What Does The Vaillant F20 Fault Code Indicate And How Do I Fix It?

Boiler F20 CODE

When error codes flash up on your boiler display screen it is a sign that something somewhere in your central heating system is not working as it should be. These codes act as warning signs, alerting you to potential malfunctions or imminent issues if left unaddressed. Essentially, your boiler is reaching out to you for help! While sometimes these alerts seem trivial, akin to a false alarm easily resolved with a simple reset, they can also signify more serious underlying issues demanding immediate attention to prevent a complete loss of heating or hot water.

Every boiler brand employs its own set of error codes to pinpoint specific faults within the system. In this article, we will focus on deciphering the F20 fault code for Vaillant boilers and outline the steps a skilled heating engineer would take to rectify it.

When your Vaillant boiler flashes an F20 code, it is sounding the alarm: it’s overheated and needs a breather to cool down and get back to business. Fortunately, Vaillant boilers are equipped with a failsafe mechanism that kicks in automatically when this error arises, preventing further damage or unexpected accidents. Ignoring it would be a mistake – it is crucial to act swiftly.

Immediately powering down allows the boiler to chill out, returning to its optimal temperature and function. While overheating is a common hiccup, easily remedied by a cooldown session, it’s wise to dig deeper into the root cause. Prolonged usage can often trigger such incidents, but there might be other factors at play behind the system’s temporary shutdown and the display of the fault code.

Even if things seem back to normal post-cooldown, underlying issues could still lurk. Neglecting these minor glitches could snowball into major disruptions down the line. So, instead of brushing it off, it’s worth ensuring the system is shipshape.

Reasons A Boiler Might Overheat And How A Professional Would Fix It

Common reasons why a boiler might overheat include:

1. Low water flow

Insufficient water flow through the boiler can lead to localised overheating, especially in components like the heat exchanger. This can be caused by pump failure, blockages in the system, or incorrect valve settings.

A professional heating engineer might tackle this by:

  • Inspecting the pump to ensure it’s functioning correctly and check for any blockages or leaks in the pipework. If necessary, they might adjust the pump speed or replace it altogether to improve water flow.

2. Thermostat malfunction

A faulty thermostat may inaccurately control the temperature of the boiler, causing it to heat beyond its intended range.

A professional heating engineer might tackle this by:

  • Testing the thermostat’s accuracy and replacing it if found faulty. Calibration adjustments might be made to ensure precise temperature control.

3. Airlock

Air trapped within the system can impede water flow and lead to overheating in certain areas of the boiler.

A professional heating engineer might tackle this by:

  • Bleeding the radiators and boiler system to release trapped air. They might also check for any obstructions in the pipework causing airlocks.

4. Scale buildup

Mineral deposits and scale can accumulate within the boiler, hindering heat transfer and causing localised hotspots.

A professional heating engineer might tackle this by:

  • Using descaling agents or chemical cleaners to dissolve and remove scale deposits from the boiler components. In severe cases, parts affected by scale buildup may need to be replaced.

5. High pressure

Excessive water pressure in the boiler system can lead to over pressurisation and subsequent overheating. This can be caused by a faulty pressure relief valve, expansion vessel issues, or the inlet valve being stuck closed.

A professional heating engineer might tackle this by:

  • The engineer would first check the pressure relief valve to ensure it’s functioning correctly. Adjustments would be made to the pressure settings, or the expansion vessel might be inspected for faults. If necessary, the inlet valve might be replaced or repaired.

6. Blocked heat exchanger

Accumulated debris or scale within the heat exchanger can restrict water flow and cause overheating.

A professional heating engineer might tackle this by:

  • Cleaning the heat exchanger thoroughly to remove any debris or scale buildup. If cleaning doesn’t suffice, the heat exchanger might need to be replaced.

7. Faulty temperature sensor

A malfunctioning temperature sensor may provide inaccurate readings to the boiler control system, leading to overheating.

A professional heating engineer might tackle this by:

  • The engineer would test the temperature sensor’s readings for accuracy and replace it if necessary. They would also check the wiring and connections for any faults.

8. Restricted flue

Blockages or restrictions in the flue or ventilation system can impede proper combustion, causing the boiler to overheat.

A professional heating engineer might tackle this by:

  • Inspecting the flue and ventilation system to identify any blockages or restrictions. Clearing obstructions and ensuring proper airflow would resolve the issue.

9. Faulty circulating pump

A malfunctioning circulating pump may fail to distribute water properly throughout the system, resulting in localised overheating.

A professional heating engineer might tackle this by:

  • Testing the pump’s performance and checking for any faults in its operation would be the initial step. Repairing or replacing the pump might be necessary to restore proper water circulation.

10. Ignition problems

Issues with the ignition system can lead to incomplete combustion, causing heat buildup within the boiler.

A professional heating engineer might tackle this by:

  • Inspecting the ignition system for faults and ensure proper combustion. Cleaning or replacing faulty ignition components would rectify the issue.


In summary, if there are blockages in your heating system or the pump isn’t working properly it can cause the water temperature to rise and overheat the boiler. Luckily, the boiler will shut down on its own automatically. Once the system has cooled sufficiently, you should be able to turn the boiler back on and things should resume to working well. But, if it keeps happening, it is important to find the root cause of the problem and that is not easy to do on your own. It is best to leave it to the experts.

These are just a few potential causes of boiler overheating, and diagnosing the specific issue typically requires the expertise of a qualified heating engineer.

While you’re here, why not explore our blog for more useful plumbing and heating advice. You never know – it might just help you make the right choice for your needs.