Error D.8

Safety precautions:

1) If you smell gas, leave the property immediately, do not operate switches, and call the gas emergency number. 2) Before opening the boiler cover or touching electrical wiring: switch off the boiler at the isolator and turn off the mains. If you are not competent with electrical work, do not open the boiler or touch terminals — call a professional. 3) Do not attempt to repair or alter gas components unless you are Gas Safe registered.

Initial homeowner checks (simple, safe, do first):

1) Check your room thermostat or smart thermostat: make sure it is set to a temperature higher than room temperature and not in Off, Frost, or Holiday mode. Replace batteries in wireless thermostats or wall stats if low. 2) Check the programmer/timer: ensure central heating is enabled on the schedule and the CH is not turned off or set to summer mode. 3) Check radiator thermostatic valves (TRVs): ensure they are not closed, and try opening one fully to see if heating calls change. 4) If you have a separate cylinder/stat or zone controls, check those are calling for heat. 5) Reset the boiler using the reset button as described in your manual; observe if D.8 appears again immediately or only when certain controls change.

If D.8 appears but system behaves normally (boiler fires when heat is needed):

1) No urgent action is required — D.8 can be an informative diagnostic showing an external demand. 2) Monitor: note when it appears (e.g., when thermostat calls, at certain times) and keep a log for the engineer if it recurs unexpectedly.

If D.8 appears and heating does not run as expected, or the code is persistent/unwanted:

1) Confirm whether the boiler shows any other fault codes or status (pump running, fan running, flame symbol). If additional faults are present, note them for the engineer. 2) Check the wiring and connections on any external control devices you can access safely (e.g., thermostat faceplate). Look for loose wires or corroded terminals. Do not touch wiring inside the boiler unless competent. 3) If you are comfortable and knowledgeable about low-voltage checks: check whether the external control output is providing a call voltage (typical values are 24V AC on many systems). Only perform this if you know the system voltage and have experience with a multimeter. If unsure, stop and call a pro.

Safe diagnostic jump/test (only if you are competent with electrical testing):

1) Locate the thermostat/central heating call terminals as described in your boiler manual and confirm the control type/voltage. 2) With mains/boiler isolated, you can visually inspect terminal condition. 3) If you know the terminals are low-voltage and are competent, you can simulate a call for heat by shorting/jumping the thermostat call terminals briefly to see if the boiler responds (restore power and observe). If the boiler responds correctly to a direct jump, the external thermostat or wiring is the likely culprit. If the boiler does not respond to a direct jump, the fault may be internal to the boiler and requires an engineer.

When to call a professional and what to tell them:

1) Call a Gas Safe registered heating engineer if: the code persists despite normal settings, you cannot identify the external control fault, the boiler will not fire when it should, or you are not comfortable performing the electrical checks. 2) Also call a qualified electrician if you suspect mains wiring faults to external controls. 3) When you call them, give the engineer the exact boiler model, the D.8 code, any other displayed codes or symptoms, what checks you have already done (thermostat settings, batteries replaced, reset attempted), and whether the boiler responds to a simulated call.

Additional notes and warnings:

1) Repeatedly resetting the boiler without addressing the root cause is not advised; it can hide intermittent faults and may cause further issues. 2) Do not open or work on gas valves, burners, or sealed components; leave these to Gas Safe engineers. 3) Taking clear photos of the wiring terminals and control labels for the engineer before they arrive can speed up diagnosis.

Summary: D.8 usually indicates an external control is calling for heat and is often not a serious boiler fault, but if it is unexpected or accompanied by lack of heating you should follow the safe checks above and call a qualified heating engineer or electrician to carry out live diagnostics and repairs.