Error S.27

Safety precautions:

1) If you smell gas or suspect a gas leak, leave the property immediately, ventilate if safe to do so, do not operate switches or phones near the leak, and call the gas emergency number. 2) Do not open sealed boiler panels, work on gas valves, or attempt internal electrical repairs unless you are a qualified gas engineer. 3) Isolate mains electricity to the boiler before removing any covers if you are checking visual wiring or connections, and only do this if you are competent and the boiler manufacturer guidance allows access for end users. 4) If in doubt, turn the boiler off and call a qualified engineer.

Initial homeowner checks (safe, non-invasive):

1) Note exactly what the display shows when the pump runs (tap symbol, any other codes such as S.17 or S.31), and how often it cycles. Take photos or a short video — this helps the engineer. 2) Check DHW settings: on the control panel, check if "comfort"/preheat for hot water is enabled. If comfort/preheat is ON, disable it and observe whether the pump stops running. 3) Check timers and external controls: ensure any external programmer/room thermostat/Smart control isn’t scheduled to call for DHW. 4) Check all hot-water taps, showers, and appliance isolating valves for tiny leaks or slow drips that might cause intermittent flow. Even a small leak or a partially open hose could trigger a flow sensor. 5) Reset the boiler: press the reset button and watch behavior for a few minutes. Note if the condition returns and on what cycle.

Specific diagnostic and basic fix steps (do not open gas sections; stop and phone an engineer if you are not confident):

1) Confirm behaviour pattern: with hot water taps closed, turn off the boiler for 30 seconds and then turn it back on and watch the display. If the tap symbol and pump start immediately and repeatedly, the boiler’s control is getting a false DHW call. 2) Turn comfort/preheat off in settings if not already off – this removes intentional periodic preheat cycles and can confirm if the feature was the cause. 3) Isolate external callers: disconnect or turn off any external HW controller or smart relay (if safe to do so) to ensure there is no external electrical signal calling DHW. 4) Check for plumbing causes: inspect visible hot-water pipework for leaks or circulating pumps/recirculation lines that could be running and returning water. If you have a domestic hot water recirculation system, ensure its timer/controller is off. 5) Electrical reset and monitor: after checking settings, leave the boiler powered and monitor for repeated S.27 cycles. Keep a log of times, frequency and any other codes.

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

1) Call a Gas Safe / Vaillant-approved engineer if the pump still cycles with no genuine hot water demand after you’ve done the checks above. Do not attempt to replace or remove the pump, PCB, diverter valve, or sensors yourself. 2) The engineer will check: DHW flow sensor/flow switch operation, temperature sensors for DHW, the hot-water demand input and wiring, pump relay on the PCB, the pump itself (electrical and mechanical), the diverter valve and actuator (sticking/position), any comfort/preheat functions in software, and the boiler wiring harness and control board for intermittent faults. 3) Provide the engineer with your notes, photos/videos, and the exact codes and symbols displayed; that speeds diagnosis.

Other notes and practical tips:

1) If the noise or continuous running is disturbing you at night, it is safe to turn the boiler off overnight but remember this disables hot water and heating until power is restored. 2) Do not ignore persistent running — it will shorten pump life and may mask an intermittent fault that will eventually cause a pump or control failure. 3) Keep a record of serial number, installation/last service date and any error/status codes — the engineer may need this for warranty or part selection. 4) This guidance intentionally does not include repair-cost estimates; call a qualified engineer for a full diagnosis and repairs.

Bottom line: S.27 indicates a pump overrun condition linked to DHW. Start with user-level checks (comfort setting, leaks, timers, reset). If the pump continues to run without a clear reason, contact a qualified Gas Safe / Vaillant-competent engineer to diagnose flow sensors, wiring, relays, diverter valve and pump — internal electrical or gas work must not be attempted by an unqualified person.