My daughters had a new Grant Vortex Combi installed 4 years ago. It has been a nightmare since the first routine maintenance . She lost all hot water, had a new large exchange unit fixed £300. Then all hot wat stopped inc CH. Same people repaired it with new air valves. Then CH stopped and the 'new' valve was found to be faulty and replaced. Now the water pressure keeps going down to zero and the boiler works only when we use the two taps to increase up to 1.5. Any idea what can be the cause of reducing water pressure...there is no leaking we can see?