Vaillant F20 Fault Code: Causes & How to Fix It

Last Updated on January 23, 2025

When your Vaillant boiler displays the F20 fault code, it can be a cause for concern. This error indicates an overheating issue, which prompts the boiler’s safety systems to shut it down to prevent further damage. 

In this post, we’ll explore the causes of the F20 fault, how to fix it, and when it’s time to call a Gas Safe engineer..

What Does the Vaillant F20 Fault Code Mean?

The F20 fault code signals that your boiler’s internal temperature has exceeded safe operating limits. Vaillant boilers are equipped with sensors to monitor temperature, and if the heat rises too high, the system automatically shuts down to avoid damage to internal components or dangerous malfunctions.

Common Causes of the Vaillant F20 Fault Code

Several issues can lead to overheating in your Vaillant boiler. Let’s look at the most common causes and how they might trigger the F20 fault code.

1. Low Pressure, High Pressure, and System Blockages

Low or high system pressure can affect water circulation, causing the boiler to overheat. Similarly, blockages in the system, whether in pipes, the heat exchanger, or the pump can restrict water flow and lead to overheating.

2. Faulty Thermistor

Thermistors monitor temperature within the boiler. If one becomes faulty, it may incorrectly sense that the system has overheated and activate the lockout.

3. Heat Exchanger Kettling

When limescale builds up on the heat exchanger, it creates hotspots that cause water to boil and produce a kettling sound. This can overheat the boiler and lead to an F20 fault.

4. Broken Fan

The fan plays a crucial role in cooling the boiler’s components. If it’s damaged, the boiler may overheat and trigger the F20 code. Older boilers are particularly prone to this issue as their efficiency declines with age.

5. Blocked Pressure Relief Valve (PRV)

A blocked or faulty PRV can disrupt pressure regulation in the boiler. This can lead to overheating and display the F20 error.

6. Limescale Near the Thermistor

Limescale deposits near the thermistor can create inaccurate temperature readings. The thermistor may sense localised hotspots which can trigger the lockout even though the overall system temperature is within range.

How to Fix the Vaillant F20 Fault Code

While some fixes can be attempted at home, others require professional intervention. Let’s break down the steps:

1. Check System Pressure

• Low boiler pressure: Re-pressurise the boiler to 1-1.5 bar using the filling loop.
• High boiler pressure: Bleed the radiators to release excess pressure. Trapped air in the system can disrupt water flow. Use a radiator key to release the air and restore circulation.

2. Inspect the Pump

If the pump is hot or noisy, it may be clogged or seized. A professional engineer can flush or replace the pump if needed.

4. Clean the Heat Exchanger

Heat exchangers clogged with limescale or debris require professional cleaning. In severe cases, the exchanger may need replacing.

5. Replace Faulty Components

• Thermistor: If the thermistor is faulty or affected by limescale, it should be replaced.
• Fan or PRV: These components should be inspected and replaced if necessary by a Gas Safe registered engineer.

6. Address Kettling

If you notice kettling sounds, turn off your boiler immediately and call a Gas Safe registered engineer. Resolving the issue early can prevent costly repairs or replacement of the heat exchanger.

Is It Time for a New Boiler?

If your boiler frequently displays the F20 fault code, it may be time to consider a replacement. Older boilers are more prone to overheating, and upgrading to a modern, efficient model can save you money on energy bills while providing reliable heating. For more information on the costs and performance of modern systems, check out our Vaillant boiler prices and review guide to help you make an informed decision.

Get Your Free Boiler Quote in Minutes

Upgrading your boiler has never been easier. At Eco Happy, we offer a quick and hassle-free process:

1. Fill out our online form.
2. Answer a few simple questions about your home and heating needs.
3. Receive a free, no-obligation quote within minutes.

Don’t let an unreliable boiler leave you in the cold. Take the first step toward a warmer, more efficient home today!

Conclusion

The Vaillant F20 fault code is a sign that your boiler is overheating and needs attention. While some issues, like low pressure or air in the system, can be resolved at home, others like a faulty thermistor, blocked PRV, or kettling heat exchanger require professional expertise.

Whether you need repairs or are ready to upgrade to a new boiler, addressing the problem promptly will ensure your heating system is safe and efficient.