Overview
The Vaillant EcoTEC Pro / EcoTEC Plus F.20 (Safety Switch Off) fault means the boiler has detected an overheat condition and has gone into a protective lockout. The boiler’s temperature sensors (thermistors) or the temperature limiter have seen temperatures outside the safe operating range, so the control shuts the burner down to protect the appliance and occupants. This can be caused by an actual rise in water temperature inside the boiler or by an incorrect sensor reading caused by a faulty thermistor or wiring. Severity: this is a safety shutdown and should be treated seriously. A single, isolated F.20 after heavy continuous use that clears after cooling is less serious than repeated or immediate lockouts. Persistent or repeated F.20 faults indicate an underlying fault (pump failure, blocked heat exchanger, limescale hotspots, fan failure, blocked PRV or flue/vent restriction, faulty sensor or wiring) that needs investigation. Because this code relates to overheat protection and internal components of a gas appliance, it is not a safe repair for an unqualified person. Basic checks can be done by a homeowner, but diagnosis and repair should be carried out by a Gas Safe registered heating engineer.
Possible Cause: Safety switch off temperature limiter
Troubleshooting Steps
Safety precautions:
1) If the boiler has locked out and you smell gas, leave the property immediately and call the gas emergency number. Do not attempt repairs.
2) Do not remove boiler panels or work on gas or electrical components unless you are a qualified Gas Safe engineer. Isolate the electrical supply to the boiler if you need to touch isolating valves or check external components, but avoid internal access.
3) If the boiler repeatedly locks out, switch it off at the programmer and at the isolator and keep it off until an engineer attends.
Initial checks a homeowner can safely perform:
1) Allow the boiler to cool for at least 20–30 minutes. Many F.20 occurrences clear after a cooldown and a single reset.
2) Try a single reset using the boiler’s reset button (follow the handbook). Do not repeatedly reset the boiler if the fault returns immediately.
3) Check the visible pressure gauge on the boiler. Very low system pressure (<0.8 bar) or very high pressure can contribute to abnormal operation. Note the reading for the engineer. Do not attempt internal repairs if you are unsure how to repressurise — follow the manufacturer’s guide or wait for an engineer.
4) Listen for unusual noises while the boiler runs before it locks out: rapid kettling/whistling suggests heat exchanger limescale or restricted flow. A short run then trip can suggest fan or airflow problems.
5) Inspect the external condensate pipe (if present) for freezing or blockage, and check the external flue outlet for obvious obstructions (birds’ nests, leaves). These are simple visual checks only.
6) Check radiators for air (cold tops) and bleed if necessary to help restore flow — only if you are confident doing this and have a towel/container ready.
Specific diagnostic notes an engineer will use (for your information):
1) Reproduce the fault and read live sensor values from the boiler control to see whether the overheat is genuine or a faulty sensor/thermistor reading. Faults that lock out immediately often point to sensor wiring or a failed thermistor. Faults that develop after running typically indicate flow problems, pump failure, scale or blockages.
2) Check the fan and flue for proper operation and airflow. A failed fan or blocked flue or air supply can cause overheating or poor combustion and will be tested by the engineer.
3) Test the circulation pump and valve positions to ensure adequate water flow through the heat exchanger. A seized or failing pump or closed isolation valves will be checked.
4) Inspect the heat exchanger for limescale, hotspots or kettling. Heavy scale or debris can create local overheating; the engineer may carry out a chemical flush or recommend component replacement if badly corroded.
5) Inspect and test the pressure relief valve (PRV), expansion vessel and inlet valve for correct operation and to rule out pressure-related issues.
6) Check thermistor sensors, wiring and connectors for degradation or intermittent faults and replace faulty sensors if required.
7) If necessary the engineer will check control board error logs, replace the fan, thermistor or other failing components, and perform a full safe combustion and leak test after repair.
When to call a professional:
1) If the fault returns after one safe reset, you should contact a Gas Safe registered heating engineer. Do not keep resetting the boiler and using it while it repeatedly trips.
2) Call a professional immediately if the boiler locks out as soon as it is switched on, if there are loud kettling noises, visible leaks, persistent loss of pressure, or if you are unsure of any of the checks above.
What to tell the engineer when you call:
1) Your boiler model (Vaillant EcoTEC Pro / EcoTEC Plus), the exact error code F.20 (Safety Switch Off), whether the code appears immediately or after running, any noises heard (kettling/whistle), the boiler pressure reading, whether you could reset it and what happened after reset, and any recent changes (new radiators, system work, freezing weather). Taking photos of the display and any external pipework or the flue outlet can help.
Final note: F.20 is a protective shutdown indicating an overheat condition or temperature-sensor problem. Basic visual checks and a single reset are reasonable, but diagnosis and repair of the causes (pump, fan, heat exchanger, PRV, thermistor or control faults) must be carried out by a Gas Safe registered engineer for safety and to maintain warranty and compliance.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Pro / EcoTEC Plus.