Boiler runs but radiators or underfloor heating remain cold; domestic hot water may still work.

Safety first: Turn off power before touching internal components. System water can be hot and under pressure.

• Check valves: Ensure the boiler's heating flow and return isolation valves are fully open. Verify any external zone valves are open when there is a call for heat.
• Bleed air: Bleed radiators and any high points in the system to remove trapped air that can stop circulation.
• Check pump operation: With power on and a heating demand present, listen and feel for the circulation pump running. If silent or excessively hot to touch, it may be seized.
• Diverter valve check: If DHW works but heating does not, the diverter valve may be stuck in DHW position. A technician can test and replace the diverter assembly.