How can i fix my voiler, vokera Sintese 25 e?

i have got hot water and heating, but even when it is off the heating keeps on. When i turn on the water the heating temperature increses as well. I have spent so much on gas because of that. So i have kept the whole sistem off from power oterwise the heating turns on by itself. Any idea what the problem is and how to fix it? Cheers