Legionella and Water Temperature: What You Need To Know
Among heating engineers, this is really going to ruffle some feathers. It's entirely possible to keep hot water permanently at 40°C, literally save 50% of your hot water bill and not worry about Legionnaires' disease.
But this is definitely not advisable for everyone! So stick around while we dive deeper into this subject, and find out what works for you — or how you or your customer could save energy.
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Where Does the 60°C Advice Come From?
The argument we often hear from engineers is that we need to keep hot water at 60°C to kill Legionella. But where does that advice come from? How risky is it — and what is Legionella?
Legionella is a bacteria that grows in water and, if consumed or inhaled, can cause a pneumonia-like illness known as Legionnaires' Disease.
On average, there are around 460 cases of Legionnaires' disease a year in the UK. Of those cases:
- 2% are from hospitals (often involving vulnerable patients)
- 51% are from people travelling
- 47% are from community settings (offices, restaurants, gyms, homes)
The most common transmission method is inhalation of contaminated aerosols — mist or spray from showers, spa pools, humidifiers, or cooling systems.
Common Sources of Legionella Contamination
- Ultrasonic humidifiers and foggers
- Spray and misting devices for food or plants
- Air washers and water scrubbers
- Emergency showers, eye wash stations
- Sprinkler and hose reel systems
- Spa pools and whirlpool baths
- Decorative fountains and water features
- Vehicle and industrial washers
- Non-disposable nebulisers
- Irrigation or fire suppression systems
How Risky Are Domestic Properties?
We could not find a single confirmed case of Legionnaires' disease contracted from a domestic property in the UK. However, bacteria are found frequently — for example, a 2017 Public Health England study detected Legionella in nearly a third of 99 shower heads tested.
Most domestic cases don’t develop into illness because the conditions for bacterial growth are rarely sustained.
How Do We Prevent Legionella Growth?
Legionella needs two things to multiply dangerously:
- Stagnant water (low or no movement)
- Temperature between 20°C and 45°C
Eliminate either, and Legionella cannot grow.
What Is Stagnant Water?
Stagnant water is relative to tank size and usage frequency. Combination boilers or instantaneous water heaters have very high turnover — so they’re low-risk. Large or seldom-used storage systems, common in commercial buildings, pose much higher risks.
Domestic homes typically turn over most of their hot water daily, so stagnation is minimal. Commercial buildings, however, can have long periods of little or no use, making them more susceptible.
Legionella Regulations
Commercial and rental properties in the UK must follow L8 Legionella regulations (HSG274), which require hot water to be stored at least 60°C. Domestic homes are exempt because the risk is so much lower.
Domestic Building Regulations only set an upper temperature limit of 60°C to prevent scalding (Part G Regulations).
So, What Temperature Should Hot Water Be Stored At?
Here’s what we know about Legionella growth rates:
- 0–20°C: dormant
- 20–32°C: slow growth
- 32–42°C: rapid growth (optimum ~37°C)
- 42–45°C: slow growth again
- 45–50°C: dormant
At 50°C, 90% of Legionella dies in around 2 hours.
At 55°C, it dies in 20 minutes and 100% in 5–6 hours.
At 60°C, it dies in 2 minutes, 100% in 30 minutes.
At 70°C, death is instant.
Scalding Risk
Water above 50°C can cause serious burns, especially for children or the elderly. At 60°C, burns occur in one second. Always fit thermostatic mixing valves (TMVs) or temperature limiters at outlets if storing water above 50°C.
Balancing Safety and Efficiency
For heat pumps or condensing boilers, efficiency improves at lower flow temperatures. A cylinder at 50°C loses around 20% less heat than one at 60°C — and your heating source will cycle less, improving overall performance.
Risk Assessment Factors
- Age: Risk increases above age 50 or with weakened immune systems.
- Turnover: Low daily water usage (less than 50%) raises risk.
- Unused outlets: Frequently used homes have lower risk since water doesn’t stagnate.
For most homes, storing water at 50°C is perfectly adequate. In fact, many hot water controls (Honeywell, Viessmann, Vaillant, Worcester) come preset at this temperature.
Minimum Safe Temperatures
If you’re low risk and use hot water frequently, you could theoretically store it at 40°C. That’s still slightly hotter than shower temperature and efficient for dishwashing (if you use a dishwasher).
Sterilisation and Anti-Legionella Cycles
If you store water at 40–49°C, consider running an occasional anti-legionella cycle — temporarily heating water to 60°C to kill bacteria. Many heat pumps include automated weekly or bi-weekly sterilisation cycles.
Alternatively, you can manually turn up your water temperature after holidays or once a year, running each outlet for several minutes.
Finding the Ideal Temperature
For most households, the best approach is to keep stored water as low as safely possible. Generally:
- Low-risk homes: 45–50°C
- With anti-legionella cycles: 45°C or lower
- Vulnerable occupants: 55°C+
Turning a combination boiler’s hot water down from 60°C to 40°C can increase efficiency by around 10%. Lowering a stored cylinder from 60°C to 45°C can reduce heat loss by 40% and make a heat pump up to 25% more efficient.
Extra Tips for Energy Savings and Safety
- Insulate any loft tanks and hot water cylinders
- Encourage full turnover by using outlets regularly
- Flush unused taps or traps periodically
- Insulate secondary circulation pipework
- Fit thermostatic limiters on outlets to prevent scalding
For my own family home, as we turn over nearly 100% of stored water daily, I keep hot water at 45°C and skip anti-legionella cycles.
For more advice, watch our video: 11 Easy Ways To Reduce Your Energy Bills | SAVE UP TO 51%!