Error D.30

Safety precautions:

1) If you smell gas or suspect a gas leak: do not operate electrical switches, evacuate the property immediately and call the emergency gas supplier or emergency services from a safe location.

2) Turn the boiler off at the programmer and at the isolator switch if safe to do so. If unsure, leave it off and call a Gas Safe engineer. Do not try to dismantle the boiler or gas valve yourself.

3) Only perform non-invasive checks (visual, pressure, reset). Any testing involving the gas valve, PCB, internal wiring, or opening covers must be done by a Gas Safe engineer.

Initial homeowner checks you can safely do:

1) Note the exact error (D.30) and any other displayed codes or lights. Photograph the display and any behaviour (e.g. constant lockout, intermittent).

2) Try a boiler reset: follow the boiler’s reset procedure (hold the reset/reset button for a few seconds). If the code returns immediately or on restart, stop and call a pro.

3) Check mains power and timers: ensure the boiler has power, the fuse/trip hasn’t operated and the programmer/cylinder thermostat hasn’t disabled the boiler.

4) Check system water pressure on the pressure gauge. If below about 1.0 bar, the boiler can go into safety modes. If you are confident using the filling loop, repressurise to about 1.0–1.5 bar and re-check. If pressure drops repeatedly, that indicates a leak or expansion vessel issue which needs an engineer.

5) Listen and look: with the boiler calling for heat try to hear the pump and fan running. Check if radiators are heating normally. If the pump is not running or radiators are cold while the boiler should be running, circulation faults can cause temperature limits and safety trips.

6) In freezing weather check the condensate pipe for freezing and the flue/air intake for obvious blockages. Thaw frozen condensate pipe carefully before restarting.

Specific diagnostic and (homeowner) fix steps to try before calling an engineer:

1) If low pressure was the only issue, repressurise to 1.0–1.5 bar, reset the boiler and observe whether D.30 clears and the boiler runs normally.

2) If the boiler overheated previously (very hot radiators or scalding flow when it briefly ran) try bleeding a few radiators to remove trapped air and improve circulation, then re-check pressure and reset.

3) If the fault follows a power cut or intermittent electrical disturbance, isolate the boiler for 1 minute then restore power and reset. Note if the fault consistently follows power glitches.

4) If you have multiple faults or inconsistent behaviour, record what actions cause the error (e.g. when calling for hot water only, when calling for CH, after a reset) — this information will help the engineer.

When to call a qualified engineer (required in most cases):

1) If D.30 stays present after the safe homeowner checks above, or if it returns repeatedly after reset. This error commonly needs electrical and gas component testing that only a Gas Safe engineer can legally and safely perform.

2) Call a Gas Safe engineer to diagnose: they will test flow and return NTC sensors and their wiring, measure sensor resistances/temperatures, test the water pressure sensor and volume-flow sensor, check pump operation and current draw, inspect and operate the 3-port diverter if fitted, test the gas valve assembly (resistance and local tightness), and check the PCB for faults or implausible signals.

3) Warn the engineer if you noticed overheating, repeated pressure loss, or for any smell of gas previously — these are important safety clues.

Why a professional is needed:

- D.30 can originate from faulty sensors, failing pump, blocked/defective diverter, a leaking or defective gas solenoid valve, wiring short/earth fault, or a defective PCB. Diagnosis requires electrical measurements, gas tightness testing and possibly replacement of gas-handling parts — all tasks for a Gas Safe engineer. Attempting these repairs yourself is dangerous and unlawful.

Final note: do not ignore the code. If the simple checks (reset, repressurise, thaw condensate) do not permanently clear the fault, arrange a Gas Safe visit and provide them with your observations and any photos of the error display to speed up diagnosis.