Overheat DHW on glowworm combi 24c

[peepod]

Hi clever people, First time user.
I have an overheat condition (cycles on limit switch o/p from primary heat exchanger) when using DHW at high volumes, gas doesn't seem to modulate, just high, CH fine, flow modulates okay. I have tried DHW sensor to no avail, pump, diverter valve seem okay. DHW okay at low volumes when DHW temperature kept under 40 degrees. Flow sensor sounds as if paddle spinning, no idea how to check o/p at sensor as three wire connection, assume o/p is developed across some kind of load on PCB. Boiler relatively new. Boiler has two inline magnetic water scale reducers.
I'd appreciate any help!

Regards, Peepod

 

[Vectra]

again late reply ( but others may find it useful)

Have you tried the resistance readings of the thermistor?? depending on what type (most are NTC) The readings will differ as the temperature changes, from low resistance to a high resistance, check these against the manufactures instructions. If the readings are ok then have you checked the plate heat exchanger?? Usually common to fail if the system has not been flushed or protected with inhibitor. Normal signs of a faulty plate is when you turn on the tap the water goes hot and cold as the boiler cycles, the boiler stuggles to clear heat as it relies on the mains passing through to dissipate the heat keeping it at full cycle. ( the boiler side of the plate, not the domestic side)

Also recently come across a 35k/w boiler in a flat when the tap was on, and the customer set dhw stat on boiler to max, the flow rate was too slow to cope and the boiler kept cycling,