Display shows E04 and the burner will not start; unit may attempt ignition several times.

Safety: Turn off power and allow the water heater to cool completely. Do not bypass any safety thermostats.

• Check system pressure and flow: On Weishaupt combi or storage water heaters, verify that the heating circuit pressure (if present) is within the recommended range and that all isolation valves are open.
• Bleed air: Use the automatic air vent or manual bleed valves on the unit and nearby radiators (if connected) to remove trapped air that can cause local overheating.
• Inspect circulation pump: Listen for pump operation when there is a heat demand. If the pump is silent or very hot to the touch, it may be seized. Some pumps have a front screw that can be opened (with power off) to manually free the rotor.
• Clean primary heat exchanger: If accessible, inspect the Weishaupt primary heat exchanger for scale or sludge buildup. Chemical descaling should be performed by a professional.
• Reset STB: Many Weishaupt units have a manual reset safety temperature limiter on the flow pipe or heat exchanger. After cooling, press the reset button if present. If it trips repeatedly, call a technician to diagnose the root cause.