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Last updated: 22nd April, 2024


Vaillant F23 Fault Code

Vaillant F23 Fault Code

Key takeaways

  • The F23 code signals a temperature discrepancy in the boiler's flow and return pipes.
  • Immediate troubleshooting is crucial to prevent further boiler damage.
  • Professional assessment by a Gas Safe registered engineer is essential.

When a boiler encounters a fault or malfunction, it communicates the problem through specific error codes displayed on its control panel. Each code corresponds to a particular issue, providing a swift method for diagnosing a malfunction.

F23 Vaillant Fault Code

When a Vaillant boiler encounters a problem, it displays a specific fault code on its digital panel, alerting users to the nature of the issue.

The F23 fault code is one such error message that pertains to Vaillant boilers, indicating an imbalance in water temperature between the flow and return pipes.

This situation usually arises due to malfunctions in the boiler’s components that prevent proper heat distribution through the central heating system.

Identifying the F23 fault code early is imperative for maintaining the health and efficiency of a Vaillant boiler. Several factors can trigger an F23 error, including a faulty pump, blockages in the system, or issues with the sensors that measure water temperature.

Users will observe that despite the boiler seemingly operating, heat isn't adequately circulated through their radiators or taps. For safety and warranty reasons, it is recommended to consult a Gas Safe registered engineer for accurately diagnosing and resolving the error.

While costs vary, repairs could range from bleeding the radiators to pump replacement, with full boiler replacements being the solution in more extreme cases.

What Is a Boiler Error Code?

Boiler error codes serve as a critical fail-safe and diagnostic tool. They provide immediate insights into the health of a boiler system.

Explain What Boiler Error Codes Are in General

When a boiler encounters a fault or malfunction, it communicates the problem through specific error codes displayed on its control panel. Each code corresponds to a particular issue, providing a swift method for diagnosing a malfunction. Vaillant boilers, like other brands, utilise a range of codes that signal to the user or a heating engineer that there is a need for inspection and potential boiler repair.

Importance of Understanding Error Codes for Boiler Maintenance

Understanding these fault codes is essential in maintaining a boiler's reliability and efficiency. It enables homeowners and engineers to identify issues early and undertake the appropriate measures to fix them. Regular boiler maintenance, informed by accurate diagnosis through error codes, can prevent extended downtime and costly repairs.

A Gas Safe registered engineer is often required to resolve the faults, ensuring that any work carried out is safe and to a professional standard. Knowing what the fault codes indicate can help expedite the repair process, as the engineer will have an initial idea of the problem before they even inspect the unit.

What Does the F23 Vaillant Fault Code Mean?

The F23 Vaillant fault code signals a problem related to the boiler's temperature differential. It is an issue that compromises the unit's operation, calling for a systematic approach to diagnose and resolve.

Detailed Description of the Specific Error Code

The F23 error on a Vaillant boiler indicates a substantial temperature difference between the flow and return pipes. In essence, the flow pipe carries heated water from the boiler to the radiators, while the return pipe brings the cooled water back for reheating. The error is typically triggered when the detected temperature disparity exceeds the boiler's design parameters.

Possible Reasons Why This Error Code Appears

There are several potential reasons for the F23 fault code appearance:

  • Thermistor Fault: This error code may signify that the heating flow NTC (Negative Temperature Coefficient) thermistor is malfunctioning, providing inaccurate temperature readings.

  • Faulty Pump: A pump not circulating water effectively could result in insufficient flow rate, causing the temperature difference.

  • Physical Blockage: Debris, such as rust or scale within the system, might be obstructing the proper flow of water.

  • Airlock: An air bubble trapped in the system could also be responsible, disrupting water circulation.

  • Safety Switch-Off: In some cases, the boiler's safety mechanisms may activate, causing it to shut down in response to what is perceived as an unsafe operating temperature difference.

The resolution of this error code may involve checking for blockages, assessing pump function, or replacing the thermistor. If the system indicates an F23 error, professional assessment and repair are advisable to ensure safe and efficient boiler operation.

Signs of the F23 Vaillant Fault Code

When a Vaillant boiler displays the F23 fault code, it's typically signalling an issue with the water's flow and return temperature disparity, potentially affecting heating and hot water availability.

List and Explain Common Signs and Symptoms That Indicate This Specific Error

  • Central Heating Inefficiency: Homeowners may discover that radiators take longer to heat up or don't reach the expected temperature, indicating potential water flow problems.

