Low
S.28 on Vaillant ecoTEC Plus 635

Your boiler has temporarily stopped the burner because it has reached the required temperature for hot water and is taking a short break to prevent overheating.

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

Hotwaterhandlingmode:Temporaryshutdownoftheburner

From this model's manual
Verified against the Vaillant Ecotec Plus 615 installation & service manual, page 38.
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 occurs when your boiler successfully heats the water in the plate heat exchanger to the set temperature and temporarily switches off the burner to prevent waste. It is a normal part of the boiler's operation, indicating that the system is simply waiting for the water temperature to drop slightly before firing up again to maintain a steady flow. This prevents the unit from overheating and ensures a consistent temperature at your taps.

This code may be more noticeable in summer when the incoming mains water is naturally warmer, meaning the boiler reaches its target temperature much faster than in winter.

DIY fix possible

Check that your hot water taps are fully closedWait for 5 to 10 minutes for the 'S' status code to clear automaticallyMonitor the boiler to see if it moves into a heating or standby state

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