The Vital Role of Correctly Setting Your Hot Water Cylinder Thermostat in the U.K.1. Keep Legionella at Bay — Set Your Thermostat at 60 °C or Above

Legionella bacteria, which cause the potentially fatal Legionnaires’ disease, thrive in warm water between 20 °C and 45 °C, and may still survive up to 50 °C . To reliably eliminate these bacteria, health authorities—including the UK’s HSE—recommend storing hot water in cylinders at a minimum of 60 °C

Multiple trusted sources reinforce this standard:

• Vaillant advises setting your hot water tank between 60 °C to 65 °C, noting that below 50 °C there's an elevated risk of harmful bacterial growth

• The Centre for Sustainable Energy confirms that 60–65 °C is the usual safe setting to kill off Legionella in domestic cylinders

• GreenSquareAccord and Rooftop Housing Group both explicitly advocate for maintaining 60 °C to neutralise Legionella

• Viessmann also emphasises setting the thermostat to 60 °C for both safety and efficiency

Bottom line: Aim for a cylinder thermostat setting of 60 °C, and not lower.

2. The Hidden Dangers of Setting the Thermostat Too Low

a) Legionella & Health Risks

If your cylinder is set below 55–60 °C, you're inviting bacterial growth. This poses a serious risk of Legionnaires’ disease, especially for the elderly or those with weakened immune systems

b) Limescale Build-Up & System Inefficiency

Lower thermostat settings, especially in hard-water areas, can foster limescale formation on heating elements. That build-up reduces efficiency and can further exacerbate temperature instability

c) Scalding Concerns—Balance with Temperature Control

While 60 °C is essential for sanitisation, water at that temperature can scald in under five seconds—particularly dangerous for children and older adults Ensuring safety at the tap is equally important.

d) Energy Waste If Set Too High

Conversely, setting the thermostat much higher than 65 °C leads to unnecessary energy consumption, increased bills, and scalding risks

3. Striking the Right Balance: Safety, Comfort & Efficiency

Recommended Settings:

Component	Suggested Temperature
Cylinder Thermostat	60 °C (kills bacteria safely)
Tap/Shower Water (via TMV)	Around 50 °C (minimises scald risk)

Thermostatic Mixing Valves (TMVs) are a smart solution—they let you maintain a high temperature in the cylinder for safety while delivering a lower, safer temperature at taps and showers

Energy Efficiency & Comfort Tips

• Insulating your hot water cylinder can dramatically reduce heat loss—saving around £50–£155 annually

• Lowering your flow temperature (especially on combi boilers) by just 3–5 °C can yield measurable energy savings without affecting comfort

4. Why It Matters to You and Your Customers

• Health & Safety: Correctly calibrated thermostats protect families from serious health risks while preventing dangerous scalding.

• Regulatory Compliance: Many regions in the UK mandate or recommend 60 °C storage for Legionella control—compliance avoids liability.

• Consumer Confidence & Energy Savings: Educating clients about thermostat settings builds trust. Plus, demonstrating energy-efficient setups can help customers reduce bills and feel good about using greener systems.

5. Final Thoughts: Best Practices for Cylinder Thermostat Settings

• Always set your hot water cylinder thermostat to at least 60 °C, ideally in the 60–65 °C range.

• Install a Thermostatic Mixing Valve (TMV) to ensure safe, comfortable temperatures at points of use.

• Insulate your cylinder to maintain warmth efficiently and lower costs.

• Educate homeowners about the dangers of under- or over-heating their water systems.

In Summary

Proper cylinder thermostat settings are more than just a technical detail—they’re a crucial element of safe, efficient, and responsible home heating. At Irving-PHG, ensuring these systems are correctly configured helps safeguard health, saves energy, and builds confidence in sustainable heating solutions.