Boiler locks out with E04, no heating or hot water, temperature rises quickly before shutdown

Safety first: Turn off power to the boiler and allow it to cool completely. Do not repeatedly reset an overheating boiler.

• Bleed radiators and air vents: With the system cool, bleed air from radiators and any automatic air vents near the ACV boiler. Low circulation due to air can cause rapid overheating.
• Check system pressure: On sealed ACV systems, verify the pressure gauge reads within the recommended range (typically 1.0–1.5 bar when cold). If low, top up via the filling loop according to the manual.
• Check pump operation (basic): When the boiler calls for heat, place a hand on the circulation pump body. You should feel vibration and slight warmth. If the pump is silent and cold, it may be seized or not powered.
• Reset overheat thermostat: Some ACV models have a manual reset overheat thermostat behind the front panel. Only reset once the boiler is cool. If it trips again, stop using the boiler and call a technician.
• Professional service: A technician should check the primary heat exchanger for scaling or sludge, verify pump performance, and test the overheat thermostat.