How to Fix the Vaillant F23 Fault Code

If your boiler displays the Vaillant F23 fault code, it usually points to a circulation problem that needs attention.

The F23 fault appears when there's a temperature difference of more than 35°C between the flow and return pipes. The boiler shuts down as a safety measure to prevent overheating or damage.

Some causes are simple, while others require professional repair. Understanding what the fault means can help you decide what to check before arranging an engineer.

What Does the Vaillant F23 Fault Code Mean?

The Vaillant F23 fault code indicates that the temperature difference between the flow and return pipes has exceeded 35°C.

Under normal operation, water circulates continuously around the heating system. If circulation slows or stops, the flow pipe heats much faster than the return pipe. When the difference becomes too great, the boiler locks out and displays F23.

Ignoring the fault can place additional strain on components and may lead to more expensive repairs.

What Causes the Vaillant F23 Fault Code?

Several faults can prevent water circulating correctly.

Faulty circulation pump

The pump moves heated water around your central heating system. If it fails or begins to wear out, water flow reduces and the boiler quickly detects an abnormal temperature difference.

Blocked heating system

Sludge, limescale or debris inside the heating system can restrict circulation. Blockages may develop within the heat exchanger, pipework or radiators.

A professional power flush or chemical flush may be required if contamination is severe.

Air trapped in the system

Airlocks reduce water circulation and can trigger the F23 fault.

Bleeding radiators may resolve minor airlocks, but recurring air usually indicates another issue within the system.

Faulty thermistor

The flow and return thermistors monitor water temperature. If either sensor gives inaccurate readings, the boiler may display F23 even when circulation is normal.

Replacing a faulty thermistor is normally a straightforward repair for a Gas Safe engineer.

How to Fix an F23 Fault

You can carry out a few basic checks before arranging a repair:

• Check whether radiators need bleeding.
• Look for obvious leaks or pressure loss.
• Reset the boiler once after completing any simple checks.

If the fault returns immediately, avoid repeatedly resetting the boiler. The underlying issue still needs diagnosing.

Problems involving pumps, thermistors, internal components or blockages should always be repaired by a qualified Gas Safe engineer. If the fault won't clear or keeps returning, arrange a boiler repair in London and we'll diagnose the cause and carry out the necessary repairs.

Typical Boiler Repair Costs

Repair costs depend on the cause of the fault.

Minor repairs are usually less expensive than replacing major components such as the circulation pump. If the boiler has suffered ongoing circulation problems, additional work may also be required.

An engineer can diagnose the fault and provide an accurate quotation before carrying out repairs.

Frequently Asked Questions - (FAQs)

Usually not. The fault is commonly caused by circulation issues, blocked systems, faulty pumps or failed sensors. Most can be repaired without replacing the boiler.

You may be able to resolve simple airlocks by bleeding radiators. Beyond basic checks, repairs should be left to a Gas Safe engineer.

Follow the reset procedure in your boiler manual. If the fault returns after resetting, arrange a professional inspection rather than repeatedly resetting the appliance.

If your boiler is showing a different error, see our guide to Vaillant boiler fault codes for explanations of other common fault messages.

Need Help With an F23 Fault?

If your Vaillant boiler continues to display the F23 fault code after basic checks, our Gas Safe engineers can diagnose the problem and carry out the repair.

Contact Serviceteam to book your boiler repair.