F.20
Vaillant Boiler
Severity: ModerateWhat Does This Error Mean?
Vaillant F.20 means the boiler shut down because the flow temperature climbed past the safety limit.
The PCB watches a high-limit thermostat on the heat exchanger and posts F.20 if that thermostat opens.
Causes split between low water flow (pump airlock, dirty filter, closed valve) and a thermostat that has aged out.
Fix the cause first, then reset — repeated F.20 resets without diagnosis stresses the heat exchanger.
Affected Models
- Vaillant ecoTEC Plus
- Vaillant ecoTEC Pro
- Vaillant ecoFIT Pure
- Vaillant ecoFIT Sustain
- Vaillant Turbomax
- Vaillant ecoMAX
Common Causes
- Pump airlocked or running too slowly
- Heating filter or magnetic filter clogged with sludge
- Frozen condensate trap restricting flow indirectly
- Heat exchanger fouled with limescale
- Overheat thermostat aged and tripping early
- TRVs all closed at once with no bypass open
How to Fix It
-
Wait for the boiler to cool.
F.20 trips at high temperature.
Wait at least 30 minutes before any reset attempt.
The thermostat closes again as the heat exchanger cools. -
Check system pressure.
Look at the gauge on the boiler.
Healthy cold pressure is 1.0-1.5 bar.
Below 0.5 bar = top up via the filling loop.
Above 2.5 bar = bleed a radiator briefly to reduce. -
Bleed the pump.
An air-bound pump cannot move water and the boiler overheats locally.
The Vaillant pump has a small bleed screw on the front face.
Slow turn anti-clockwise with a small screwdriver until water trickles out, then close.
Power-cycle the boiler. -
Inspect the magnetic filter.
If the system has a Magnaclean or similar filter, clean it.
Iron sludge from old radiators reduces flow dramatically.
Most installs need this every 1-2 years. -
Open at least one radiator fully.
If every TRV in the house is closed and there is no automatic bypass, the pump pushes water against a wall and the boiler overheats.
Open at least one radiator (often the bathroom towel rail) fully so flow always has a path. -
Press the reset button.
On most ecoTEC models, the reset button shows a flame symbol with a slash.
Press once.
The boiler attempts a fresh ignition.
Do not press repeatedly — that does not bypass an underlying fault. -
Schedule service for repeats.
If F.20 returns within hours, the heat exchanger is likely scaled or the overheat thermostat has aged out.
Both are jobs for a Gas Safe engineer.
A power flush is 400-700 GBP; thermostat replacement is 80-150 GBP.
Frequently Asked Questions
Is F.20 dangerous?
The shutdown protects the heat exchanger from cracking under thermal stress.
The danger is not the alarm — it is repeated F.20 events without finding the root cause.
Each overheat event shortens the heat exchanger's life.