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Quick answer
Many households find a combi hot water setting of around 50 to 55°C gives comfortable, efficient hot water. Higher settings mean you mix in more cold at the tap, increase scale build-up and raise scalding risk; lower settings can feel lukewarm in winter when mains water is colder. Combis do not store hot water, so they do not have the same Legionella storage concerns as a hot water cylinder, which should be kept at 60°C. For bath safety, new homes must limit bath hot water to 48°C, usually with a thermostatic mixing valve.
Key takeaways
- Around 50 to 55°C is a sensible starting point for most combis.
- Setting it too hot wastes energy through mixing and increases limescale.
- Combis do not store water, so the 60°C cylinder storage rule does not apply the same way.
- Hot water cylinders should store water at 60°C to control Legionella.
- Building Regulations limit bath hot water to 48°C in new dwellings.
Why not just set it to maximum?
When the combi runs at maximum temperature, you add cold at the tap to make it comfortable. The boiler has heated water hotter than needed, and the extra heat goes to waste. Higher temperatures also make the boiler less likely to condense efficiently on hot water and speed up scale formation in the plate heat exchanger.
Winter vs summer
Mains water is colder in winter, so the combi has to raise it by more degrees. That reduces the flow of water at a given temperature. If your shower feels less hot in winter, a slightly higher setting can help, but check flow first. See combi hot water fluctuating.
Legionella and combis vs cylinders
Legionella bacteria grow in stored warm water. Hot water cylinders should heat water to 60°C to control risk, and landlords have duties to assess this. Combis heat water instantly and do not store it, so the risk profile is different. See cylinder pasteurisation and Legionella.
Scalding and children
Water at 60°C can scald in seconds. For bathing children and older people, lower temperatures and a thermostatic mixing valve or thermostatic bath and shower valves reduce the risk. Building Regulations Approved Document G requires bath hot water in new homes to be limited to 48°C.
Heating temperature is separate
The combi’s heating dial controls the temperature of water sent to radiators. That is a separate setting. See boiler flow temperature settings.
Book with Gasmaster
Want your combi set up properly? Gasmaster Plumbing & Heating, Gas Safe registered (220932), services combis across Stoke-on-Trent, Newcastle-under-Lyme and Biddulph. Book an annual boiler service.
Frequently asked questions
What temperature should a combi be set to for hot water?
Around 50 to 55°C suits most homes. Adjust to comfort.
Does a lower hot water setting save money?
Usually slightly, by reducing mixing and heat loss, and it reduces scale.
Why is my hot water not hot enough even at max?
Low flow rate, scale, a diverter valve fault or low gas pressure could be the cause. See no hot water but heating works.
Sources
- Building Regulations Approved Document G (gov.uk)
- Health and Safety Executive, Legionella guidance L8 (hse.gov.uk)
- 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.
