heat exchange valve valiant turbo max boiler?

i had a boiler installed by warm sure-ideal a few years back. the gas valve circuit and cable have been changed twice and have now been told problem is with heat exchange unit. they say it is my fault it broke becaue of hard water but the system has always been used with an inhibitor. is hard water really likely to be the cause. also should i cut my losses and change the whole boiler or keep mending this system. the grant peolpe are now saying hard water problems not covered by grant. please help!!!