The benefit of any type of water-based radiator is that it is very cost-effective and does not deliver dry, heated air to the rooms, as does a forced-air system. Unlike steam radiators, hot water radiators do not have the added benefit of increasing the humidity in the rooms--often a necessity in dry winter months.
Hot water radiators can be either the one-pipe or two-pipe system. In the one-pipe system, hot water leaves the furnace and moves in a continuous loop, returning back to the furnace as cooler water. This cooler water is re-heated and sent out on its journey again. The two-pipe system moves hot water to the radiators in one pipe and returns it to the boiler in another pipe.