Why does your tank heat to full every two weeks?

If your tank heats to 100% about once every two weeks, even though you normally ask for less hot water, it is usually running its automatic cleansing cycle. This is a regular water hygiene function, rather than a change to your everyday heating plan.

You normally do not need to do anything. Leave the tank switched on and allow the cycle to finish.

What is the cleansing cycle?

Your tank normally prepares the amount of hot water you need, starting at the top. Water lower down can remain cooler. The cleansing cycle heats water throughout the tank to help control harmful bacteria. It runs automatically, so you don’t need to request a full tank yourself.

The cycle runs at least once every two weeks by default. Your exact day and time depend on the settings for your tank.

What might you notice?

• The hot water level rises towards 100%, even if you normally request less. This describes how much hot water is prepared, not how full the tank is: the tank already contains water.
• The tank heats outside your usual schedule.
• Heating may continue for longer than a normal small boost.
• Boost may be temporarily unavailable while the cycle is running.
• The app may show that the tank is cleansing or fully heating.

These signs do not usually mean that the schedule is broken or that someone has changed your settings.

How long does it take?

How long the cycle takes depends on the tank size, the starting temperature and the way your water is heated. A boiler may complete it more quickly, while a heat pump usually heats more gradually. Some systems use the electric heater inside the tank to help finish the cycle.

Leave the tank on until the cycle finishes. Turning it off can interrupt the process, and the tank may try again later.

What happens if the cycle is interrupted?

The cycle may stop if the tank loses power, the boiler or heat pump is unavailable, or the water does not reach the required temperature. The tank normally tries again automatically, so you may see it heat to full at another time.

A one-off full heat every two weeks is normally expected. Repeated attempts, an error message or no usable hot water may need checking.

Can you choose when it runs?

If you use the app, select your tank and look for Tank Settings. Open it, then look for Cleansing. Where these controls are available, choose the day and time and save any changes.

Choose a time when the tank can stay switched on and the system that heats it, such as a boiler or heat pump, is available. If someone else manages your tank, ask them before changing the timing.

The cleansing temperature is normally set during installation to suit your system. Leave it at the installed setting unless your installer, landlord, housing provider or Mixergy advises you to change it.

Will it increase your energy use?

The cycle uses energy because it heats more water than a small everyday target. It runs periodically for water hygiene, not every day. You can reduce inconvenience by scheduling it for a suitable time and by keeping your tariff details correct in the app, where available.

Do not try to save energy by turning the tank off before the cycle. An interrupted cycle may retry later and can make the heating pattern harder to understand.

When should you ask for help?

• The tank repeatedly tries to cleanse without finishing.
• An error message appears in the app.
• The tank heats to full much more often than every two weeks and it is not caused by your schedule, a boost or solar energy.
• You have no usable hot water after the cycle.

If you rent, contact your landlord or housing provider first. Homeowners can contact Mixergy's Homeowners & Residents team on 01865 595 875, Monday to Friday, 9am to 5pm. Tell them how often the cycle has restarted, which heat source the tank uses and any message shown in the app.

Common questions

Why has my tank ignored its schedule?

The cleansing cycle temporarily takes priority over normal schedules so the full usable volume can reach the required temperature.

Why does the app show 100%?

The cycle heats water throughout the tank. A 100% hot-water level describes the amount available, not the water temperature or how full the tank is. Normal heating control resumes afterwards.

Why can’t I use Boost?

Boost may be unavailable during cleansing because the tank is already heating towards full.

Can I turn the cleansing cycle off?

Do not disable it without advice. It is a water-hygiene feature and the installed settings should normally be left in place.

Why did the cycle start again?

A previous attempt may have been interrupted or may not have reached the required temperature, so the tank is trying again.

Related guides

How Mixergy chooses when to heat