i got a small problem at home with my gas ch/hw system..its a gas fired open vented system with an expansion/header tank in the loft....when the heating is on the expansion tank overflows, been up to the loft and its full of steaming hot water..the overflowing is a drip,drip drip not spurts..you can also hear drip drip drip in the loft too..ive tried the ball valve thats all working fine..ive turned the pump down to one..and although its probably slowed it down its still dripping..any ideas?