Vaillant F20 Fault Code Explained
Hertfordshire, Bedfordshire & Cambridgeshire
An F20 fault on a Vaillant boiler means the overheat safety cut-out has operated. The boiler has got hotter than it should and shut itself down.
There is one thing worth checking yourself, and then it is engineer work.
What F20 actually means
Boilers have a safety thermostat that trips if the water temperature climbs beyond a safe limit. Overheating almost always means the heat is not being carried away properly, which points to circulation rather than to the burner.
So F20 is usually a symptom of a flow problem, not a fault with the safety device itself.
What you can safely check yourself
Check the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when the heating is cold. If it is low, repressurise using the filling loop, and bleed your radiators if any are cold at the top.
If the pressure is correct and the fault stays, stop there. The rest is inside the appliance.
When to stop, and why
Do not keep resetting an overheat lockout. The cut-out has operated for a reason, and repeatedly overriding a safety device while the underlying circulation problem is still there risks damaging the heat exchanger, which is the most expensive part of the boiler.
What causes it beyond the simple checks
A failed or seized pump, sludge restricting flow through the heat exchanger, closed or stuck valves, a failed thermistor giving a wrong reading, or air locks. Where sludge is the cause, a power flush deals with the underlying problem rather than the symptom.
How Vantage can help
We are Gas Safe registered and cover Hertfordshire, Bedfordshire and Cambridgeshire, with an emergency line answered 24 hours a day.
Call us on 01462 506374. See also boiler repair and Vaillant boiler installation.
Frequently asked questions
Can I just keep resetting it?
No. It is a safety cut-out and something is making the boiler overheat.
Is it expensive?
It depends on the cause. A stuck valve is cheap, a pump is more, a heat exchanger is the one to avoid by not ignoring it.
Would a power flush help?
If sludge is restricting circulation, yes, and it prevents a repeat.
Related fault codes
Other Vaillant codes: Vaillant F22 · Vaillant F75 · Vaillant F28 · Vaillant F29 · Vaillant F27 · Vaillant F23 · Vaillant F83 · Vaillant F32 · Vaillant F62 · Vaillant F61 · Vaillant F72
The same problem on other makes (overheat cut-outs): Baxi E110
See also our guide to boiler making banging noise, the full boiler advice section, and boiler repair.
In Vaillant’s own words
Vaillant describes F20 as the safety cut-out interrupting the gas valve control: the gas valve was closed because the temperature at the flow or return temperature sensor exceeded the maximum limit.
That is worth understanding. The boiler did not break. It measured a temperature it will not run at and shut the gas off. Something stopped the heat leaving the boiler, and that is what needs finding.
When to call an engineer
Anything past the free checks above needs a Gas Safe engineer with the right test kit. Ring Vantage on 01462 506374, any hour, any day of the year. You will get an honest arrival time rather than a comforting one, and the price before the work.
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