Error S.17

Safety first

1) If you smell gas or suspect a gas leak: leave the building immediately, call the gas emergency number and do not operate electrical switches or the boiler. 2) For routine checks: switch the boiler off at the programmer and at the mains isolator before touching wiring or internal parts. Allow the boiler to cool. Do not attempt any gas, combustion chamber, or sealed-component work unless you are a Gas Safe / qualified engineer.

Initial homeowner checks (safe, no internal access required)

1) Confirm heating and hot water settings: check the boiler is in the correct mode (summer/DHW only or winter). If 'comfort' or 'preheat' is on, turn it off and see if the behaviour stops. 2) Watch behavior after a simple reset: press reset on the boiler, note if S.17 reappears and exactly when the tap symbol and pump run. 3) Check for visible water leaks in the hot water pipework and under the boiler; a continuous small leak or return flow can sometimes simulate demand. 4) Check system pressure on the boiler display; note normal pressure (around 1–1.6 bar when cold) and report if very low or fluctuating. 5) Check other controls: timers, room stats, smart controls, motorized valves or external relays that may be calling DHW.

Simple diagnostic tests you can safely try

1) Isolate DHW briefly: if you have an accessible hot water isolating valve (usually under a sink), close it and observe the boiler. If the pump stops and S.17 clears, this suggests the boiler is detecting a false flow (flow sensor or wiring) or an external leak/flow path. 2) Power-cycle test: switch the boiler off at the fused spur for 60 seconds and power back on. Observe whether the intermittent tap symbol/pump activity returns immediately. 3) Observe patterns: does the overrun only happen when the boiler is still hot (restarting hot) or continuously from cold? If only when hot, that can point to sensors or internal hydraulic/heat-exchange behaviour.

What different results suggest

1) Pump still runs with DHW isolator closed -> likely electrical/PCB, flow sensor false signal, or internal pump wiring/short. 2) Pump only runs when boiler hot and cycles to higher temp before S.17 -> could be temperature sensor (NTC) drift or heat exchanger issue causing incorrect temperature reading. 3) Pump runs and diverter valve clicking or not moving -> could be diverter/3‑port valve sticking or not seating correctly and confusing the control logic.

Things you should not do (and why to call a professional)

1) Do not remove the boiler casing, alter gas connections, or work on the burner, gas valve or sealed waterways unless you are qualified. 2) Do not repeatedly reset the boiler and run it indefinitely while S.17 persists; prolonged operation under fault conditions can damage the pump, heat exchanger or PCB.

When to call a qualified engineer and what to ask them to check

1) Call a qualified Gas Safe (or local competent) engineer if basic checks do not isolate the issue. Provide them with the exact behaviour, when S.17 appears, whether isolating the DHW stops the problem, and the boiler model/age. 2) Ask the engineer to check in this order: flow sensor operation and wiring, DHW temperature sensor (NTC), pump operation and motor bearings/impeller, diverter/3‑port valve movement and microswitches, any external control wiring, and the boiler PCB for faults. 3) Request that they look at the installer diagnostic menu/log for any correlated error data and test live signals rather than replacing parts blindly.

Final notes

S.17 is a status/protection message and not necessarily an immediate danger, but it should be diagnosed because it leads to component wear, noise and inefficient operation. Many causes are electrical/sensor related and need a competent engineer to test and replace components safely. Take notes/photos of the display and sequence of events to give the engineer accurate information before they visit.