Error D.2

Safety first:

- Do not open or attempt internal repairs on the boiler unless you are a competent, Gas Safe qualified engineer. Isolate electrical supply and gas supply before any internal work. When checking connectors, turn power off and wait until the appliance is safe to touch.

- If you smell gas, evacuate, ventilate and call your gas emergency number immediately.

Initial homeowner checks you can do (no internal access required):

1) Record what you see: note the display codes and whether the boiler goes to S.7 (pump overrun) or S.8 (anti‑cycle) after burner off, and when the micro‑firing happens (time of day, thermostat changes, hot water demand, controller adjustments).

2) Check the room/stat and external controller behaviour: ensure the room thermostat or smart controller isn’t rapidly changing setpoints or running in an adaptive/learn mode that causes the boiler to request different flow temperatures.

3) Check system basics: confirm system pressure is correct (typically ~1–1.5 bar cold), all radiator TRVs/valves are open as required, and radiators are bled of air. Ensure pump is running (you can hear/feel vibration) when heating is demanded.

4) Power‑cycle the boiler: switch the front main switch off for 10–20 seconds then back on. That resets the anti‑cycle timer and can show whether restarts are then normal.

Specific diagnostic steps and practical fixes (safe checks for homeowners):

1) Use the boiler display Live Monitor to observe the heating flow setpoint vs actual flow temperature and the status sequence when it switches off. If the unit goes to S.8 (anti‑cycle) but you still see restarts sooner than expected, record timestamps to show apparent mismatch.

2) If you have an external controller with configurable output modes, try switching it from an analog/modulating output to a simple on/off (two‑point) mode or set a stable fixed flow temperature on the controller. Modulating/analog controllers that change requested flow temperature frequently are a common cause of short/micro‑firing.

3) Try a stable higher flow temperature temporarily: if the boiler’s calculated anti‑cycle time is very short at low flow requests, increasing the target slightly (for example to the boiler front dial max allowed or a higher set point on the controller) can increase the effective anti‑cycle time and reduce micro‑firing — this is a diagnostic check rather than a fix.

4) Check for sensor/wiring symptoms: look for fault codes (e.g., F codes for NTC issues) and any obvious loose external plug connections to the controller. Do not open the casing; you may check accessible plug‑in room/stat connectors or controller wiring at the thermostat end if you are competent.

5) Pump and hydraulics: ensure the boiler pump is not set too low and that there are no closed service valves or blockages causing rapid temperature swings at the flow sensor. If radiators take very long to heat and burner cycles off early, a slow pump speed or flow restriction can contribute.

6) If you suspect the boiler is not entering S.8 and is instead showing S.7 pump overrun, the boiler may be stopping because of flow/return delta limits or the hydraulic temperature sensors are triggering shutdown; note these patterns to give to the engineer.

When to call a professional (must do):

- If you want d.2 adjusted (changing the maximum anti‑cycle parameter), ask a Gas Safe engineer. Accessing and changing diagnostic parameters should be carried out by a qualified engineer who understands your heating system dynamics.

- If you see persistent fault codes related to NTC sensors, mass flow, flame detection, temperature limiter trips or any fault that repeats after resetting, call a Gas Safe engineer.

- If you suspect wiring, PCB, pump replacement, or mechanical blockages, or if the boiler repeatedly short‑cycles despite controller and system checks.

Why a pro is required and what they will do:

- A Gas Safe engineer can safely access diagnostic menus and change d.2 and other parameters, interpret the internal anti‑cycle table, tune pump speed, verify flow/return NTC resistances, test wiring and replace sensors if faulty. They will examine controller-to-boiler interactions and can reconfigure the controller (two‑point vs analog) or adjust installer settings to stop micro‑firing without compromising safety or efficiency.

Important notes:

- D.2 itself is not an emergency fault but frequent micro‑firing should be investigated to avoid reduced efficiency and premature wear. Avoid repeatedly resetting or bypassing safety timers as a permanent workaround.

- Pressing the main switch will reset/cancel the current anti‑cycle timer temporarily, but this is only a diagnostic action; do not use it as a regular fix.

If in doubt, document the pattern (times, display codes, when micro‑firing occurs) and contact a Gas Safe registered heating engineer to inspect, diagnose and, if needed, adjust d.2 or repair sensors/electronics.