I just turned on the heater after last winter. Before I turned it on, the hot water is fine. But after I turned it on, the pressure drops quickly to zero (in a few minutes). With the boiler on, when I bleed the radiator yesterday, there is air comes out, and then I wait until the water come out. However, this morning, after I turned the boiler on, I need to bleed the radiator again, and there is also a fair amount of air come out. After lunch time, again, there is a fair amount of air come out from several radiators... Then I turned off the boiler, and when the radiator become cold, I intend to bleed them again, but then, there is no water or air comes out (for all 5 radiators in the house). I checked the radiator, and the boiler, and the pipe outside of the house connecting to the boiler, and I cannot find any leaks, I also cannot find any dark/damp patches on any part of my floor (most of them are covered by carpet). I just don't know what the problem is, and have been driven mad... I am wondering whether anyone could give me some clue or advise... thank you very much.