Boiler runs but radiators remain cold or only lukewarm.

Safety first: Turn off power before working on valves or pump.

• Check boiler temperature: Confirm the boiler flow temperature rises to the setpoint; if it does, the issue is likely in the distribution system.
• Verify pump operation: Feel the pump; it should vibrate and be warm. If not, free or replace it as described earlier.
• Bleed radiators: Bleed air from all radiators, starting with the highest.
• Check motorised valves: Ensure any zone or diverter valves are moving to the open position when heating is called. Manually move the lever to test.

When to call a professional: If circulation cannot be restored, a heating engineer should assess the system.