Legionnaires' disease and hot water temperature: what a home owner should know
Legionella thrives in lukewarm water. A hot water system that stays properly hot is part of the answer, but you do not adjust it yourself.
Legionella bacteria multiply in warm water, roughly between 20 and 60 degrees Celsius. Water kept clearly hotter than that. Or clearly cold, is a much poorer place for them to grow. That is the whole reason hot water temperature comes up in Legionnaires' disease guidance.
For a typical house the risk is low. It is still worth knowing how the pieces fit together.
How do people catch Legionnaires' disease?
By breathing in tiny droplets of contaminated water, such as spray from a shower. Health authorities say it does not spread from person to person, and you do not catch it by drinking the water.
The bacteria are normally present in the water supply in very low numbers. Trouble starts when they find a warm, still place to multiply.
Why does temperature matter so much?
Storage that sits in the warm band is the problem. A tank held hot enough keeps the bacteria in check. A tank that is switched off for months, or set low, can drift into the range where they grow.
Victoria's Department of Health describes the risk as rising in systems that store water between 20 and 60 degrees. That is guidance for the premises it regulates, such as aged care and health services. Private homes are not on its list, so treat it as background, not as a rule for your house.
Is the water at my tap meant to be that hot?
No. Storage tanks run hot, and a tempering valve mixes in cold water before it reaches the bathroom. That keeps the tap water at a safer scalding temperature.
It also means the pipes after the valve carry water in the warm band. Our guide to the tempering valve explains why that design is a deliberate trade-off.
When is a home more likely to need attention?
- A system that has been off or unused for a long stretch, such as a holiday house.
- A tank that never seems to get properly hot, which is also a fault in its own right.
- Someone in the household with a weakened immune system or a chronic lung condition.
If any of those fit, ask a licensed plumber to check the system. Do not wait for symptoms.
Should you turn the temperature down to save energy?
No, not on a storage system, unless a licensed plumber confirms it is safe for your model. Lowering the stored temperature to save energy is exactly how a tank ends up in the warm band. The saving is small compared with the risk.
Better savings come from insulation and sensible use, which we cover elsewhere. See insulating a hot water system.
What about heat pump and solar systems?
The same principle applies. Heat pump and solar storage tanks should be able to reach a properly hot temperature, and many have a booster for the days when they cannot. If your tank rarely gets hot, that is worth raising with the installer.
Continuous flow gas units hold almost no stored water, so there is no tank for bacteria to settle in. The pipes between the unit and the tap still matter, but the heater itself is not the place to worry about.
What should you ask a plumber?
- What temperature is the stored water kept at, and does it match the manual for this model?
- Is the tempering valve working and was it tested at the last service?
- Is there any dead pipe, a spur that no tap uses, where water sits and cools?
Dead legs are a quiet cause of trouble. Water that never moves cools into the warm band, and a plumber can remove or shorten one.
This far you can go yourself
You can use the hot water regularly, tell your plumber about any long period of disuse, and notice if the water never gets properly hot. You can also ask about the stored temperature when the system is serviced.
Adjusting temperature settings, replacing a thermostat or element, and any work on a gas control is licensed work in every Australian state and territory. Your state or territory health department publishes Legionella guidance, and your plumbing regulator lists licensed tradespeople.
Frequently asked questions
Can you get Legionnaires' disease from a home hot water system?
It is uncommon, but possible if bacteria multiply in warm stored water and you breathe in the spray, for example in a shower. A system that stays properly hot lowers the chance.
What temperature does Legionella grow at?
Roughly between 20 and 60 degrees Celsius. Clearly hot or clearly cold water is a poorer place for it to grow.
Can I catch Legionnaires' disease by drinking tap water?
No. Health authorities say it is caught by inhaling small droplets of contaminated water, not by drinking it and not from another person.
Can I adjust my hot water thermostat myself?
No. On an electric system the thermostat is inside the electrical enclosure, and gas controls are gasfitting work. Ask a licensed tradesperson.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- Department of Health, Victoria. Legionella and water delivery systems
- Australian Government, energy.gov.au. Hot water systems: types, installation and energy performance