Low
D.23 on Vaillant ecoTEC Regular

Your boiler is currently set to summer mode, which means it is only heating your tap water and has temporarily turned off the central heating.

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.

What the manual says

Manufacturer manual

Summer/Winter function

From this model's manual
Verified against the Vaillant Ecotec installation & service manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

What causes this fault?

This status occurs when the heating system has been manually or automatically switched to its summer setting to save energy. It physically prevents the boiler from sending heat to your radiators while still allowing it to provide hot water for your taps and showers. The boiler is not actually broken; it is simply following an instruction to prioritise hot water over room heating.

This code is most frequently seen during the spring and summer months when homeowners turn off their heating as the outdoor temperature rises.

DIY fix possible

Locate the radiator temperature dial or button on the boiler control panelTurn the dial clockwise or press the '+' button to increase the target heating temperatureCheck that the display no longer shows 'Summer' or the radiator icon with a cross through it

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