Error S.15

Safety first

1) If you smell gas, or see flames, soot, or hear unusual mechanical noises, turn the gas off at the meter, isolate electrical power to the boiler if safe to do so, leave the property and call the gas emergency number and a Gas Safe engineer. Do not attempt internal repairs.

2) Before doing any checks, switch the boiler off using the power button and wait 10–20 seconds. Only perform visual and non-intrusive checks when the boiler is cold. Do not open the boiler casing — that work requires a qualified engineer.

Initial homeowner checks you can do

1) Reset the boiler: press the reset button once and observe the display. If S.15 appears just once and does not return, monitor the boiler for a day — it may have been a temporary glitch.

2) Note when the code appears: after hot water demand, after filling/repressurising, after a power cut, or continuously. Record times/duration and any noises. This information helps the engineer.

3) Check for obvious causes: ensure no taps are left slightly on causing repeated DHW calls, ensure the heating controls/timers are set correctly, and ensure electrical supply to the boiler is stable (no recent power surges or frequent power loss).

4) Check the boiler pressure on the display: if pressure is extremely low or very high it can cause unusual behaviour. Repressurise only if you know how and follow the manual instructions.

Diagnostic checks (homeowner-friendly)

1) Observe pump/fan behavior: when a hot tap is turned off, listen—do the pump and fan stop after a short run-on (a few minutes)? If they keep running indefinitely, that supports S.15 being valid. Note whether the burner is running or only pump/fan.

2) Check for display messages or other fault codes alongside S.15 (e.g., F32 fan, F75 pump or pressure sensor). Multiple codes together usually indicate a specific failed component and should prompt calling an engineer.

3) Power-cycle test: isolate mains to the boiler for 1–2 minutes and then restore power. If the code does not reappear immediately and the system behaves normally, it could have been an electronic glitch. If S.15 returns repeatedly, stop further attempts to reset and contact a pro.

What an engineer will check and do (do not attempt yourself)

1) Verify fault history and perform live diagnostics on the control board to see why the DHW sequence overran.

2) Test relays and control outputs for the pump and fan (a stuck relay on the PCB can hold those devices on).

3) Inspect and test the pump (electrical supply, current draw, mechanical seizure/free movement) and the flue fan for correct operation.

4) Check sensors and flow switches associated with DHW mode (flow sensor, temperature sensors, pressure sensors) and the diverter/three-port valve that switches between CH and DHW modes.

5) Clean or replace blocked filters or hydroblock parts that could be affecting flow detection, and inspect hoses and clips for leaks or degradation.

6) If required, replace faulty components: pump, fan, relays, PCB, sensors or associated wiring. Confirm correct software/firmware and settings.

When to call a professional

1) If S.15 recurs after a reset or if it is accompanied by other fault codes. 2) If pump or fan run continuously or the boiler shows unusual noises or smells. 3) If you are unable to identify a clear, temporary cause (like a one‑off power surge) from the simple checks above.

Final notes

Do not open the boiler or attempt electrical or gas repairs yourself. Collect as much information as you can (photos of the display, timing of events, other error codes, any recent work on the system) and contact a Vaillant-trained or Gas Safe registered engineer. Provide them with the S.15 status code and your observations — that will speed up diagnosis and repair.