I have a Alpha CB24 boiler, that works fine for about 30 minutes when central heating is turned on, then turns itself of for about 15 minutes then comes on again for about 5 minutes. The h/w out of the hot water tap is boiling hot and you can not put your hands in the hot water. you can here a hissing sound from the boiler when you turn on the central heating. I have repressured the system 2 times in a week, and both times there was a lot of air in the radiators in the upstairs bed rooms. When the central heating is turned of the h/w from the taps is fine.