Boiler shuts down with E04; display may indicate overheat or high limit; unit very hot to touch

Safety: Turn off power and allow the boiler to cool completely. Do not open any pressurized components while hot.

• Check system valves: Ensure all NTI boiler isolation valves on the supply and return piping are fully open. Verify zone valves are not manually closed.
• Verify circulation: During a call for heat, feel the pipes on the boiler supply and return. If the supply is very hot and the return is cool, the system pump may not be running or there is an airlock.
• Bleed air: Use automatic air vents or manual bleed valves on radiators and near the boiler to remove trapped air. Keep system pressure within the recommended range (typically 12–20 psi for residential systems).
• Check pump operation: Listen and feel for vibration at the primary circulation pump. If it is silent or extremely hot to the touch, it may be seized or failed. A technician should test voltage at the pump and replace it if defective.
• Reset high limit: Some NTI models have a manual reset high‑limit thermostat on the heat exchanger. After cooling, press the reset button if present. If E04 recurs, further diagnosis of flow and heat exchanger condition is required.