Error S.20

Safety precautions:

1) If you smell gas, do not touch the boiler, do not operate electrical switches, open windows and doors, turn off your gas supply at the meter if safe to do so and call the gas emergency service immediately. Evacuate the property if necessary.

2) Do not remove covers or attempt repairs that expose the gas valve, burner, ignition electrode or PCB. These parts are live and dangerous. Only a Gas Safe qualified engineer should work on gas and high-voltage components.

3) If you temporarily isolate the boiler for safety (e.g., main switch), be aware that in freezing weather you may need to keep it powered to avoid frozen condensate or pipework; follow the manufacturer’s guidance or call an engineer.

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

1) Check the boiler display and note the exact code (S.20) and any additional fault or F.xx codes shown. Take a photo of the display for the engineer.

2) Try a single fault clear/reset: press and hold the fault clearance/reset key for one second (as per the manual) to prompt another ignition attempt. If the boiler restarts and runs normally, monitor for recurrence.

3) Check that the gas supply to the house is on. Ask neighbours or check outside if there is a known gas supply issue. If you suspect no gas supply, contact your gas supplier.

4) Check mains electricity to the boiler and the boiler’s external isolator fuse/switch is on.

5) Check the condensate pipe/tundish is not blocked or frozen (common in cold weather). A blocked or frozen condensate can cause faults. Thaw frozen condensate with warm (not boiling) water if safe and accessible.

6) Ensure the boiler water pressure is within the normal range (typically around 1.0–1.5 bar on many Vaillant models). If pressure is below 0.8–1.0 bar, top up following the manufacturer instructions for your filling loop. Low pressure can cause other faults and prevent correct operation.

7) Verify DHW demand: open a hot tap to ensure the boiler is actually being called for DHW. Confirm any room or cylinder thermostat/controller settings are not preventing burner operation.

8) Ensure air intake and flue terminals are not blocked by debris, snow, or bird nesting.

Specific diagnostic steps and recommended fixes (actions to try and information to gather for an engineer):

1) After a reset attempt, if S.20 returns, record whether any other error codes appear (e.g., F.28, F.27, F.xx) and the sequence of events (e.g., shows S.20 immediately when DHW is demanded, or after a short time running).

2) If the fault occurs only during DHW but not heating mode, note that detail — it helps the engineer target burner/flow-switching, diverter or sensor issues.

3) Check for signs of ignition activity: do you hear the boiler attempting to fire (fan running, sparking)? If you hear multiple ignition attempts with no flame, that suggests ignition electrode/lead or flame detection problems or gas valve not opening.

4) Check external wiring and connectors that you can access without opening sealed covers: make sure the boiler’s low-voltage control cables (room thermostats, timers) are seated and not damaged. Do not undo sealed PCB plugs.

5) If condensate was blocked/frozen and thawing cleared the fault, monitor for recurrence. A recurring blocked condensate may need routing or insulation improvements by an engineer.

6) If there are signs of water leaks, corrosion in the boiler casing or around electrical connections, switch the boiler off and call an engineer — moisture can cause false ignition/flame detection faults.

When to call a professional and what to tell them:

1) Call a Gas Safe registered engineer if S.20 is not cleared after a single reset, if the fault repeats, if there are additional F.xx fault codes, if you smell gas, if you are unsure about the gas supply, or if you are uncomfortable performing any of the checks above.

2) When you contact the engineer, give them: boiler model and serial number, exact error S.20 and any other codes shown, what you’ve already checked (power, gas supply, pressure, condensate, reset attempts), and whether the fault is constant or intermittent. Photos of the display and of the condensate/pressure gauge can be very helpful.

3) The engineer will safely inspect ignition electrode and leads, gas valve operation, flame detection circuit, fan and air/flue route, wiring harness and PCB diagnostics. They have the appropriate tools and certification to test and replace combustion parts and to check safe operation.

Final notes and precautions:

1) Do not attempt to replace ignition electrodes, gas valves, PCBs or fuses yourself. These are gas or live-electrical components requiring qualified intervention.

2) If the boiler locks out frequently, cease using it for DHW until a Gas Safe engineer has diagnosed and repaired the underlying cause. Repeated lockouts indicate a persistent fault that could affect safety and reliability.

3) Keep a record of faults, resets and engineer visits — this helps with diagnosis and with warranty/service history.