  • Fluctuations in Hot Water Temperature: Fluctuating hot water can suggest the boiler struggles with temperature regulation, a symptom often associated with the F23 error.

  • Boiler Overheating Messages: The appliance may exhibit overheating, which in turn triggers safety mechanisms and displays the fault code.

Can I Fix This Boiler Error Code Myself?

  • DIY Possibility: Simple tasks like bleeding radiators are within the capabilities of most individuals, which may sometimes alleviate this error.

  • Professional Intervention: However, issues like a faulty pump or serious flow and return temperature differences typically require a Gas Safe registered engineer's expertise.

Do I Need to Contact a Gas Safe Engineer to Fix This Error Code?

When confronted with a Vaillant F23 fault code, a homeowner might wonder about the necessity of professional intervention.

Advice on When It's Necessary to Call a Professional

The appearance of the F23 fault code on a Vaillant boiler suggests a circulation problem, typically arising from a faulty pump or air within the system. While bleeding radiators can sometimes be done independently to resolve minor airlocks, the intricacies of boiler repair often require expert attention. If a boiler reset does not rectify the issue or a homeowner is not confident in performing diagnostics, contacting a Gas Safe registered heating engineer is the recommended course of action. Safety is paramount, and DIY repairs on gas appliances can pose severe risks.

Explanation of What a Gas Safe Engineer Is and Why Their Expertise Is Important

A Gas Safe Engineer is a professional who has gone through rigorous training and assessment to obtain certification from the Gas Safe Register—the United Kingdom's official gas registration body. They are qualified to work safely and competently on gas appliances. Employing a Gas Safe registered professional ensures that boiler repairs, services, and gas work conform to the latest safety standards. Their expertise is crucial to not only mitigate the immediate risk associated with the F23 fault code but also to maintain the longevity and efficiency of the heating system.

How Much Does Fixing the F23 Vaillant Fault Code Cost?

When faced with a Vaillant boiler displaying the F23 error code, you can expect a range of costs depending on the nature of the problem and the necessary repairs.

General Cost Range for Fixing This Error (Can Be an Estimate or Based on Average Rates)

Repair costs for the F23 fault code, which points to a temperature differential issue within the flow and return pipes of a Vaillant boiler, can vary. General estimates for repairs include:

  • Bleeding radiators: Approx. £80

  • Replacing a faulty pump: From £400

It's important to note that these are indicative prices, and actual costs can fluctuate based on your specific situation and the prices of the engineer or company providing the service.

Factors That Might Affect the Cost

Several factors can influence the overall cost of fixing the F23 fault code on a Vaillant boiler:

  • Extent of the damage: Minor issues may only require adjustments, whereas more significant problems could necessitate part replacements.

  • Labour rates: The cost for a technician's time can vary widely depending on the region and the professional's experience.

  • Parts: If a replacement part is required, the price depends on the specific component and its availability.

  • Boiler warranty: Should the boiler still be under warranty, the cost may be reduced or covered, provided that the warranty terms include the F23 fault.

In cases where the boiler is beyond economical repair, a full boiler replacement might be the more cost-effective solution in the long term, with prices ranging from around £1,950 to over £5,000. Assessing whether to repair or replace is a critical decision, factoring in the boiler's age, condition, and the potential for future issues.

Do I Need a New Boiler?

Encountering the Vaillant F23 error code can be distressing, leaving homeowners to question whether a repair will suffice or if a full boiler replacement is unavoidable. Below, key considerations and information are outlined to aid this important decision.

Criteria to Decide Whether a Repair Is Sufficient or If a New Boiler Is Required

  1. Age of the Boiler: If the Vaillant boiler, such as the EcoTec series, is within its first decade of operation, repairs tend to be more cost-effective.

  2. Frequency of Breakdowns: Should the unit demand frequent repairs, investing in a new Vaillant model, whether an EcoTec Pro, Plus, or Exclusive, might be more economical in the long run.

  3. Energy Efficiency: Modern Vaillant boilers, including the trusted EcoTec range, offer superior energy efficiency compared to older units. If the existing boiler significantly lags in efficiency, considering a replacement could result in long-term savings.

  4. Cost Comparison: A detailed cost analysis can highlight when the cumulative expense of repairs outweighs the one-time cost of a new installation.

