Does hot water have to come out of the tap below 50 degrees?

These two requirements look contradictory and are not. They solve different problems, and the device that reconciles them is a tempering valve.

Stored water needs to be hot enough that Legionella bacteria cannot establish in the tank, which is why storage systems are set around 60°C. Water at that temperature will scald a child in well under a second, so it cannot be delivered to a shower or basin at storage temperature. The plumbing standard therefore caps delivery to outlets used for personal hygiene at 50°C, and a tempering valve blends cold water in to hit that figure.12

What this means for you

• If your system is a storage unit installed or replaced in recent years, it will have a tempering valve. It is a small brass valve near the heater.
• Tempering valves wear out. Water that has become either scalding or unable to get hot is a common symptom of one failing, and replacing the valve is far cheaper than replacing a heater.
• Instantaneous systems preset at or below 50°C may not need a separate valve, because they are not storing water at 60°C in the first place.
• Turning your storage thermostat down to save energy is not a safe workaround. Below about 60°C you are creating the conditions the standard exists to prevent.