A Vaillant F20 fault code means the boiler has detected overheating and gone into a safety lockout to protect its components. The fault is commonly caused by poor water circulation, a faulty component or limescale inside the system, and the underlying cause usually needs diagnosing before the boiler can safely return to normal operation.
- Content Guide
- » What Does the Vaillant F20 Fault Code Mean?
- » What Causes a Vaillant F20 Fault Code?
- - Faulty Fan
- - Faulty Thermistor
- - Heat Exchanger Kettling
- - Blocked Pressure Relief Valve
- - Limescale Around the Thermistor
- - Circulation Pump Problems
- » How to Fix a Vaillant F20 Fault Code
- » Can You Fix a Vaillant F20 Fault Yourself?
- » Professional Vaillant F20 Boiler Repair
What Does the Vaillant F20 Fault Code Mean?
The Vaillant F20 fault appears when the boiler temperature rises above its safe operating limit, usually above 97°C. When this happens, the boiler automatically shuts down to prevent damage to internal components.
Possible reasons include:
- Low or excessive system pressure
- Poor water circulation
- Component failure
- Heavy limescale build-up
Resetting the boiler without fixing the cause may only result in the fault returning.
If you're seeing a different error, browse our complete guide to Vaillant boiler fault codes to find the meaning of each code.
What Causes a Vaillant F20 Fault Code?
The most common causes include:
Faulty fan
A failed fan can prevent the boiler from cooling correctly, leading to overheating.
Faulty thermistor
If the thermistor gives inaccurate temperature readings, the boiler may lock out even when temperatures are normal.
Heat exchanger kettling
Limescale inside the heat exchanger creates hot spots, causing overheating and noisy operation.
Blocked pressure relief valve
A faulty or blocked PRV can affect pressure control and contribute to overheating.
Limescale around the thermistor
Scale deposits can interfere with accurate temperature readings.
Circulation pump problems
A worn or seized pump can restrict water flow, allowing temperatures to rise too quickly.
How to Fix a Vaillant F20 Fault Code
Start with the simplest checks.
- Try a boiler reset using the manufacturer's instructions.
- Check whether the boiler pressure is within the recommended range.
- If the fault returns, stop resetting the boiler repeatedly.
A Gas Safe registered engineer may need to:
- Test the thermistor
- Inspect the fan
- Check the circulation pump
- Clean or flush the heat exchanger
- Inspect the pressure relief valve
- Power flush the heating system if sludge or limescale is present
- Add inhibitor and fit a magnetic filter where appropriate
Can You Fix a Vaillant F20 Fault Yourself?
Beyond checking the pressure and carrying out a single reset, most F20 faults should be left to a Gas Safe registered engineer.
The fault often involves internal components and overheating. Attempting repairs yourself can be unsafe and may damage the boiler further.
Regular servicing also helps reduce the risk of overheating faults developing.
If the F20 fault keeps returning after a reset, book a boiler repair in London and we'll diagnose the underlying cause safely.
Professional Vaillant F20 Boiler Repair
If your Vaillant boiler keeps showing an F20 fault code, our Gas Safe engineers can identify the cause and carry out the necessary repairs. We diagnose overheating faults, replace failed components where needed and test the boiler before leaving your property.
If your boiler has locked out or the F20 fault keeps returning, book a Gas Safe engineer today. We'll diagnose the cause, explain the repair and get your heating working safely again.
