Why does hot water come out of the overflow in my central heating system?
If your heating system has an expansion tank on it the bladder in it may be defective leaving expanding (expands when the heat comes on) water no place to go, except out a pressure relief valve. When the heats turns off and cools down the water contracts dropping the pressure in the system and at that point there is a valve that will let water in to maintain the pressure the system is supposed to have starting the cycle over again. I am not sure of all the particulars of your system but this is one definite possibility.