Information on the Lifespan of Boilers and When Replacement Is Typically Recommended

  • Average Lifespan: Vaillant boilers, akin to those from Worcester, Viessmann, and Ideal, have an expected life of around 10 to 15 years.

  • Replacement Indicators: Diminished efficiency, persistent malfunctions, and obsolescence in technology suggest the boiler is nearing the end of its service life.

New Boiler Costs

Boilers can be a daunting purchase for many people as they’re an appliance with a lot of responsibility, providing heat for you and your family is something you want to get right. Boilers aren’t exactly a quickly disposable item either, potentially lasting you a decade.

Effectively, New boiler cost can be split into two segments: the first is the actual boiler itself (unit price), and the second is the cost of the boiler being installed (set up) in your property by an expert engineer. 

Here at iHeat, we want to remove all of this undue stress and make the decision making process of upgrading to a new central heating system, as easy as possible.

Boiler costs can vary depending on a number of factors including their brand, model, fuel, output, warranty, labour and installation type. Typically a new boiler will cost between £1,845 and £3,500, below is a list of average boiler installations offered by iHeat (guide only).

Installation Type

Price (inc VAT)

Combi to combi swap


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System to combi conversion


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New boiler install


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Back boiler to a combi


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System to system


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This section encapsulates the crucial insights about the Vaillant F23 fault code and suggests a definitive course of action. The guidance offered aligns with the experiences documented by users and underpins the importance of expert intervention.

Summarise the Key Points Discussed in the Blog Post

  • The F23 error signifies a temperature differential between the flow and return pipes within a Vaillant boiler system, indicating poor circulation.

  • Professional assessment generally reveals issues like faulty pumps or the necessity for bleeding radiators, with repair costs potentially ranging from £80 to £400+.

  • The cost for a boiler specialist's diagnosis may oscillate between £55 and £75, while hourly rates for a Gas Safe engineer tend to be within £40 to £80.

  • In circumstances mandating complete boiler replacement, expenses may escalate to between £1,950 and £5,000.

  • iHeat and other boiler-related platforms maintain a wealth of information and user feedback, including reviews on Trustpilot.

  • When selecting a service provider, it's advisable to look for those offering fixed prices to avoid unexpected financial burdens.

Frequently Asked Questions

How do I fix the F23 on my Vaillant boiler?

To fix the F23 error on your Vaillant boiler, it's important to first find the underlying cause. Potential issues could be dirty system water, faulty pumps, blocked heat exchangers, or defective sensors. In some instances, you may clear the error by resetting the boiler. However, it's advisable to consult a Gas Safe registered engineer for proper diagnosis and repair.

What does F23 mean on my Vaillant boiler?

The F23 error code on your Vaillant boiler refers to a temperature difference between the flow and return pipes. This suggests that the system is experiencing issues such as overheating or insufficient circulation due to blockages or faulty components.

How do I reset my Vaillant error code?

To reset the error code on your Vaillant boiler, switch the boiler off using its main power switch. Wait a moment, and then switch it back on. In many cases, this will clear the error code. However, if the error persists or reoccurs, it's best to consult a qualified engineer to ensure any issues are adequately addressed.

What is error F32 on Vaillant?

Error F32 on a Vaillant boiler refers to a fault with the fan or the connection to the fan. This can be caused by a defective fan, poor electrical connections, or issues within the wiring harness.

How do I fix F32?

To fix the F32 error on your Vaillant boiler, a thorough inspection of the fan and its connections is required. This might involve examining the wiring, assessing the fan, and replacing any faulty components. It's highly recommended to seek assistance from a Gas Safe registered engineer to ensure proper resolution.

What are the fault codes on a Vaillant boiler?

Fault codes on a Vaillant boiler help to identify specific issues affecting the boiler's function. These codes range from F01, which indicates a lack of ignition, to F84, signalling a faulty sensor. Each code corresponds to a particular problem, which can guide the engineer in diagnosing and repairing faults.

What is the error code on the Vaillant gas valve?

There isn't a specific error code solely related to the Vaillant gas valve. Various error codes could indicate issues with the valve or its components, such as F01 (ignition problem) or F28 (gas supply problem). If a specific error code relates to the gas valve, it would be best to consult the boiler's user manual or seek help from a Gas Safe registered engineer.

Stephen Day profile photo
Article by
Stephen Day | Co-founder
Gas Safe registered and FGAS certified engineer with over 20 years experience in the heating and cooling industry.