Low
d.25 on Vaillant ecoMAX 635

Your boiler's internal timer is currently trying to trigger a hot water boost, which is a normal operational status rather than a breakdown.

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.

What the manual says

Manufacturer manual

Hot–water activation via warm–start clock

From this model's manual
Verified against the Vaillant Ecomax 613 installation & service manual, page 47.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

DIY fix possible

Check your boiler's control panel settingsReview your external timer or programmer settingsDisable the 'warm start' or 'pre-heat' function if you do not require instant hot water

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