my daughter c/h header tank is over flowing and its not due to a sticking ball cock or the vent pipe over the tank thats causing the problem. It only happens when she puts the timer switch to H/W only and it would seem that the filler pipe at the bottom (as if there was a back pressure) of the tank is forcing water back into the tank. If she turns on a hot water tap the o/f pipe stops leaking. Its a 2 pump system, the cylinder stat controls the hot water pump and the boiler stat controls the C/H pump. this problem has only occured in the last couple of days. Doe's anyone have any ideas as to why.The rads have TRV's on them and there are 2 rads ( without TRV's) on the hot water system.