Vaillant F23 fault code: what it means and what to do

When your Vaillant boiler displays F23, it has detected an excessive temperature difference between the water leaving the boiler and the water returning to it.

The boiler performs a safety shutdown when this temperature spread becomes too great.

What does the Vaillant F23 fault code mean?

Vaillant defines F23 as a safety shutdown caused by the temperature spread between the flow and return becoming too great.

The flow carries heated water away from the boiler, while the return brings water back after it has circulated through the heating system.

An unusually large difference can indicate that water isn't circulating through the boiler and heating system as expected.

What causes a Vaillant F23 fault?

Vaillant lists several possible causes of F23, including:

• air in the boiler

• a circulation fault

• a blocked or underperforming pump

• a valve or blockage restricting circulation

• a flow or return temperature sensor fault

• an electrical connection problem affecting the sensors

• an internal expansion vessel fault

The code doesn't identify which of these is responsible in your particular system.

Professional diagnosis is therefore needed to establish the underlying cause.

Is F23 a circulation fault?

F23 is closely associated with circulation because restricted water movement can create an excessive difference between flow and return temperatures.

For example, a pump that is blocked or operating with insufficient output may prevent water from circulating at the required rate.

However, circulation isn't the only possibility. Temperature sensors and their electrical connections can also be involved.

Avoid assuming that the pump needs replacing based on the F23 code alone.

Can air in the heating system cause F23?

Air is one of the possible causes identified by Vaillant.

Air within the boiler or heating system can interfere with normal water circulation and contribute to an excessive temperature difference.

However, F23 doesn't prove that trapped air is responsible.

If you also have symptoms associated with air in your radiators, such as cold areas towards the top, bleeding the radiator may be appropriate. Follow the normal safe procedure and check system pressure afterwards.

Should you check the boiler pressure?

Checking the pressure gauge is a safe external check.

If your Vaillant boiler also has unusually low system pressure, follow the instructions for your particular model.

However, low pressure isn't the primary definition of F23. Vaillant uses other fault codes, including F22, specifically for low water pressure.

Don't repeatedly repressurise the boiler unless the displayed pressure actually requires it.

Can you reset a Vaillant boiler showing F23?

Vaillant states that resetting can sometimes clear a fault or temporarily restore boiler operation.

Follow the reset procedure in the user manual for your specific model.

If F23 returns, don't repeatedly reset the boiler. The excessive temperature difference still needs investigating.

A reset doesn't repair a blocked pump, circulation restriction or defective sensor.

Can you fix Vaillant F23 yourself?

Most causes of F23 aren't suitable for DIY repair.

You can make external observations, such as checking the boiler pressure and whether your radiators are heating normally.

Don't remove the boiler casing or attempt to inspect or repair the pump, internal valves, temperature sensors, electrical connections or expansion vessel.

Vaillant recommends installer involvement for F23 because the fault can relate to how the boiler is operating with the heating system.

What will an engineer check?

The exact checks depend on the symptoms and boiler model.

Potential areas for investigation include:

• water circulation

• pump operation

• restrictions or blockages

• air within the system

• flow and return temperature sensors

• sensor connections

• the internal expansion vessel

The engineer can use the F23 code alongside system measurements and physical checks to identify the actual cause.

This is more reliable than replacing a component simply because it appears on a list of possible causes.

How much does an F23 repair cost?

There isn't one reliable price for repairing an F23 fault.

F23 can have several causes, ranging from circulation issues to sensor or component faults. The work required therefore depends on the diagnosis.

An estimate based purely on the fault code can be misleading because F23 doesn't tell you which component, if any, needs replacing.

Does F23 mean you need a new boiler?

No. F23 doesn't automatically mean your Vaillant boiler needs replacing.

The fault should first be diagnosed to establish what is causing the excessive temperature difference.

If the underlying problem can be repaired appropriately, replacing the entire boiler may not be necessary.

Any decision about replacement should be based on the diagnosed fault and the wider condition of the boiler, not the F23 code alone.

What to do when your Vaillant boiler shows F23

F23 means the boiler has detected too great a temperature difference between its flow and return.

You can check external information such as the system pressure and observe how the radiators are heating, but don't open the boiler or attempt internal repairs.

If an appropriate reset doesn't clear the fault, or F23 returns, arrange professional investigation of the boiler and heating system.