Chaffoteaux Britony Help please

[Oldbones]

Hope someone can help as getting most frustrated .
My sister has a Chaffoteaux Britony 80 Combi boiler that has developed a few noises.
It happens with the hot water, after a few mins you get a humming noise that continues until its like a plan overhead then the boiler shuts down and you then get a loud sound like a propeller slowing down.
Have done the usual thing of diverter valve and flow switches .?

 

[kevplumb]

dhw plate heat ex is full of carp

 

[nicknox]

The primary thermistor will also cause this fault. Overheating.

 

[bengasman]

Simple way to find out if the system (and consequently the hex) is sludged up.
Run heating for half an hour and feel along the bottom of the rads (careful, hot! ) if the middle feels colder than the sides, move your hand up along the rad and see where it gets warm.

 

[Oldbones]

Hum heat exchanger full of carp that sounds fishy kev, but on a serious note have had the dhw exchanger off and ran water through it seems to flow ok but with the resistance of the fins in the exchanger its hard to tell.
The thermister may be the problem will replace it tomorrow anyway. Just out of interest if the primary thermister was the fault is there another on the boiler that shuts it down when the temp gets to high but not the pump, as the second noise seems to be the pump spinning pushing the water but with no flow..