ravenheat whitestar LS80T?

Hi I have a Ravenheat whitestar LS80T and I have no central heating but still have hot runing water, if i turn the C.H on and give it a slap the boiler fires up and the heatin comes on all toasty. I have changed the diverter valve as i was told by a friend of a friend that this was probably the fault, turns out no its not. Could it be the Air Pressure Switch that is causing the prob and if so is there a tempary fix to help me out till i get a new one (i.e cleaning it out perhaps).
Any answers with my prob would be a great help
Thanks