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Vaillant F23 fault

A Vaillant F23 is about poor circulation causing overheating in the boiler. It is not a common fault on Vaillant boilers as they normally cope with this quite well.

f23 fault on a Vaillant boiler — F23 fault on a Vaillant boiler

What do Vaillant say about F23?

Vaillant suggest to check the following things.

cable to heating pump/water pressure sensor,
air/insufficient water in the heating circuit,
flow and return temperature sensor inverted,
strainer in the hydraulic block, water
pressure sensor,
heating pump.

Most common cause of F23?

The only time I’ve seen a Vaillant F23 fault (instead of F22) is when the plate heat exchanger is blocked. This is normally on very corroded and dirty heating systems. It can also mean a blocked main heat exchanger.

Is this different to F22?

Yes it’s a different fault. F22 is always when the boiler thinks the pressure is below 0.5 bar.

What causes an F23 fault?

The boiler shows this fault when the temperature inside the boiler rises faster than it expects .

The boiler has several temperature sensors on pipes and other components. When the boiler runs the temperatures are constantly monitored to make sure the boiler is working properly.

Normal operation will see the sensors heating up and cooling down at different speeds and to different temperatures depending on how the boiler is running.

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