Error D.2

Safety precautions:

1. Do not open the sealed boiler casing or attempt gas or electrical repairs unless you are a registered Gas Safe engineer. Working inside a gas boiler is hazardous.

2. For simple checks you may view the display and menus with the boiler powered on, but avoid touching internal components. If you must isolate the appliance for any reason, turn off the electrical supply at the external switch and close the gas isolator cock only if you are competent to do so.

Initial homeowner checks:

1. Note the exact display messages and any fault codes (F.xx) before resetting. If there are F-codes or persistent faults, stop and call an engineer.

2. Confirm the boiler is configured for the system controller in use (e.g. VRC 430). Check that the room controller settings (mode: modulating vs two-point, minimum flow temperature, heating curve, AUTO_OFF) match your intended setup.

3. Make sure system valves are open, the system pressure is correct, and radiators are bled of air. Check the pump is running (you can often hear it) and there are no obvious flow restrictions.

Specific diagnostic steps you can perform (view-only and non-invasive):

1. Enter the boiler diagnostics and note these values while you reproduce the micro-firing behaviour: d.2 (current maximum anti-cycle parameter), d.5 (target flow temperature), d.67 (remaining anti-cycle time), d.1 (pump overrun), and watch the status codes S.x (S.7 = pump overrun, S.8 = anti-cycle active, S.2..S.5 sequence for start-up). Record timestamps of when the burner stops and when it restarts.

2. Observe whether the boiler goes to S.7 only, or S.8. Anti-cycling applies only if the boiler actually enters the anti-cycle state (S.8) — if it never reaches S.8 then the d.2 parameter will not delay restarts.

3. If the target flow temperature is changing dynamically from a room controller, note how quickly it drops after burner stop. Rapid drops in requested flow can reduce the effective remaining anti-cycle time and allow immediate re-starting.

Simple non-invasive fixes to try:

1. On the room controller (VRC 430 or similar) try switching from analog/modulating mode to a two-point/static flow mode or increase the minimum flow temperature so the controller does not reduce the flow demand so quickly. This can reduce rapid target changes that upset the anti-cycle calculation.

2. If you are comfortable checking menus, view d.2. Increasing d.2 (the maximum anti-cycle value) can lengthen calculated lockouts and reduce micro-firing; decreasing it shortens the lockout. Only change this if you understand the impact and preferably after discussing with a qualified engineer.

3. Check the pump overrun parameter d.1 and system pump behaviour. Ensure pump speed/operation is correct so heat is removed from the heat exchanger as expected — incorrect pump operation can change flow/return temps and trigger early shutdowns.

When to suspect a fault and call a professional:

1. If the boiler does not enter S.8 when it should, or the anti-cycle timer (d.67) behaves inconsistently, or the boiler restarts immediately despite the diagnostics indicating remaining anti-cycle time, this can indicate a control, sensor or PCB fault.

2. If there are any F.xx fault codes (sensor interruption, thermistor faults, mass flow sensor, overheating protection etc.), stop attempting DIY adjustments and call a Gas Safe engineer.

3. If changing controller behaviour or d.2 does not stop the micro-firing, you likely have a wiring issue, failed NTC thermistor (flow or return), faulty controller communication (VRC/remote stat), or a PCB/pump fault — these require an engineer.

What the professional will check/do:

1. A Gas Safe engineer will verify sensor resistances (NTC flow/return), wiring and connectors, check for blocked heat exchanger, airlocks or closed isolation valves, test pump operation and speeds, and review the controller communication and parameters.

2. They can safely change internal parameters (d.2, d.1, pump settings), replace defective thermistors, harnesses or PCB components, and prove correct operation after adjustements.

3. They will also advise on the correct balance between anti-cycle time and system responsiveness for your heating system to avoid excessive wear or poor comfort.

Final note: Viewing and recording diagnostic values and adjusting user-level room controller settings are reasonable DIY actions. Changing internal boiler parameters or doing any repairs or part replacements must be performed by a qualified Gas Safe engineer. If you are unsure at any stage, call a professional and give them the diagnostics you recorded (d.2, d.5, d.67, status codes and sequence timings) to speed diagnosis.