My heating system has two expansion tanks. It's a radiator hot water system that evidently at one time worked on a gravity feed system. (It has four inch pipes coming from the furnace which reduce down in size at the furnace, which is fairly new.) There is an old original expansion tank up between the joists that is about four feet long, and then there is a smaller, newer one (the kind with a diaphram inside) at the furnace. Do I need two of these things? I'd like to get the old one out of there if I can.