Chaffoteaux combi boiler

[Manivelle]

Hello, Can anyone help with this
I have a Chaffoteaux wallmounted combi boiler, heating works perfectly but when using hot water, boiler goes on and off about every 15 seconds ie 15 secs on, 15 secs off so the water temperature is varying all the time. I live in a very hard water area, could this be limescale in the heat exchanger making the thermostat think its overheating, if not what could it be? This has only recently started, I'v had boiler from new about 6 years.
The exact model is Elexia II, I live in France, although this is not a uk model I assume the principles are the same. I called an engineer who only seemed to want to sell me a new boiler not interested in fixing it
I am extremely technically minded (retired electronics endineer) and very diy capable
Thanks in advance for any info,
Alex

 

[PEDANTICVINDICTIVEMAN]

Most likely blocked DHW plate heat exchanger, I would take it off and flush it through with a hose pipe, could also be hw sensor, I have had to do both at the same time in the past.

 

[45yearsagasman]

It does sound like a scaling/restriction type of fault.These are not marketed in the UK and can only guess at your fault.