Heater shuts down with E04; may occur during long draws of hot water, sometimes accompanied by very hot outlet temperature.

Safety: Turn off electrical power and close the water supply. Allow the unit to cool completely (at least 30–60 minutes) before touching internal components.

• 1. Reset safety thermostat: Remove the cover and locate the manual reset thermal cut-out (often on the heat exchanger block). Press the reset button if present. If it immediately trips again, do not continue to reset without finding the cause.
• 2. Check water flow: Ensure inlet filters and strainers are clean. Fully open isolation valves. Low flow can cause overheating.
• 3. Inspect for scale: In hard-water areas, limescale can restrict flow inside the heat exchanger. Look for signs of overheating (discoloration) on the copper or stainless block. Descale the heat exchanger according to Kospel instructions or replace if severely blocked.
• 4. Element control check: If elements remain energized when they should be off, a stuck relay or triac on the main PCB may be causing overheating. This requires board diagnosis and likely replacement.
• 5. Verify sensor readings: Confirm that NTC sensors are correctly installed and reading accurately; incorrect readings can delay shutoff and cause overheat.

If E04 recurs frequently, have a professional check the hydraulic and electrical system; persistent overheating is a serious safety issue.