What temperature should hot water be stored at?
The short answer
Stored hot water should be kept at 60°C or above, and should reach the taps at 50°C or more, within about a minute of running. That's hot enough to control Legionella bacteria in the cylinder. Because water at that temperature will also scald, the answer is not to store it cooler but to limit the temperature at the outlet — with a thermostatic mixing valve.
The two risks pull in opposite directions, and that's the whole point of the rule. Legionella bacteria thrive in stagnant water roughly between 20°C and 45°C — which is precisely the temperature a cylinder sits at if someone turns the thermostat down to save money. Storing at 60°C keeps the cylinder out of that range.
But 60°C water at a tap is dangerous, particularly to children and older people, so the fix is applied at the outlet rather than at the tank: store hot, deliver safe. A thermostatic mixing valve blends the stored hot water down to a safe temperature at the bath or basin, without ever letting the cylinder itself drop into the risky band.
This is why "just turn the cylinder down a bit" is bad advice, and why a plumber will push back on it. If your hot water is too hot at the taps, the answer is a mixing valve, not a colder tank.
Combi boilers heat on demand and don't store water, so the storage rule doesn't apply in the same way — but the scalding risk at the tap still does.
- Store at 60°C or above
- Distribute at 50°C or above, reaching the tap within about a minute
- Bacteria multiply in stagnant water roughly between 20°C and 45°C
- Control the risk of scalding at the outlet, not by cooling the tank
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