combination boiler

Hi. i have a combination boiler. When i switch it to hot water and turn the tap on, the boiler does not fire up. If i switch boiler to hot water and heating then the boiler will fire up. if i turn the thermostate control for the heating down to 0 (which is in the dinning room)then the boiler will cut out. So that leaves me with the problem of i cannot have hot water unless i have the heating on as well. Could someone please help and tell me why they think the boiler does not fire up on the hot water switch and why it cuts out on the hot water/heating switch