E04 displayed and unit will not operate; sometimes accompanied by tripped safety thermostat

Critical safety warning: Overheat faults can indicate dangerous conditions. If there is any sign of scorching, burning smell, or water boiling/venting, isolate power and heat source immediately and call a qualified engineer.

Basic checks:

• Isolate power: Turn off the fused spur and, if applicable, isolate the boiler or immersion circuit.
• Locate overheat stat: On many Gledhill units, the manual reset overheat thermostat is mounted on the cylinder front or on the immersion heater boss, with a small red reset button.
• Reset: Once the unit has cooled, press the reset button firmly until a click is felt. Do not repeatedly reset if the fault recurs quickly.
• Check circulation: Ensure the primary pump (store pump or boiler pump) runs when there is a heat demand. A seized or air‑locked pump can cause rapid overheating.
• Inspect motorised valve (if fitted): A closed or stuck 2‑port/3‑port valve on the primary side can also cause local overheating. Verify valve operation.
• Thermostat test: If the overheat stat will not reset or remains open‑circuit when cool, it must be replaced like‑for‑like.

Do not bypass any safety thermostat. If the unit overheats again after reset, stop using it and arrange professional diagnosis.