Boiler displays A04 and will not start, often after power interruption

Safety: Ensure the boiler is cool before adding water. Overfilling can cause relief valve discharge.

Steps:

• Read system pressure: Check the pressure gauge or digital reading. If below the recommended minimum (often 0.8–1.0 bar cold), the boiler will lock out.
• Top up the system: Locate the filling loop (flexible hose or integrated filling tap) under the De Dietrich boiler. Slowly open the valves until the pressure rises to the target range (usually 1.2–1.5 bar). Close both valves firmly afterwards.
• Bleed radiators: After filling, bleed air from radiators and towel rails. Re‑check pressure and top up again if needed.
• Inspect for leaks: Look for visible water around radiators, valves, and under the boiler (especially around the hydraulic block and safety valve discharge). Persistent pressure loss indicates a leak.
• Reset the boiler: Once pressure is correct and stable, reset the boiler via the control panel.

Professional intervention: If pressure drops repeatedly, or if the pressure sensor reading is erratic, a technician should check the expansion vessel pre‑charge, pressure sensor, and the system for hidden leaks.