Error S.24

Safety precautions:

- If you smell gas: evacuate the property immediately, do not operate electrical switches, and call the gas emergency number for your area. Do not try to use or relight the boiler.

- If the boiler displays any safety lockout or you see signs of overheating (scorching, unusual smells, pipes extremely hot), switch the boiler off at the mains and isolate gas if you know how to safely do so; otherwise leave it off and call a professional.

- Do not open the boiler casing or attempt to access electrical, ignition or gas valve components yourself unless you are a qualified Gas Safe engineer.

Initial checks a homeowner can safely do:

1) Note the exact code(s) shown (S.24 and any F or other S codes), when they appear and what the boiler was doing (starting for hot water, running central heating, immediately after a reset). Take photos.

2) Check the boiler display and any reset button guidance. If the boiler shows a temporary status, try a single reset: switch off at the programmer, then turn the boiler off at the mains for 1–2 minutes, switch back on and allow a restart. Do not repeatedly reset if the fault returns.

3) Check house gas supply and other gas appliances. If no other gas appliances work, contact your gas supplier — do not attempt internal repairs.

4) Check the boiler pressure gauge. If pressure is very low (well below 1.0 bar on many Vaillant models) this can cause circulation/overheat protection events. If you know how to use the filling loop safely and you have a readable pressure gauge, bring pressure up to the recommended level (usually around 1.0–1.5 bar) and then reset. If you’re unsure, stop and call an engineer.

5) In cold weather check the condensate discharge pipe for freezing. A blocked condensate can cause ignition and safety faults — thaw carefully with warm (not boiling) water if you suspect it is frozen.

Specific diagnostic steps to report to an engineer (do not attempt these unless qualified):

- If S.24 occurs with an overtemperature safety code (examples: F20, F24, F25), this commonly means flow/return sensor or circulation fault (pump seizure, blocked heat exchanger, 3‑port valve or trapped air). An engineer will check NTC sensors, continuity of wiring, pump operation and for blockages or scaling in the heat exchanger or system.

- If S.24 appears with flame detection/ignition related codes (F27, F28, F29 or similar) this suggests issues with ignition electrode/lead, gas valve leakiness or PCB/flame detection circuit faults. An engineer will test the ionisation/flame signal circuitry, ignition transformer and the gas valve for correct operation and leak tightness.

- If flue temperature or flue limiter has triggered, the engineer will inspect the flue, flue gas temperature limiter (STB), wiring harness and check for flue blockage or recirculation.

What you can try before calling a pro (safe, limited actions):

- Perform one controlled reset after the boiler has cooled down and only if there are no gas smells or visible damage.

- Top up water pressure if it is clearly low and you know how to use the filling loop correctly.

- Thaw a frozen condensate pipe with warm water if accessible and safe to do so.

- Bleed a radiator if you suspect airlocks in your system to improve circulation.

When to call a professional and what to tell them:

- Call a Gas Safe registered engineer if the fault returns after a single reset, if you observed overheating, persistent ignition or flame faults, or if you smell gas. Do not attempt internal repairs to the ignition system, gas valve, PCB or sensors.

- Provide the engineer with the boiler model, serial number, exact error codes (S.24 and any F or S codes), the sequence of events, whether the fault occurred on heating or hot water, the results of your pressure check and any reset attempts. This information speeds up diagnosis.

Final note: S.24 itself can be just a status message for burner ignition, but when it coincides with safety or flame detection errors it points to potentially serious combustion, sensor or circulation problems that need a qualified Gas Safe engineer or Vaillant service support to diagnose and repair safely.