Vaillant TurboMax Problem - Help please?

Hi,

I have a Vaillant Boiler Turbo Max around 11 years old.

Now the boiler has stopped working as it should.

If you open the hot water tap, you get no hot water and the radiators start heating up, thats with the central heating off.

So, I called a plumber, he came around, quotes me £180 for a diverter valve, which he believed to be faulty. The price was abit much for me, so I called another engineer, who came around, checked the boilder and said the diverter valve was fine and that the Heat exchanger needed replacing.

So, he replaced the heat exchanger, and bingo, we have hot water not as hot as it used to be, before the boiler conked out, bearable, I would say.

BUT, the rads still heat up when the hot water is running.

I have told the engineer, he believes it may be a censor, but assured me that he will sort it out.

Can anybody shed some light on this?

If any additional info is required, please let me know.

Thanks.