Error D.8

Safety precautions:

1) Do not open the sealed boiler combustion chamber or attempt gas or heat-exchanger work yourself. 2) Do not touch live mains wiring inside the boiler unless you are a qualified electrician/engineer. 3) If you are unsure about any step that involves wiring, switching off mains or joining wires, stop and call a Gas Safe engineer. 4) Do not repeatedly reset the boiler more than a few times — repeated resets can mask an underlying issue and may damage components.

Initial homeowner checks (safe, no tools required):

1) Look at the boiler display: note any other symbols (central heating symbol, summer mode symbol, fault lamp). If the CH symbol is absent or a little radiator symbol has an X/Off, the boiler is not being asked to heat. 2) Check your room thermostat: set it higher than room temperature and switch it to HEAT/ON mode. If it’s programmable, make sure the schedule currently permits CH. 3) Check the programmer/timer: ensure the CH is turned on in the current time period. 4) If you have a wireless thermostat, check batteries and that the thermostat is paired and showing it is calling for heat. 5) Check TRVs on radiators are not all closed. 6) Make sure the boiler is not in summer mode (this disables CH but leaves hot water working). 7) Try a simple reset: press the boiler reset button once (short press) and wait for the boiler to attempt to fire. Do not repeatedly reset more than 2–3 times.

Simple diagnostic steps to identify source of problem:

1) Manual boiler heating demand test: On the boiler controls set the heating flow temperature to a value above current water temperature (e.g. 50–60°C). If the boiler fires and heats, the boiler internal parts are fine and the problem is almost certainly an external control (thermostat, programmer, wireless link, zone valve). If the boiler still does not fire and D.8 remains, move to the next checks.

2) Wireless thermostat check: replace batteries, place the thermostat next to the boiler (or base) to ensure it is in range and call for heat — confirm on boiler display whether CH demand appears. If pairing fails or it looks intermittent, the wireless link may need resetting or an engineer to fault-find.

3) Check motorised valve(s) or zone indicators: if your system has a 2- or 3-port motorised valve, check the valve head position indicator (some show ‘mid’ or ‘heat’). If a valve is stuck or not moving to the CH position, it can prevent a heat call. Do not remove valve caps or dismantle; only observe position. If stuck, call engineer.

4) Visual wiring inspection (non-invasive): With the boiler powered and the outer cover closed, visually inspect thermostat wiring where it enters the boiler (external, not inside the sealed chamber) for obvious damage, corrosion or loose clips. Do not prod or disconnect live terminals unless you are qualified.

5) Controlled bypass test (ONLY if you are competent and comfortable with low-voltage controls): isolating the boiler mains is recommended before any contact with internal terminals. If you know where the low-voltage thermostat wires terminate at the boiler, and you are competent with basic electrical safety, you can temporarily short the two thermostat wires together to simulate a call for heat (this should be done with power off when connecting, then power restored). If the boiler fires when the thermostat is bypassed, the thermostat or its wiring is at fault. If you are not fully confident with this procedure, do not attempt it — call a Gas Safe engineer.

When to call a professional and what they will check:

1) Call a Gas Safe registered heating engineer if the D.8 message remains after you have confirmed programmer/thermostat settings, replaced batteries, checked wireless links, and tried a reset. 2) A qualified engineer will safely test continuity and voltage on the external control terminals, inspect and test motorised valves and zone controls, check eBUS/communication with external controllers, and test the boiler PCB inputs for correct detection of the external call-for-heat. 3) If faults are found with wiring, the motorised valve, the external controller, the eBUS communications or the boiler PCB, these require diagnostic equipment and qualified replacement/repair.

Final notes:

- D.8 is usually not an emergency or combustion safety fault, but it will leave you without central heating while active. - If you have no heating in cold weather and cannot resolve the control issue, contact a Gas Safe engineer promptly. - Avoid attempting mains or gas-side repairs yourself; for any wiring, valve or PCB work, always use a qualified professional.