The simplest all-water system is a system of hydronic
(hot-water) convectors. In this system hot water
from a boiler or steam-operated hot water converter is
circulated through a finned tube, usually mounted horizontally
behind a simple metal cover which provides an
air inlet opening below the tube and an outlet above.
Room air is drawn through the convector by natural
convection where it is warmed in passing over the
finned tube.