The boiler fires but quickly overheats or shuts down; radiators remain cold at the top, and the pump is noisy with rushing water sounds.

Safety: Allow hot components to cool before bleeding. Protect surrounding areas from water spills.

User actions:

• Bleed radiators starting from the lowest to highest points using a radiator key until water flows steadily.
• Monitor and maintain system pressure via the filling loop while bleeding.

Engineer actions:

• Check the automatic air vent near the Grant boiler and pump; ensure the cap is loose (open) and vent is not blocked.
• Manually vent high points in the pipework and the pump housing if it has a bleed screw.
• Verify correct pump orientation and speed setting to ensure adequate circulation.