Vaillant F23 Fault Code: Causes & What to Do

Vaillant F23 fault code? F23 means the boiler has detected too great a temperature difference between the heating water leaving the boiler and the water returning to it.

In simple terms, heat is being produced but the water may not be circulating through the boiler and heating system as expected.

Possible causes include trapped air, a circulation problem, a pump or valve fault, a blockage, temperature-sensor problems or an expansion-vessel issue. F23 normally requires professional diagnosis rather than simply resetting the boiler repeatedly.

What Does the Vaillant F23 Fault Code Mean?

Vaillant describes F23 as:

The temperature spread between the flow and return is too great.

The flow is the heated water leaving the boiler for the heating system. The return is the cooler water coming back to the boiler after travelling through the system.

There should naturally be a temperature difference between them, but if that difference becomes excessive, the boiler can shut down and display F23.

This is a protective response rather than something to ignore and continually reset.

Vaillant F23 Fault Code: What Should I Check First?

You should not remove the boiler casing, but there are a few useful observations you can make before arranging an engineer.

• Check the boiler pressure display.
• Write down the exact F23 code.
• Check whether the radiators are warming normally.
• Listen for unusual gurgling or circulation noises.
• Note whether some radiators remain completely cold.
• Check whether heating and hot water are both affected.
• Note whether F23 returns after a normal manufacturer-approved reset.

These observations can help identify whether the fault may be related to circulation, air, pressure or another boiler component.

5 Possible Causes of a Vaillant F23 Fault

1. Poor Water Circulation

Poor circulation is one of the main areas associated with F23.

If heated water is not moving away from the boiler quickly enough, the flow temperature can rise while the return remains much cooler.

This creates the large temperature difference detected by the boiler.

Possible circulation problems can involve:

• the heating pump;
• a valve;
• a restriction or blockage;
• air in the system;
• another hydraulic problem.

The actual cause needs to be identified rather than assuming the pump automatically needs replacing.

2. Air in the Boiler or Heating System

Vaillant lists air in the appliance as one possible cause of F23.

Air can interfere with normal water circulation and heat transfer.

You may also notice symptoms such as:

• gurgling noises;
• radiators cold at the top;
• uneven heating;
• changes in boiler pressure after radiators are bled.

If your boiler or radiators are making unusual sounds, see:

Boiler Making Noise? Common Sounds & What They Mean

If you bleed radiators, remember to check the boiler pressure afterwards because releasing air can also lower system pressure.

3. Pump Problem

The heating pump is responsible for circulating water through the boiler and heating system.

Vaillant technical information for some models specifically lists a blocked pump or insufficient pump output as possible causes of F23.

A pump problem may result in insufficient water movement, allowing the flow temperature to rise much faster than the return temperature.

Possible symptoms can include:

• the boiler becoming hot quickly;
• radiators failing to heat properly;
• unusual circulation noises;
• repeated F23 lockouts.

Pump testing requires access to internal heating equipment and should be carried out by an engineer.

4. Flow or Return Temperature Sensor Problem

The boiler uses temperature sensors to monitor the water leaving and returning to the appliance.

Vaillant lists faults involving the electrical connections for the flow and return sensors, as well as defective sensors themselves, as possible causes of F23.

Some model-specific Vaillant technical manuals also identify flow and return NTC sensors being incorrectly connected as a possible cause.

This means an F23 fault does not necessarily prove that water circulation itself is poor.

If the temperature information reaching the boiler is incorrect, the appliance may detect an abnormal temperature difference even though another electrical or sensor problem is responsible.

Diagnostic point: F23 tells us what abnormal condition the boiler has detected. It does not tell us which part should automatically be replaced.

5. Expansion Vessel or Another Heating-System Fault

Vaillant’s current fault guidance also lists an internal expansion-vessel fault as a possible cause associated with F23.

The expansion vessel helps manage changes in system pressure as heating water warms and cools.

If you also notice large pressure changes while the heating is operating, tell the engineer because this can provide another useful diagnostic clue.

For pressure problems, see:

Boiler Pressure Too High? Causes & What to Do

Is Vaillant F23 a Water-Pressure Fault?

Not primarily.

This is an important distinction between F22 and F23.

F22 relates mainly to low water pressure.

F23 relates to an excessive temperature difference between the flow and return.

Pressure can still be relevant to how the heating system operates, but F23 should not simply be treated as a “top the boiler up” fault.

If your Vaillant boiler is actually showing F22, see:

Vaillant F22 Fault Code: Causes & What to Do

What Is the Difference Between Vaillant F22 and F23?

The two faults can sometimes appear to homeowners as similar heating failures, but they represent different detected conditions.

• F22: low water pressure in the heating system.
• F23: the temperature difference between flow and return is too great.

With F22, checking and correcting genuinely low pressure may sometimes be a normal homeowner task.

With F23, Vaillant recommends installer investigation because circulation, sensors, the pump, valves, blockages or another system problem may be involved.

What Is the Difference Between Vaillant F23 and F24?

F23 and F24 are also closely related but describe different conditions.

F23 means the difference between the flow and return temperatures is too large.

F24 means the temperature is increasing too quickly.

Both can involve circulation-related faults, but the detected condition is different.

If your display shows F24, use information specifically for F24 rather than assuming the F23 diagnosis applies.

Can a Blocked Heating System Cause F23?

Yes, a circulation restriction or blockage is one possible cause listed by Vaillant.

Heating-system sludge, debris or another restriction may reduce water movement through the system.

However, seeing F23 does not prove that the heating system needs flushing.

The pump, valves, temperature sensors and other possible causes should also be considered before deciding on treatment.

Can a Faulty Pump Cause Vaillant F23?

Yes.

Vaillant’s technical information lists a blocked pump or insufficient pump output as possible F23 causes on relevant models.

