Either system uses the pipe to supply radiators with hot water and return the cool water to the boiler. A circulator is used to pump the water through the pipes. The vertical tube, cast iron type, is installed standing on the floor. They are located at floor level against the outside wall in an enclosure. Convector radiators also have finned tubing enclosed in a rectangular box, usually metal.
Air circulates up from the floor level through the tubing and then flows out the top into the room.
Some convector systems use a fan to force the heat out of the convector. The hot exhaust gases travel through the vent and out the chimney. Because of design improvements, there are other types of gas-powered furnaces and boilers. Check with a reputable dealer for more information. Whatever type of gas unit you buy, be sure the models you are considering are Design Certified by the American Gas Association.