What is the F23 Fault Code in Vaillant Boilers?
What is the F23 Fault Code in Vaillant Boilers?

What is the F23 Fault Code in Vaillant Boilers?

The F23 fault code is a common issue encountered by Vaillant boiler owners. When this error code appears, it signals a problem with the system and requires immediate attention.

The F23 code is triggered when the temperature difference between the flow and return pipes exceeds 35°C. This situation causes the boiler to shut down for safety reasons. The flow pipe carries hot water from the boiler to circulate through the heating system, while the return pipe carries water back to the boiler for reheating.

To fix this issue, homeowners are advised to seek the services of a qualified heating engineer. DIY methods may not be enough to address the underlying problem and could cause further damage to the system. A professional heating engineer will diagnose and resolve the problem, ensuring the safe and efficient operation of your boiler.


What Does the F23 Fault Code Indicate?

The F23 error code is triggered when the difference in temperature between the flow and return pipes exceeds 35°C. As a safety measure, your boiler will shut down when this occurs.

The flow pipe carries the hot water from the boiler to the heating system, while the return pipe transports the cooled water back to the boiler for reheating. The flow pipe will always be warmer due to the freshly heated water, but if the difference in temperature exceeds 35°C, it can cause both functional and safety issues, leading to the F23 fault code display and the shutdown of your system.


What is the F23 Fault Code in Vaillant Boilers?

The F23 fault code on Vaillant boilers appears when the difference in temperature between the flow and return pipes exceeds 35°C. As a result, the boiler will shut down for safety reasons. The temperature difference between the flow and return pipes should never be exactly the same.

The hot water that circulates through the heating system exits through the flow pipe, while the water that returns to the boiler for reheating travels through the return pipe. The flow pipe is naturally warmer as it supplies freshly heated water to the heating system, while the return pipe temperature will be lower after the water has circulated and cooled.

However, if the temperature difference surpasses 35°C, this creates functional and safety problems, leading to the shut-off of the system and the display of the F23 fault code


What Causes an F23 Error Code? An F23 error code is triggered when the temperature difference between the flow and return pipes of your boiler exceeds 35°C. This can be caused by various factors, including:

  1. Faulty Pump - If the pump is not functioning properly, it may not circulate water fast enough, causing the central heating water to be cooler than it should be by the time it reaches the return pipe. Possible causes of a faulty pump include a stuck pump shaft, incorrect speed setting, or a circuit board issue in electric boilers.
  2. Physical Blockage - Any physical blockage in the heating system can limit the water circulation and increase the temperature difference between the flow and return pipes. This may be caused by central heating sludge or limescale accumulation.
  3. Airlock - An airlock anywhere in the heating system can affect the water circulation and cause an F23 error.
  4. Thermistor Fault - The thermistor is responsible for regulating the temperature of your boiler. If it fails to do so, it may result in boiler faults, including an F23 error code.


Can I Resolve an F23 Error Myself?

The ability to fix this error code will depend on the root cause. If uncertain, it's best to seek assistance from a professional.

Here's the answer based on the cause:

Faulty Pump - No If a pump in your boiler is not working properly, it needs to be replaced. Unfortunately, this is not something that can be done as a DIY project.

Only a gas safe registered professional is legally and safely allowed to replace a faulty pump. Therefore, you should hire a qualified heating engineer to handle this task.


Can I Remove an Airlock from my Heating System DIY?

If an airlock is causing your F23 error code, it can generally be removed easily by bleeding your radiators. However, it is important to turn off your heating and ensure your radiators are cold before beginning.

Here's how to bleed your radiators:

  1. Place a towel and basin under the bleed valve of the first radiator.
  2. Insert the bleed key into the valve and turn it counterclockwise slowly.
  3. You will hear a hissing noise as the air escapes.
  4. Once the water starts to flow out, turn the key clockwise to close the valve.
  5. Repeat the process for additional radiators.
  6. A reset of your boiler may be necessary for the error code to disappear.

Note: If you are unsure about the process, it's best to consult a professional to avoid potential harm or damage to your heating system.


Removing Physical Blockage - Usually,

Yes If there is a physical blockage in your heating system, flushing your radiators can help resolve the issue. The process is similar to bleeding your radiators, but it's specifically designed to remove physical blockages.

