Potential overheating in hot water circulation area

Question from David Phillips, via email:
I have an Ideal Classic FF250 boiler running a fully pumped system. For some weeks now, when it is heating the hot water the circuit, there is a rumbling and racing water sound in the pipes and the boiler will cut out by activating the overheat button.

Sometimes it will work fine for four or five days then do it again. It does it less when the boiler thermostat button/adjuster is turned down to less than half it’s potential.

I have replaced the pump, the PCB, the overheat thermostat and the programmer – but still the problem persists. Are you please able to point me in the right direction regards this problem please?

Answer from Martyn Bridges, director of marketing and technical support at Worcester, Bosch Group:
This sounds like an overheating situation, specifically in the primary hot water circulation area.

The obvious “non-boiler” consideration would be to investigate the motorised valve to the cylinder and inspect it for partial blockage or restriction. If that appears clear and the valve energises to design when the cylinder thermostat requests, then the boiler control system may need investigating.

In this instance I would suggest the boiler manufacturers technical advice line is contacted.