Hi, As title says, my boiler it consistently losing all pressure overnight when cold. (Been doing it for months now so, no signs of any water contamination on ceiling etc) I have had my local gas engineers out four time now, I've had a leaking pipe inside the boiler replaced, I've had the expansion vessel recharged and all visible pipes checked for leaks. The boiler works perfectly all day/night until it goes off at night then won't fire in the morning as the pressure is so low. The last engineer that came out, I mentioned a knocking noise to him, which he said is the pump on the way out (never mentioned it until I said to him). Could this be the cause of the drop of pressure overnight? I also don't think it's the PRV as I don't seem to have any leaking water outside where the drain valve is... And the pressure only rises slightly when the boiler is running. Have you guys any ideas? As I am fed up of constantly having these guys out for them to do nothing to solve it! Cheers.