How the Nest thermostat helps prevent Legionella bacteria

Legionnaires’ disease is a potentially fatal type of pneumonia that’s contracted by inhaling water droplets that contain 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 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 2 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 Bacteria Prevention mode is running by showing a water droplet inside of a circle  on the temperature screen.

Note: The Bacteria Prevention icon  (water drop with circle) is different than the hot water boost icon  (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.

Turn Bacteria Prevention on or off

Most hot water systems are compatible with the Nest thermostat’s Bacteria Prevention mode, so it’s recommended that you keep 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 you only use during the summer.

1.Press your thermostat ring to bring up the Quick View menu 2.Select Settings . 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.