The unit displays E04 and will not fire, often immediately after power-on.

Safety: Allow the water heater to cool completely before touching any internal components. Never bypass safety thermostats.

• 1. Power off: Switch off the main electrical power supply and close the gas isolation valve.
• 2. Locate safety thermostat: On many ACV storage water heaters, a manual reset safety thermostat is mounted on the tank or heat exchanger. Remove the front panel and locate the small red reset button.
• 3. Reset thermostat: Press the reset button firmly until you hear a click. If it immediately trips again on the next operation, do not continuously reset it; instead, thoroughly investigate the underlying cause.
• 4. Bleed air from primary circuit: Check automatic air vents and manual bleed points on the ACV primary heating circuit. Bleed any radiators or the primary loop if the heater is part of a combined heating system.
• 5. Check circulation pump: Verify that the ACV primary circulation pump is running when there is a heat demand. Listen for its operation and feel the pump body for vibration. If the pump is seized, try freeing the shaft via the pump vent screw (if provided); otherwise, replace the pump.
• 6. Check for scale: In hard water areas, excessive mineral scale buildup on the heat exchanger can cause localized overheating. If the unit frequently overheats, arrange for professional descaling or heat exchanger replacement.

If the E04 error recurs after the reset and circulation checks, contact an ACV service technician to inspect the heat exchanger and control system.