......sorry FF..........but I don't understand a word of this :o)
Your warm air system - is it electric, or perhaps a boiler with some kind of heat exchanger..........
Your small expansion tank should only be connected to the indirect primaries that flow through your cylinder..........if the supply is cut off, then it can only fill sort of backwards from the boiler primaries..............(still with me?)
Do you have a bigger tank in the roof that supplies the HW cylinder?
Need more info I'm afraid