Vovlo 240 heating

I have a Volvo 240 estate that is using a lot of water in its cooling system. I cant find where but it seems to warm the haeting system up and looks almost as if ssteam is coming through the vents into car at times - I topped up the overflow water system yesterday and then when i had been out I checked the water - some had gone but the water that was left was cold. Does this water not circulate and could it be a thermosta jammed open or closed? Thanks Rich Westerly 2219 DAVICO