Low
S.109 on Vaillant ecoTEC plus

Your boiler has temporarily stopped working because it has entered a standby mode, usually while it waits for a signal from your thermostat or because it needs to take a short break before restarting.

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

Product'sstandbymodeactivated

From this model's manual
Verified against the Vaillant Ecotec Plus 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 code appears when the boiler reaches its target temperature or doesn't receive a call for heat from your home controls, causing it to pause operations for several minutes to prevent constant on-off cycling. It is often triggered by an external thermostat that has satisfied the room temperature or has lost its wireless connection to the boiler. This 'anti-cycling' state is a protective feature designed to save energy and reduce wear on internal components.

This status is more frequently observed in the spring and autumn when the weather is milder, as rooms reach their target temperature quickly and the thermostat cycles the boiler off more often.

DIY fix possible

Check that your room thermostat is turned up and calling for heatEnsure your programmer or heating app is set to an 'On' periodWait up to 15 minutes to see if the boiler restarts automatically

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