Unit displays E04 and will not fire, often immediately after power-on

Safety: Allow the water heater to cool before touching any components. Do not bypass safety thermostats.

• 1. Power off: Switch off electrical power and close the gas 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 red reset button.
• 3. Reset thermostat: Press the reset button firmly until it clicks. If it immediately trips again on the next run, do not keep resetting; find the cause.
• 4. Bleed air from primary circuit: Check automatic air vents and manual bleed points on the ACV primary circuit. Bleed radiators or primary loop if the heater is part of a combined system.
• 5. Check circulation pump: Verify the ACV primary circulation pump is running when there is a heat demand. Listen for operation and feel the pump body for vibration. If 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 scale on the heat exchanger can cause overheating. If the unit frequently overheats, arrange for descaling or heat exchanger replacement.

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