Here's how to flush your radiators:

  1. Turn off the heating and wait for the radiators to cool down.
  2. Place towels underneath the radiator.
  3. Turn off the thermostatic radiator valve by twisting it to the off or "0" position.
  4. Remove the plastic cap from the lockshield valve and turn it clockwise to turn it off, taking note of how many times it was turned.
  5. Place a bucket under the thermostatic radiator valve.
  6. Use the bleed key to open the bleed valve and allow air into the radiator.
  7. Allow the water and sludge to drain into the bucket.
  8. Loosen the lockshield valve to let more water out. Tilt the radiator if necessary, with help from a second person.
  9. Close the bleed valve using the key.
  10. Remove the radiator from its brackets and tilt it to drain any remaining water into a bucket.
  11. Go outside and attach a hose to the pipe inlet to flush out the sludge.
  12. Keep flushing the radiator until the water coming out is clear.
  13. Reattach the radiator by reversing the steps mentioned earlier.
  14. Turn the lockshield valve counterclockwise the same number of times it was turned off earlier.
  15. Open and then close the bleed valve to expel trapped air.

With these steps, the physical blockage should be resolved.


If you've tried removing the airlock and flushing the radiators, but the F23 error still persists, a power flush may be necessary. Although, it's possible to perform this task DIY, it requires a great deal of caution, as well as following all safety measures and instructions from the manufacturer.

It's also important to note that a power flush can address build-ups of sludge and limescale not only in radiators but throughout the entire piping system.

However, when it comes to a thermistor fault, it is not recommended to attempt a DIY repair. Regardless of whether the thermistor requires repair or replacement, a professional should handle the issue.


The Cost of Hiring a Heating Professional for an F23 Fix When it comes to fixing an F23 error, you may decide to hire a professional. The cost of their services will depend on various factors, including your location. The average hourly rate for a Gas Safe Register engineer is around £30 to £60. Here's a rough estimate of what you may expect to pay for specific fixes:


If your boiler is outdated, fixing it may not be the best option. Replacing a broken pump or a faulty thermistor can be costly and a full replacement may be more cost-effective in the long run. While the cost of a new boiler can be high, there are options available to make it more manageable. Many companies offer financing plans, allowing you to pay for the replacement in monthly instalments over a period of one to two years. This can help ease the burden of a large upfront cost.


If your Vaillant boiler is showing common fault codes, you'll find plenty of information in our detailed guides. We've covered a range of Vaillant boiler error codes to help you resolve the issue.


Vaillant F22 Error Code.

Vaillant F23 Error Code.

Vaillant F24 Error Code.

Vaillant F25 Error Code.

Vaillant F27 Error Code.

Vaillant F28 Error Code.

Vaillant F29 Error Code.

Vaillant F54 Error Code.

Vaillant F62 Error Code.

Vaillant F72 Error Code.

Vaillant F75 Error Code.

Vaillant F76 Error Code.

Vaillant F83 Error Code.


Does my Vaillant Boiler Warranty Cover Repairs for an F23 Fault?

The coverage of a Vaillant boiler warranty only extends to repairs that are due to defects in the manufacturing process. Repairs resulting from misuse or other factors not related to manufacturing will not be covered.

It's crucial to ensure that your boiler's warranty is still valid, even if the repair is related to a manufacturing defect. Vaillant boiler warranties can vary, so it's important to be aware of the length of your specific boiler's warranty and the terms and conditions that apply.

For example, a domestic Vaillant boiler must meet the following criteria for a valid warranty:



Warranty Coverage for F23 Error Code in Vaillant Boilers The likelihood of a Vaillant boiler warranty covering repairs or replacement work for an F23 fault is low, as the warranty only applies to defects resulting from the manufacturing process.

To maintain a valid warranty, certain conditions must be met such as annual servicing by a Gas Safe Registered engineer, and a proper installation and commissioning within six months of the boiler's dispatch.

It's crucial to understand the specifics of your boiler's warranty, including its length and any terms and conditions. It's essential to keep in mind that warranties for Vaillant boilers can vary, and an F23 fault is unlikely to be covered under such warranties.


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