Low
S.15 on Vaillant Home System

Your boiler is simply cooling itself down for a few minutes after heating your water, which is a normal part of its operation.

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

DHW mode: Pump/fan overrun

From this model's manual
Verified against the Vaillant Home System installation & service manual, page 32.
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

Wait 5 to 10 minutes for the boiler to finish its cooling cycleCheck that your hot water taps are fully turned offEnsure there is no 'constant' demand for hot water on your external controls

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