Error F.11

Safety first

1) If you smell gas or suspect a gas leak, leave the property immediately and call your gas emergency number. Do not try to fix gas faults yourself.

2) Before doing any checks, switch off the boiler electrically at the isolator and at the mains if you will be touching valves or fittings. Allow the boiler to cool.

3) Do not attempt to open or repair internal boiler components (gas, burner, combustion chamber, heat exchanger, pump internals) unless you are a Gas Safe registered engineer.

Initial, safe homeowner checks

1) Note the exact error code and any other display messages. Try a single reset of the boiler following the manufacturer instructions (usually by switching off and on or using the reset button). If the code clears but returns, do not keep resetting repeatedly.

2) Check the boiler water pressure gauge. For most domestic systems the target is around 1.0–1.5 bar when cold. If pressure is very low (well below 1.0 bar) that can cause flow problems. If you are confident and familiar with your filling loop, you can top up to the correct pressure and then reset once. If unsure, stop and call an engineer.

3) Check visible isolation/service valves on pipework into and out of the boiler are fully open (flow and return valves). These are often under the boiler; if they are partly closed this will restrict flow.

4) Check radiators: if most radiators are cold or only the top is cold, that indicates poor circulation or airlocks. Bleed radiators to remove trapped air (turn off boiler power first and follow radiator-bleeding safety steps). After bleeding, check system pressure and re-pressurise if needed.

5) Listen to the boiler when trying to run CH demand: can you hear the pump running (a low hum or vibration)? If the pump is silent when it should be running that points to a seized pump or electrical/pump fault.

Specific diagnostic steps a careful homeowner can consider (only if comfortable and confident)

1) Check any accessible inline filters, magnetic filters or strainer baskets on the system (usually fitted externally on pipework or at the filling loop). Isolate the water supply, relieve pressure by opening a hot tap, then remove and inspect the strainer for debris. Have a container and towels ready. If in doubt stop and get a pro.

2) If the boiler has an external magnetic filter on the return, check and clean it per the filter manufacturer instructions (isolate and depressurise first). This can remove sludge that blocks flow.

3) If you are able to access the boiler pump head cover safely (not recommended for most homeowners), you can check for obvious seized rotor or foreign objects; however, do not dismantle the pump motor or electrical connections. If the pump feels seized or is difficult to rotate by hand, call an engineer.

When to call a professional

1) If the F.11 code remains after the basic checks (reset, pressure, valves, bleed radiators, clean accessible filters), stop and contact a Gas Safe registered engineer. Internal blockages, heat exchanger blockages, pump replacement, pump servicing, or internal sensor faults require trained diagnostics and safe handling of gas and sealed components.

2) Tell the engineer the exact fault code, the boiler model, the system pressure reading, any recent work or symptoms (cold radiators, noises, leaks) and what you have already tried. That will speed diagnosis.

Final notes and cautions

1) Repeatedly forcing resets can hide progressive faults and risks further damage; use reset only as a single diagnostic step.

2) Do not attempt to work on gas or combustion components, or dismantle the boiler casing and internals unless you are a Gas Safe registered engineer. Many causes of F.11 are mechanical (pump, sludge, heat exchanger) and need professional tools and certification to repair safely.

3) Getting a professional diagnosis promptly is the safest course when the fault does not clear after the simple checks above.