Should I keep resetting F.20?

No. F.20 is a safety shutdown for overheating. Leave the boiler off and book an engineer.

F.20

Vaillant fault code

Vaillant F.20: the safety cut-out has stopped the gas because the boiler overheated

Engineer only

The water temperature went above the boiler's safe limit, so a safety device closed the gas valve. Repeatedly resetting it isn't safe.

There is nothing you can safely do yourself with this one. Leave the boiler off and book an engineer.

Find a Gas Safe engineer near you Opens the official Gas Safe Register search.

Applies to: Vaillant boilers (Vaillant's fault code list). Codes can differ between models, so check your manual.

Model	Meaning
Vaillant boilers (Vaillant's fault code list)	The safety cut-out has closed the gas valve because the flow or return temperature exceeded the maximum

Model

Meaning

Vaillant boilers (Vaillant's fault code list)

The safety cut-out has closed the gas valve because the flow or return temperature exceeded the maximum