I had the same problem 2 years ago. The hot water also used to pour out of the overflow pipe. I woke up one day and thought it was raining, only to find water pouring thru the ceilings. A plumber reckoned the coil had gone inside the copper cylinder and the water was just syphoning into the expansion tank. I renewed the tank and also increased the height of the breather pipe and touch wood i have had no problems since. Act now as i had to have new ceilings throughout.