Control displays E04 and the furnace shuts down; often occurs after overheating or poor circulation, heat exchanger very hot to the touch.

Safety: Turn off power and allow the Viadrus furnace and system water to cool completely before touching any safety thermostats or pipework.

• Check system valves: Ensure all flow and return valves near the furnace and on main heating circuits are fully open. Verify radiator valves are not all closed.
• Clean system filter: If your Viadrus installation includes a Y‑strainer or magnetic filter on the return line, isolate it, place a container underneath, and remove the cap. Clean out sludge and debris, then reassemble and open valves slowly to vent air.
• Reset safety thermostat: Locate the manual reset safety thermostat on the flow pipe or heat exchanger (usually a small device with a red button). Press the button firmly until it clicks. Do not repeatedly reset without finding the cause of overheating.
• Check circulation pump: With power restored, call for heat and verify that the circulation pump on the Viadrus furnace or external pump station runs and vibrates slightly. If it is hot but not turning, the rotor may be seized. Many pumps have a front screw you can remove (with power off) to manually free the shaft with a screwdriver.
• Bleed air: Bleed air from radiators and any automatic air vents near the furnace. Low flow due to air locks can cause rapid overheating.
• Persistent E04: If the thermostat trips again quickly, the pump or system design may be inadequate. Contact a heating professional to check pump sizing and system hydraulics.