Why does my central heating expansion tank fill up with hot water?
Because you have the boiler turned up high and only one rad, the pump for the heating circuit is pumping the water up the expansion pipe into the expansion tank. Your pump was designed to pump for 12 rads not one. You must be careful if your expansion tank is a plastic type because the prolonged discharge of boiling water into it, can soften the plastic, and you may have very hot water coming through the ceiling. I have seen this happen.
