Why stored hot water is kept hot and the delivered temperature is limited separately, and what that means for a vulnerable bather.
Hot water storage carries two opposite risks, and the answer to both is the same: keep the store hot, and control the temperature actually delivered at the tap separately.
Why the store is kept hot
Water held lukewarm for any length of time lets bacteria such as legionella multiply, so stored hot water is kept hot enough to control that risk rather than turned down to save money. Guidance on controlling legionella in hot and cold water systems works to stored hot water at 60°C or above, reaching at least 50°C at the outlet within a minute of running. Turning a cylinder thermostat down to lukewarm to cut bills removes that protection and should be advised against.
Why the tap is limited separately
Delivering water at storage temperature is itself a scalding hazard, so the delivered temperature is controlled at the outlet rather than by under-heating the store. Guidance on scalding treats hot water above 44°C at a bath or shower as a real risk of serious injury, and the risk is greatest during whole-body immersion and for anyone who cannot react quickly or move away, such as an elderly or disabled bather or a young child. Where a vulnerable person could be scalded, fit and commission a thermostatic mixing valve to limit the outlet temperature, and check what it actually delivers rather than assuming the setting is correct — a pressure-reducing valve limits pressure, not temperature, and a manual mixer can still be turned to full hot, so neither protects against scalding on its own.