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Quick answer
Cylinder pasteurisation is the process of periodically heating stored hot water in a cylinder to a temperature high enough to kill legionella bacteria, which can otherwise multiply in water left at lukewarm temperatures for extended periods. Most modern hot water cylinders are designed to reach this temperature automatically as part of their normal heating cycle, but cylinders set too low, used infrequently, or fitted with certain thermostatic mixing arrangements can sometimes fall short, which is worth understanding if you have a stored hot water system.
Key takeaways
- Legionella bacteria multiply most readily in water sitting between roughly 20C and 45C, which is why stored hot water systems are designed to periodically exceed this range.
- Pasteurisation temperature for domestic cylinders is generally around 60C, a level most cylinder thermostats are set to reach as standard.
- Risk is low in a typical, regularly used domestic system, but rises in cylinders set too low, rarely used, or serving properties left empty for extended periods, such as holiday homes.
- Landlords and anyone responsible for a rented, communal, or infrequently occupied property have a more formal duty of care around hot water system risk than an owner-occupier in daily use of their own home.
- A cylinder thermostat check is a routine part of a boiler and system service, not something that normally needs separate attention.
Hot water cylinders are a background fixture in many UK homes, quietly storing and reheating water, and most people never think about the biology of what happens inside one, until legionella comes up as a topic and raises a reasonable question about whether their own system is doing what it should.
What legionella is and why cylinders matter
Legionella is a type of bacteria found naturally in small numbers in most water systems, which becomes a genuine health risk primarily when it multiplies to significant levels in water that is then inhaled as a fine mist or aerosol, most commonly associated with showers, spray taps, and similar fittings. The bacteria multiply most readily in water sitting in a favourable temperature range, roughly between 20C and 45C, for an extended period, which is why stored hot water systems, where water sits in a cylinder rather than being heated on demand, are specifically designed with this risk in mind.
How pasteurisation works
Pasteurisation in this context means heating the entire volume of stored water in the cylinder to a temperature high enough to kill legionella bacteria, generally understood to be around 60C for domestic systems, for long enough that the whole tank reaches that temperature, not just the top layer near the immersion element or coil. Most modern cylinders are designed to reach and maintain this temperature as part of their normal heating cycle, controlled by the cylinder thermostat, which is why a correctly set up and regularly used domestic system carries a low baseline risk.
Where the risk increases
Risk increases in specific, identifiable circumstances rather than being a general concern for every home: a cylinder thermostat set too low, below the temperature needed for effective pasteurisation, in the interests of saving energy; a cylinder that sits largely unused for long periods, such as in a rarely visited second home or holiday let, where water can sit at intermediate temperatures for extended stretches; and certain thermostatic mixing valve arrangements, which deliberately reduce the temperature of water at the point of use for scald protection but need to be fitted correctly so the stored water in the cylinder itself still reaches full pasteurisation temperature upstream of the mixing valve. Landlords with tenanted or communal hot water systems, and owners of holiday homes and static caravans, carry a more formal duty of care here than an owner-occupier using their own system daily.
What to check or have checked
For most homeowners in regular daily use of a standard cylinder system, the practical takeaway is straightforward: have the cylinder thermostat checked as part of a routine boiler and system service, avoid setting it unnecessarily low purely to save energy, and if the property is going to sit unused for an extended period, running the hot water through a full heating cycle before regular use resumes is a sensible, low-effort precaution. If you have any specific concern about your system’s setup, particularly around thermostatic mixing valves or a communal system, raising it with a qualified engineer at your next boiler service is the straightforward way to get a proper answer.
Book with Gasmaster
Have a question about your hot water cylinder’s setup or temperature settings? Gasmaster Plumbing & Heating serves Stoke-on-Trent, Newcastle-under-Lyme and Stafford for cylinder checks and servicing. Book through our plumbing services page.
Frequently asked questions
Is legionella a common problem in UK homes?
No, it is genuinely uncommon in normally used, correctly set up domestic hot water systems; the main risk factors are specific, identifiable circumstances such as very low thermostat settings or long periods of disuse rather than routine daily use.
What temperature should my hot water cylinder be set to?
Around 60C is the generally recommended setting for effective pasteurisation on a standard domestic cylinder, balanced against scald risk at point of use, which thermostatic mixing valves are designed to manage separately.
Do combi boilers carry the same legionella risk as cylinders?
No, because a combi boiler heats water on demand rather than storing it, there is no significant volume of standing water at intermediate temperatures in the way a cylinder holds, which is one of the reasons combi systems are considered lower risk in this specific respect.
Should landlords do anything specific about this?
Landlords with cylinder or communal hot water systems generally have a more formal responsibility to manage this risk as part of overall property safety; a qualified engineer can advise on what is appropriate for a specific rented property.
How do I know if my cylinder thermostat is working correctly?
A functioning thermostat should be checked as part of a routine service visit; contact Gasmaster if you have not had your cylinder checked recently or have any concerns about your system’s temperature settings.
Sources
- Health and Safety Executive, legionella and hot water system guidance
- CIBSE domestic hot water system guidance
- Energy Saving Trust (energysavingtrust.org.uk)
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
