The boiler fires but quickly overheats; the flow pipe gets very hot while the return pipe stays cool, and some radiators remain cold.

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Seized pump rotor, Electrical failure of the pump motor, Blocked pump impeller due to sludge, Incorrect pump speed setting

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

User checks:

Place a hand on the pump body (if accessible) to feel for vibration or noise indicating operation (do not touch if hot).

Engineer steps:

Confirm voltage at the pump terminals when there is a call for heat.
Remove the pump head and check the impeller for blockage; free it or replace it as necessary.
Set the pump speed according to system design and Grant recommendations.
Replace the circulating pump if it has electrically failed or is excessively noisy.

Medium 3/5

Circulating pump

Check Price on Amazon

We may earn a commission from links on this page.