The calorifier should have 2 coils in, one for the central heating and heating the water connected to the header tank and another totally separate for the engine to heat the water.
But it sounds as though you only have one coil with everything connected, engine coolant, central heating, expansion tank and whatever heat source runs the heating. If so its a daft system and I have never seen one like it.
How many connections are on the calorifier? There should be at least six, 2 + 2 for 2 coils, cold in and hot out.