other icons that appear on your thermostat screen, visit our article about what you'll see on your Nest thermostat.

Bacteria Prevention

Nest thermostat temperature screen prevention

Hot water boost

Nest thermostat temperature screen boost

Bacteria Prevention with combi boilers

Combi boilers don’t store hot water in a tank. They produce domestic hot water on demand, which keeps bacteria from growing. Even modern combi boilers that have a small buffer of domestic hot water for comfort purposes do not require the Nest thermostat’s Bacteria Prevention feature, because they regularly heat the small amount of water stored in the buffer.

Turn Bacteria Prevention on or off

Most hot water systems are compatible with the Nest thermostat’s Bacteria Prevention mode, so we recommend keeping it on. But you can turn it off if you’ll be away for an extended time, such as if you have a holiday home that you only use during the summer.

1.Press your thermostat ring to bring up the Quick
View menu
2.Select Settings Nest settings icon.
3. Select Equipment and Continue.
4. Choose Bacteria Prevention.
5. Turn Bacteria Prevention on or off.

Note: You can only switch Bacteria Prevention on or
off from the thermostat. This option isn’t available in the app.

Nest thermostat Bacteria Prevention setting

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