Error S.25

Safety precautions:

1. If you smell gas, stop immediately: turn off the gas supply at the meter, do not operate electrical switches or the boiler, ventilate the area and call your gas emergency number and a registered engineer.

2. If the boiler is locked out or showing faults, do not open the boiler casing. Internal access to gas and electrical components must only be done by a competent engineer.

3. Before doing any basic external checks, switch the boiler off at the room thermostat and then at the isolating switch or fuse. Only touch external controls and visible pipes. If you need to isolate electrical supply, switch off the dedicated boiler fuse or the main consumer unit.

Initial homeowner checks you can safely do:

1. Note the exact code (S.25), what was happening when it appeared (hot tap, central heating, recently serviced), and whether the code clears on reset or recurs.

2. Try a single reset: switch the boiler off, wait 30 seconds, then switch it back on and request hot water. Observe whether the code returns. One reset that clears and does not recur can indicate a transient issue; repeated reappearance needs a professional.

3. Check basic boiler readings: confirm system water pressure is within the recommended range (typically around 1.0–1.5 bar on Vaillant units). Low or flashing pressure can cause other faults.

4. Listen for abnormal operation when calling for hot water: can you hear the pump running continuously, or the fan running loudly or continuously after hot water demand ends? Make a note of noises and timing.

5. Ensure external controls are not causing continuous demand: check room thermostat, cylinder thermostat (if fitted), programmer or smart controls that may be stuck calling for hot water.

Diagnostic and simple remedial steps (limited DIY):

1. Reset and monitor: after a controlled reset, run a hot tap and watch the boiler. If the pump/fan stops shortly after the hot water demand ends and no S.25 returns, monitor for a few days to confirm.

2. Check for blocked condensate or flue restrictions only if you are comfortable visually inspecting external pipes: ice or a blocked condensate trap can sometimes cause strange running behavior. Do not dismantle condensate traps unless competent.

3. Check external wiring and plugs you can safely access: ensure the boiler’s isolating switches and fused spur are secure, and that there are no obvious loose connectors on external system controls (e.g. room thermostat, cylinder thermostat, programmer). Always isolate electrical supply before touching wiring.

4. Cycle heating to see if the problem is DHW-only: call for central heating and then for hot water. If the overrun only happens in DHW mode, it points toward DHW control circuits, diverter/zone valves or DHW-specific sensors rather than the whole system.

5. Record evidence: take photos of the display, note the frequency/timing of the fault, record any unusual noises. This information helps the engineer diagnose quickly.

When to call a professional and what they will check:

1. Call a Gas Safe (or local qualified) engineer if S.25 persists, if the fault returns after reset, or if you are not comfortable with the steps above. Also call an engineer if there are other fault codes, loss of gas supply, electrical issues, or continued lack of hot water.

2. The engineer will safely open the boiler, check the pump and fan operation and speed control, inspect relays and wiring, test the PCB and sensor inputs (impeller sensor/fan sensor), and check for blockages or mechanical seizure of the pump or fan. They will use diagnostic menus/logs to see how long the pump/fan are running and why the controller flags overrun.

3. The likely professional fixes include repairing or replacing a faulty pump, fan, relay or wiring, replacing a faulty PCB or sensor, clearing blockages or addressing failed controls. The engineer will also test for correct operation and clear/verify the fault codes.

Final notes:

1. Do not attempt internal repairs on the boiler yourself: gas, electrical and sealed hydraulic systems are hazardous and must be worked on by a competent engineer.

2. When you contact an engineer, give them the boiler model, serial number, the S.25 code, what you observed, and how often it occurs. This speeds diagnosis and ensures they bring likely parts.

3. If the boiler shows any additional safety codes, loss of gas, or dangerous smells, isolate and get immediate professional help rather than attempting further checks.