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Understanding the Vaillant Boiler F23 Fault Code: Causes and Solutions

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Vaillant Boiler F23 Fault Code

Understanding the Vaillant Boiler F23 Fault Code: Causes and Solutions

When your Vaillant boiler displays an F23 fault code, it’s alerting you to a temperature discrepancy between the flow and return pipes. This error can cause your boiler to shut down as a safety precaution, leaving you without heating or hot water. But what exactly does this fault code mean, and more importantly, how can you fix it? Here’s all you need to know.

What is the F23 Fault Code in Vaillant Boilers?

The F23 error code is specific to Vaillant boilers and indicates that the temperature difference between the flow (the pipe sending heated water into your heating system) and the return (the pipe bringing water back to the boiler) is too great. Typically, a difference exceeding 35°C will trigger this fault code

Common Causes of the Vaillant Boiler F23 Error

Several issues can lead to an F23 fault code:

Faulty Pump: If the pump is not circulating water quickly enough, the water in the return pipe cools excessively, causing a significant temperature difference. In this case, the Central Heating Pump needs to be replaced.

Blockages: Sludge or debris in the system can restrict water flow, leading to temperature imbalances.

Air Locks: Air trapped in the system can prevent water from circulating properly, again causing a temperature differential.

Sensor Issues: Faulty sensors may incorrectly report temperatures, resulting in an F23 error.

Water Circulation Issue: Water circulation issues can also result in an F23 error.

Fixing the Vaillant Boiler F23 Fault Code

Addressing an F23 fault code involves several steps:

Check the Pump: Ensure it’s set to the correct speed and is free from blockages.
Bleed Radiators: Release any trapped air in the system to allow for proper water circulation.
Flush the System: Remove sludge and debris that could be causing blockages.
Inspect Sensors: Have a professional check the temperature sensors for accuracy.

When to call our Gas Engineers

While some steps, like bleeding radiators, can be done by yourself, others require the expertise of our Gas Safe registered engineers. If you’re unsure about any aspect of fixing the F23 error, it’s best to Call Us Now. We can accurately diagnose the issue and ensure repairs are carried out safely and effectively.

Preventing Future F23 Errors in Vaillant Boilers

Regular maintenance is key to preventing F23 and other fault codes. Annual servicing by a qualified engineer can keep your boiler running smoothly and identify potential issues before they become serious problems.


The F23 fault code on your Vaillant boiler is a sign that something isn’t right with the temperature balance in your heating system. By understanding the common causes and solutions, you can take proactive steps to fix the issue or we are here to help you with any of your Boiler related issues. Call Us Today and we’ll take care of the rest.

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