Error 20

Safety precautions:

1) If you smell gas or see a major water leak, turn the gas off at the supply, turn off electrical power to the boiler at the fused spur and call your gas emergency number or a Gas Safe engineer immediately.

2) For any work that involves opening the boiler cover, testing live circuits, changing sensors, or touching gas parts, get a qualified Gas Safe engineer. Do not attempt complex electrical or gas-side repairs yourself.

3) Allow the boiler to cool before touching pipework or components and isolate electrical power at the isolator switch or fused spur for any internal inspections.

Initial homeowner checks you can safely do:

1) Reset the boiler: switch it off, wait 30 seconds, and switch back on or use the boiler reset button. Note whether the code returns and when (immediately or only when CH/DHW is called).

2) Check the system pressure gauge on the boiler. Compare with the pressure recommended in your manual (typical cold pressure is often around 1–1.5 bar, but confirm your model’s spec). If pressure is very low, the boiler may behave oddly. If you know how to repressurise safely with the filling loop, you may do so; if not, call an engineer.

3) Look for visible signs of leaks, corrosion, or water on the floor around the boiler and pipework. Check that the central heating flow and return valves (at the boiler and any external isolation valves) are open.

4) Bleed radiators to remove trapped air (airlocks can stop flow and confuse the sensor). After bleeding, re-check system pressure and top up if needed.

5) Listen for the pump when the heating calls: you should hear/feel the pump running briefly. If the pump is quiet when heat should be flowing, that is a likely cause to report to your engineer.

Specific diagnostic and fix steps (for a homeowner to attempt without opening live electrical parts):

1) Note the exact conditions when code 20 appears (on demand for CH, on DHW, after a restart). This information is useful for the engineer.

2) Visually inspect any accessible pipework and the external wiring harness and connectors to the boiler (do not disconnect anything while the boiler has power). Look for loose plugs, corroded connectors or damaged cables.

3) If you are comfortable and it is clearly described in your manual, you can power down the boiler, remove the cover and visually inspect the flow sensor wiring and connector for corrosion or disconnection. Only do this after isolating mains power. If you find a loose connector, re-seat it securely, replace the cover and power up to see if the code clears. If you are unsure, stop and call an engineer.

4) Clean any accessible sensor tip gently if it appears dirty or covered in scale (only if it is easy to access and the boiler is cold and powered off). Some thermistors can be cleaned rather than replaced, but do not force or damage the sensor.

5) Check and clean system filters (magnetic or strainer) if your system has them and you can access them safely. A blocked filter can reduce flow and trigger the fault.

6) If the boiler shows low pressure and you know how to use the filling loop, repressurise to the correct cold pressure, secure the filling valves, and then run the system. Re-check whether code 20 returns.

When to stop and call a professional:

1) If the code 20 remains after the basic checks above, if you find damaged wiring, if you need to test sensor resistances, or if the fault appears intermittent, book a Gas Safe registered engineer. Replacing sensors, pump fault diagnosis, or replacing electronic control boards (PCBs) requires professional skills.

2) Call urgently if you smell gas, the pressure exceeds safety limits, water is coming from the pressure relief valve, or you cannot restore heated water when needed.

What the engineer may do:

The engineer will power-up tests the flow sensor and wiring, measure thermistor resistance against ATAG values, check pump operation and flow rates, inspect for blockages or scale in heat exchanger or filters, clean or replace the flow sensor if required, and if necessary test or replace the boiler PCB. They will also run the boiler to confirm the fault is cured and check system pressure and safety devices.

Summary: do basic safe checks first (reset, pressure, valves, bleed radiators, visual wiring check). Do not attempt internal electrical or gas work — if simple checks don’t clear the code, or if you find damaged components, call a Gas Safe registered heating engineer to diagnose and repair the flow sensor or associated components.