If the pump cannot circulate enough water, the heated flow can become significantly hotter than the return.

That said, do not automatically replace the pump based on the code alone. The complete circulation system needs checking.

Can Low Boiler Pressure Cause F23?

F23 is not Vaillant’s normal low-pressure fault code — that is F22.

However, boiler pressure is still worth checking because water quantity and circulation can affect heating-system operation.

If your pressure is repeatedly falling, you may have a separate problem that also needs attention.

See:

Why Does My Boiler Pressure Keep Dropping?

Can I Reset a Vaillant F23 Fault?

If the user instructions for your particular Vaillant boiler allow a normal reset, you can follow that procedure.

Write down the F23 code before resetting.

A reset may temporarily restore operation, but it does not repair a circulation, sensor, pump or expansion-vessel problem.

If F23 returns, repeatedly resetting the boiler is not the correct solution.

Can I Fix Vaillant F23 Myself?

F23 is generally not a DIY boiler repair.

You can safely:

• check the pressure display;
• record the fault code;
• observe whether radiators are heating;
• listen for unusual gurgling or circulation noises;
• follow the normal reset instructions for your boiler if appropriate.

You should not:

• remove the boiler casing;
• test or replace the pump;
• test internal temperature sensors;
• alter electrical connections;
• adjust the expansion vessel;
• dismantle internal valves or boiler components.

How Is a Vaillant F23 Fault Diagnosed?

An engineer should use the F23 code together with the symptoms rather than treating the code as a parts list.

Depending on the boiler and heating system, diagnosis may include checking:

• system pressure;
• water circulation;
• pump operation;
• air within the boiler or system;
• valves and possible restrictions;
• flow and return temperature sensors;
• sensor wiring and connections;
• the expansion vessel;
• manufacturer-specific diagnostic information.

Engineer point: changing a pump because F23 appears would be guessing. The pump is one possible cause, not the definition of the fault.

Why Does F23 Keep Coming Back?

If F23 disappears after a reset but later returns, the underlying condition may still be present.

The boiler may continue to detect an excessive difference between the flow and return temperatures.

Record:

• how long the boiler runs before F23 appears;
• whether the radiators become hot first;
• whether the fault happens during heating only;
• the system pressure;
• whether you hear unusual noises;
• whether some radiators remain cold.

This information can make professional diagnosis more useful.

Can F23 Cause the Boiler to Stop Heating?

Yes.

F23 is a safety shutdown fault, so the boiler can stop normal operation when it detects the excessive temperature spread.

This can leave the property without central heating and, depending on the boiler and system, may also affect hot water.

If your boiler repeatedly starts and then shuts down, see:

Boiler Keeps Turning Off? Causes & What to Do

How Much Does a Vaillant F23 Repair Cost?

There is no accurate single price for every F23 repair because F23 can have several different causes.

The eventual repair may involve:

• air or circulation problems;
• a pump;
• a valve or blockage;
• a temperature sensor;
• sensor wiring;
• an expansion-vessel problem;
• another heating-system fault.

The sensible approach is to diagnose the actual cause before pricing the repair.

Does Vaillant F23 Mean I Need a New Boiler?

No.

An F23 fault does not automatically mean the boiler needs replacing.

Circulation, pump, sensor and heating-system faults can often be repaired once the actual cause is identified.

If the boiler is older and already has several significant faults, then it may be worth comparing a confirmed repair quote with replacement.

For that comparison, see:

New Boiler Cost Guide

When Should I Call a Boiler Engineer?

Arrange professional diagnosis if:

• F23 keeps returning after a normal reset;
• the boiler will not provide heating;
• some or all radiators remain cold;
• the boiler is making unusual circulation noises;
• pressure is also behaving abnormally;
• the boiler repeatedly overheats or locks out;
• you suspect a pump, sensor, valve or internal fault.

For professional boiler fault diagnosis, visit:

Boiler Repairs

Vaillant F23 Fault in Cardiff?

If you’re in Cardiff and your Vaillant boiler is displaying F23, repeatedly locking out or has a suspected circulation problem, visit:

Boiler Repair in Cardiff

When booking, tell us:

• your Vaillant boiler model;
• the F23 fault code;
• the current pressure reading;
• whether the radiators heat normally;
• whether the boiler makes unusual noises;
• whether F23 returns after a reset.

Quick Questions

What does F23 mean on a Vaillant boiler?

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

What causes Vaillant F23?

Vaillant lists possible causes including air in the appliance, circulation faults involving the pump, valves or blockages, defective flow or return temperature sensors or wiring, and an internal expansion-vessel fault.

Can a faulty pump cause F23?

Yes. A blocked pump or insufficient pump output can reduce circulation and contribute to the excessive flow-to-return temperature difference detected by the boiler.

Is F23 caused by low boiler pressure?

Not necessarily. Vaillant’s main low-pressure fault is F22. F23 specifically relates to an excessive flow-and-return temperature difference, although overall system conditions still need checking.

Can trapped air cause F23?

Yes. Vaillant lists air in the appliance as a possible cause because it can interfere with normal circulation.

Can I reset a Vaillant F23 fault?

You can follow the normal reset procedure in your boiler’s user instructions where appropriate. If F23 returns, the underlying circulation or temperature problem needs diagnosing.

Need Help With a Vaillant F23 Fault?

Let’s Heat can investigate Vaillant F23 faults, circulation problems, recurring boiler lockouts and heating-system faults.

You do not need to identify the failed component yourself before booking.

Book Boiler Repair →

Sources Checked

This guide has been checked against current Vaillant fault-code guidance and Vaillant technical documentation.

Vaillant – Official Boiler Fault Codes

Vaillant – ecoTEC Pro Installation & Maintenance Instructions