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A2:1 on Viessmann Vitodens 300-W

Your boiler is temporarily prioritizing the heating of your hot water over your radiators, meaning your heating may feel cool for a short period while the water tank refills.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description

With DHW priority applicable to mixer: The mixer is closed whilst DHW is loaded; the heating circuit pump operates

What causes this fault?

This status occurs because your boiler is designed to prioritise heating your hot water tank over your radiators. It physically closes a mixing valve to direct all available heat to your water cylinder, ensuring you have a full tank of hot water as quickly as possible. Once the water tank reaches its target temperature, the boiler will automatically switch back to heating your home.

DIY fix possible

Wait up to 30 minutes for the hot water cylinder to reach its target temperatureCheck if the hot water demand setting on your control panel is set unusually highMonitor the radiators to see if they begin to heat up once the hot water light goes off

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