Error E5 218

Safety precautions:

- If the boiler is locked out, switch the boiler off and leave it in a safe, ventilated area until you follow the steps below. Do not attempt gas-side repairs. If you smell gas, evacuate and call the gas emergency number.

- Isolate electrical supply (switch off at the isolator) and turn off external controls before opening casing — unless instructed by your boiler manual. Most internal checks and repairs must be performed by a Gas Safe registered engineer.

Initial homeowner checks (safe, simple checks you can do):

1. Note exactly when the code appeared and what the boiler is doing (CH only, DHW or both). Press the boiler reset button once as per the manual and observe. If the code clears and does not return, monitor for recurrence.

2. Check system pressure on the gauge; typical pressure is about 1–1.5 bar cold. If pressure is very low, follow your boiler manual to repressurise. Low pressure can cause pump issues and irregular operation.

3. Check that the room thermostat, timer/programmer and any TRVs are calling for heat or set correctly. If the boiler is not being asked to heat but pump/valves are incorrectly positioned it can cause odd behaviour.

4. Check radiators: are they heating normally? If most radiators are cold but boiler flow is hot, there may be a flow restriction or closed zone valve. Open radiator valves and ensure TRVs are not stuck closed.

5. Listen for the pump running when there is a demand. A seized or non-running pump can cause local overheating. You can sometimes hear or feel the pump running; if the pump is silent when it should run, do not force it — call an engineer.

If the code persists after the initial checks, do not attempt internal repairs. Prepare this information for the engineer: when the fault started, whether reset helped, system pressure, which radiators are hot or cold, and whether you have hard water or scale issues in the property.

What an engineer will (or should) do during diagnosis and repair:

- Visually inspect wiring and connectors to the flow thermistor and safety sensor, and check for loose, corroded or damaged leads. Repair or replace damaged wiring/connectors.

- Measure the NTC thermistor resistance at known temperatures with a multimeter to confirm the sensor is within specification. Replace the flow sensor if it is open, shorted or out of range.

- Check the safety temperature limiter and other temperature sensors (safety/limit sensor, return sensor) for faults or incorrect readings; replace if required.

- Confirm pump operation and flow rate, test pump speed, check for seized pump or blocked impeller and clear or replace pump if necessary.

- Inspect for partial blockages or scale in the heat exchanger or system pipework. If scale or sludge is suspected, the engineer may recommend a powerflush or localized flush, fit a magnetic filter, and advise on water treatment or scale prevention. If the heat exchanger is badly scaled or damaged, further remedial work may be required.

- Check for stuck zone valves, closed isolating valves or incorrect valve positions that restrict flow.

- If wiring and sensors are OK, the control PCB may be checked for intermittent faults; replacement or repair may be needed only by the manufacturer/qualified engineer.

Important notes and final actions:

- Replacing sensors, working on the gas valve, PCB or heat exchanger, and performing powerflushes must be done by a Gas Safe registered engineer. Do not work on the gas supply or internal gas components yourself.

- If the same error returns after basic resets and checks, stop attempting repeat resets and book a Gas Safe engineer — persistent overheating or sensor faults can damage the boiler.

- When the engineer completes the job, ask for a written report explaining cause, work done and any follow-up recommendations (e.g., magnetic filter, inhibitor, water softening, annual service).

Call a professional: If your boiler shows E5 218 after you have done the simple checks above, contact a Gas Safe registered heating engineer or Worcester Bosch accredited service agent to carry out the full diagnosis and safe repair.