How to fix a Vaillant boiler fault code F20

The F20 Vaillant boiler fault often indicates that the boiler has overheated and its safety cut-out has activated. The boiler constantly monitors the temperature of the water using sensors on the flow and return pipes. If these sensors detect that the water has become too hot, the boiler will lock itself out to protect the heat exchanger and other internal components. There are multiple possible reasons for this issue, and it is likely that you will need the help of a heating engineer to fix it. However, we will explain the cause of the problem and provide repair suggestions that you, as a homeowner, can attempt.

Here’s a list of what could be causing the problem:

• Low water pressure, restricting flow around the system
• Blocked or restricted pipework, reducing circulation
• Trapped air in the system after a restart
• Faulty pump not circulating water correctly
• Limescale build-up inside the heat exchanger
• Faulty flow or return temperature sensor giving a false reading

Firstly, as a homeowner, you can check the pressure, bleed any obviously cold radiators, and try resetting the boiler.

Check water pressure:

• Locate the pressure gauge on your boiler. It is usually on the front and may have a green and red section, or it may be shown on the digital display.
• Check if the needle is within the recommended range, which is typically between 1 and 1.5 bar. If it’s below this range, top up the pressure using the filling loop.
• If you notice any radiators that are cold at the top, or one radiator that’s noticeably cooler than the rest, bleed it to release any trapped air.

Reset the boiler:

This is a round or oblong button on the face of the boiler. It will have a flame with a cross through it. Press for 5-10 seconds until the display resets. Let the boiler cool down for a few minutes first if it feels very hot to the touch. This may get your boiler working again on a temporary basis.

If the previous attempts to fix the issue have been unsuccessful, the problem is more likely to be a faulty pump, a failing temperature sensor, or limescale restricting flow through the heat exchanger. An engineer will typically check the pump is spinning freely and moving water at the correct rate, test the flow and return sensors for accurate readings, and inspect the heat exchanger for signs of blockage or scale. A replacement temperature sensor is one of the cheapest parts on the boiler, with the part itself typically costing around £20-£30, plus labour for fitting which varies depending on where in the UK you are based. A new pump usually costs £300-£500 fitted, including parts and labour, again depending on location and the model of pump required. If magnetite (sludge/dirty system) is the culprit, a power flush may be recommended to clear the system – this is a larger job, typically £300-£800 depending on the size of the property and number of radiators, but it can prevent repeat breakdowns.

Repeated F20 lockouts without addressing the underlying cause can put extra strain on the heat exchanger over time, so it’s worth having a professional diagnose the root cause rather than resetting the boiler repeatedly.

Remember to follow the specific instructions provided in your boiler’s manual, as different Vaillant models may have slight variations in their procedures. If you’re unsure or uncomfortable performing these steps, it’s best to consult a professional to avoid causing further issues or voiding your boiler’s warranty.