The boiler has exceeded a safe temperature limit and shut down on its safety cut-out.
Most common on: ecoTEC plus, ecoTEC pro
F.20 means the safety cut-out has activated: the boiler has sensed the temperature rising beyond a safe limit and shut the burner down to protect itself. It is a deliberate protective shutdown.
Overheating is almost always a circulation problem — if the heated water is not moving away from the heat exchanger fast enough, its temperature spikes. It can also be caused by a faulty temperature sensor or the safety cut-out device itself, so a persistent F.20 needs a heating engineer.
Top up to about 1.0–1.5 bar (cold) if the gauge reads low, then reset.
Press reset after checking the pressure. If F.20 clears and stays clear, it may have been a one-off.
Cold spots or radiators slow to heat can indicate sludge or a circulation problem worth flagging to an engineer.
Persistent F.20 needs a heating engineer to check the pump, sensors and heat exchanger, and possibly a system flush.
Work on the gas side of a boiler must be carried out by a Gas Safe registered engineer. The checks above are limited to safe, non-invasive things a homeowner can look at (pressure, resets, the gas supply) — if in doubt, stop and call an engineer.
TradePlanr turns the callout into paid work — log the job, quote the parts and labour, raise the gas safety record and send the invoice from your phone, even with no signal. Free to start, works in your browser, no download needed.
The boiler has exceeded a safe temperature limit and shut down on its safety cut-out. F.20 means the safety cut-out has activated: the boiler has sensed the temperature rising beyond a safe limit and shut the burner down to protect itself. It is a deliberate protective shutdown.
You can try a single reset and the safe checks below, but Vaillant F.20 usually needs a Gas Safe registered engineer. Never work on the gas side of a boiler yourself — it is illegal and dangerous unless you are Gas Safe registered.
A recurring F.20 means the underlying cause has not been fixed — most often poor circulation from a failing pump or an airlock. Resetting repeatedly will not solve it; get the boiler diagnosed properly.