I have a Worcester Bosch Greenstar 24i boiler with strange pressure fluctuations. For the last couple of days I noticed that the pressure was reading zero in the morning when cold. I also noticed a slow drip coming out of the drain valve thanks to it only being hand tight. So, I assumed that the pressure had dropped due to the water leak (albeit small). After tightening the drain valve I re-pressurised the system (when cold) back to 1.5. However, when the boiler is on, the pressure goes up to 3 bar and into the red. I've bled the nearest radiator to reduce the pressure back to 1.5 when the boiler is on (not sure if I should do that while the boiler is running?). And then it sits happily around 1.5-2 bar. However, when the boiler has been off for a while, the pressure is near enough back to zero after cooling right down which has forced me to re-pressurise to get the pressure back up to 1. I've been through this process a few times of re-pressurising when cold but then having to reduce the pressure when hot. Normally the boiler pressure is 1 and goes up to 1.5 when warm. I'm not especially technical, but from what I've read could it be my expansion vessel? Or could there be trapped air causing this? (FYI I've bled most of the radiators and none at the time had air in them). Any help would be appreciated.