When your Vaillant boiler suddenly shuts down and displays the F20 fault code, it’s easy to imagine the worst. No heat. No hot water. And a message that’s more confusing than helpful.
But here’s the good news – the F20 error, which signals overheating, is something I come across regularly. And it’s often resolved quickly with the right approach.
I’m Tony from Advanced Heating and Plumbing – a local, Gas Safe registered engineer with more than 25 years of experience working on boilers like yours. Here’s what you need to know.
Need help fast?
If you’re in Watford, London, Middlesex or Hertfordshire, I can usually be with you the same day. I fix F20 issues on a regular basis and carry many parts on the van.
📞 Call now on 0800 328 0449
Request a fast quote onlineWhat Does the Vaillant F20 Fault Code Mean?
The F20 error is your boiler’s way of saying it’s getting too hot – and it’s shut down to protect itself.
This doesn’t always mean your boiler is on its last legs. In fact, overheating can be triggered by relatively minor issues, such as low water flow or a faulty sensor. But it does need investigating quickly, as continued overheating could damage internal parts.
What Causes an F20 Error?
Here are the most common reasons I see when called out to fix F20 faults:
🔹 1. Low or Restricted Water Flow
Your boiler needs a constant flow of water to keep itself cool. If the system is full of air, sludge, or has a faulty pump, the water can’t move as it should – and things get hot, fast.
🔹 2. Faulty Temperature Sensor (NTC)
The temperature sensor monitors how hot your boiler’s getting. If it’s giving incorrect readings, the boiler might think it’s overheating when it isn’t – or fail to spot that it’s too hot until it’s too late.
🔹 3. Pump Problems
If the pump isn’t running properly, water won’t circulate. This leads to trapped heat, especially around the heat exchanger.
🔹 4. Blockages in the System
Over time, radiators and pipework can get clogged with sludge or limescale. This reduces water flow and causes hot spots.
Can You Do Anything Yourself?
If your boiler has shut down with the F20 fault, try these first:
- Check the system pressure – Should be around 1.2–1.5 bar when cold.
- Bleed your radiators – Air in the system can restrict flow.
- Turn the boiler off – Let it cool before attempting a reset.
⚠️ Important: If you hear banging or gurgling sounds, or the boiler keeps shutting down repeatedly, stop using it and call an engineer. Overheating can damage key components if left unresolved.
Why Call a Professional?
Diagnosing the exact cause of an F20 error often involves testing sensors, checking pump operation, and sometimes inspecting the system for internal blockages or buildup. These are tasks best left to a qualified engineer.
When I attend these callouts, I:
- Check water flow and pump operation
- Inspect and test temperature sensors
- Look for blockages or restrictions
Make sure your system is safe and running efficiently
Need Help? I Can Sort It – Fast and Fairly
Whether it’s a blocked system or a faulty sensor, I’ll get to the bottom of it. I offer:
✅ Same-day callouts (when available)
✅ Fixed, fair pricing – no hidden extras
✅ Over 25 years of trusted, local service
📞 Call me now on 0800 328 0449
I’m always happy to offer advice – and if I can save you a visit with a tip over the phone, I will.
Book your repair or request a quote here
