I have a duel heating system, oil and multi fuel stove/fire, these are fed into a duel circuit cylinder for heating rads and water on a direct system, recently the cold water supply froze without us realising, and now when I use the solid fuel stove,the moment the pump kicks in I have warm water flowing up the expansion pipe and into the header tank,( it is a separate tank to the main water tank and the oil fired system is uneffected) the expansion pipe is connected to the pipe on the top inlet on the side of the tank(sorry I don't know if this is the supply inlet or the return) could the freeze have blown the cylinder? And could I not just cap off the expansion pipe and allow any overflow to escape through the expansion pipe on the oil side of the cylinder, the water level in the header tank stays the same when the expansion runs into it, so I am assuming the system is re- circulating the water that's coming into the tank.