How F20 shows up
- The boiler stops abruptly while running
- A 'water hammer' or rumbling noise before it shuts down
- Code F20 stays on display after a reset
- Radiators may feel unusually hot just before shutdown
- The fault returns after a few minutes of running
What error code F20 means
- Overheating detected — the temperature sensor measured a value beyond the safety threshold, the boiler shuts down to protect itself.
- Insufficient water flow — weak pump, partially closed valve, or a sludged-up system.
- Scale build-up on the heat exchanger — common in hard-water areas, it slows down heat transfer.
- Air in the system — an air pocket can create a localised hot spot.
- Faulty temperature sensor — reporting overheating that isn't actually happening.
What you can safely check yourself
Let the boiler cool down
Don't reset immediately after an overheat shutdown: wait a few minutes for the temperature to drop.
Check that the radiators are open
Radiator valves that are all closed at the same time can significantly reduce water flow.
Fault keeps repeating? Call a professional
Recurring overheating needs a prompt diagnosis: scale, pump or sensor — only a licensed technician can confirm which.
Which brands does this code apply to?
This code is used as-is on Vaillant and Bulex boilers, two brands that share the same technical platform. On Junkers, Viessmann, Atlantic and Remeha boilers, an equivalent fault shows under a different code — our technician identifies the fault on site, whatever the brand.
A clear price, no surprises
A fixed 35 € call-out fee in Brussels, Wallonia and Flanders. The repair price is given to you before any work starts. Free, no-obligation quote on 0488 02 77 07.