Error D.24

Safety precautions:

- If the boiler has locked out or there is an urgent safety concern (smell of gas, burning smell, visible leaks), turn the appliance off at the mains and the gas supply and call your gas supplier or a Gas Safe engineer immediately.

- Do not open the boiler casing or attempt to work on gas or mains electrical components unless you are a qualified Gas Safe engineer. Electrical shock and gas hazards can be fatal.

- Ventilate the boiler area if you suspect any gas smell. Do not use naked flames or switch electrical equipment on/off near a suspected gas leak.

Initial homeowner checks you can safely do:

1. Observe and record: note the exact display text, whether the boiler is locked out, and any other fault codes shown. Note symptoms (no hot water, no heating, unusual noises, leaks).

2. Simple reset: try a single reset using the boiler reset button or switch off at the mains for 1–2 minutes and restart. Only reset once or twice—repeated resets can mask an underlying fault and may not be safe.

3. Check system water pressure: look at the pressure gauge. Cold system pressure is normally around 1.0–1.5 bar. If it is significantly below 1 bar the system may be low on water. If you are confident and have the boiler manual, you can top up using the filling loop until the pressure is in the correct range—but if you are unsure, leave this to an engineer.

4. Visual checks: look for obvious leaks, blocked condensate pipe (especially in freezing weather), frozen/blocked external condensate discharge, or blocked flue/ventilation at the terminal. Ensure vents around the boiler are not blocked.

5. Check whether radiators or hot-water outlets get warm and whether the pump is running (you may hear it or feel pipes warming). Note any abnormal noises from the boiler.

If D.24 appears but the boiler is otherwise normal:

- No immediate remedial action is usually required. Make a record of the code and monitor the boiler. If it does not reappear and the boiler runs normally, no further action is usually needed.

When to call a professional and what the Gas Safe engineer will typically check:

- Call a Gas Safe registered engineer if D.24 is persistent, if it appears with other fault codes, if the boiler has locked out, or if you have loss of heating/hot water or signs of overheating or leaks.

- The engineer will read the full fault log and service/diagnostic entries, then carry out systematic checks such as:

- Enter service mode and inspect logged diagnostics and fault history.

- Test flow and return NTC/thermistor resistances and wiring continuity (compare to expected values at known temperatures).

- Check the water pressure sensor (signal range), filling loop operation and internal pressure circuitry.

- Inspect and test the pump (rotation, flow, electrical supply) and check for airlocks or blockages in the system and heat exchanger.

- Test the 3‑port diverter valve operation if present and check for blockage or sticking.

- Inspect ignition system, electrodes and ignition leads if there are related flame/ignition symptoms.

- Check gas valve control signals and coil resistance, fan operation, flue temperatures and for any flue blockages or recirculation issues.

- Inspect PCB for communication errors, DSN/software assignment issues and any signs of moisture or damage.

- Check condensate trap/discharge and clear if blocked.

Final advice:

- D.24 by itself is typically a benign diagnostic entry, but if anything else is wrong with the boiler or the code is repeated, get a Gas Safe engineer to inspect and diagnose properly. For Vaillant-specific support you can also contact Vaillant customer service (the number in your boiler documentation or the Vaillant helpline). Do not attempt internal electrical, gas or sealed-system repairs yourself.