Boiler locks out with E02, burner stops, circulation pump may continue to run for a short time. Flow temperature rises rapidly before shutdown.

Safety first: Switch off electrical power to the boiler. Allow the boiler to cool completely (at least 30–45 minutes) before touching any internal components. Do not bypass or short any safety thermostat.

Step-by-step checks:

• 1. Check system pressure: On sealed Viadrus systems, verify the pressure gauge reads typically 1.0–1.5 bar when cold. If below 0.8 bar, top up via the filling loop according to the manual and bleed radiators.
• 2. Reset the overheat thermostat: Locate the manual reset safety thermostat on the flow pipe or heat exchanger (often a red button). Press firmly until you feel a click. Restore power and test. If it trips again, do not keep resetting—find the cause.
• 3. Bleed air from the system: Bleed air from the boiler’s automatic air vent (if fitted) and from the highest radiators. Air locks can stop circulation and cause rapid overheating.
• 4. Check circulation pump operation: With a call for heat, listen for the Viadrus circulation pump running. Lightly touch the pump body (careful, may be hot). If silent and cold, check the pump fuse and wiring. If the pump hums but does not circulate, the rotor may be seized—remove the pump front screw and try to turn the shaft with a screwdriver.
• 5. Inspect primary heat exchanger: If the boiler overheats quickly even with good circulation, the cast-iron or steel primary heat exchanger may be partially blocked with sludge or scale. Check for uneven temperatures across sections. A chemical flush or mechanical cleaning may be required by a professional.

When to stop: If the overheat thermostat trips repeatedly, or if you suspect a blocked heat exchanger or failed pump motor, contact a qualified heating engineer familiar with Viadrus boilers.