Display shows H1, furnace continues to run at reduced output or locks out

Safety first: Isolate power and allow the system to cool before touching pipework.

• 1. Check system pressure and circulation: Ensure system pressure is correct and that all radiator valves are open. Listen for the pump running and check that flow and return pipes warm up evenly.
• 2. Bleed air: Bleed radiators and the boiler's automatic air vent to remove trapped air that can reduce circulation.
• 3. Pump speed setting: On models with adjustable pump speed, set the pump to a higher speed and observe if H1 occurs less frequently.
• 4. Flow NTC check: Test the flow temperature sensor resistance and replace if it reads incorrectly high compared to actual pipe temperature.
• 5. Heat exchanger cleaning: If overheating persists, the primary heat exchanger may be partially blocked by scale or sludge. Cleaning or replacement should be done by a qualified technician.