A steam heating system is still a hydronic system, but instead of feeding hot water to the radiators, it sends steam. The steam enters the radiator and fills the radiator up. In doing so, it pushes back air in the system with either a vent or return lines.
Radiators are usually convertible. A hot water rad can operate as a steam rad most of the time. A dedicated steam rad cannot be converted to hot water. A steam radiator does not need to be connected at the top, so if you try to convert that to water, there is no way for the water to come back in the radiator. The hot water comes in and stops because it has nowhere to go. But the steam will fill up the rad whether or not the top passage is connected.
Steam heating systems are generally used on large buildings, schools, and churches, etc., because of their ability to get the energy a long way in a speedy fashion. It is not an efficient system for homeowners because it consumes significantly more energy, and has significantly less control than a standard hydronic system. In Toronto, there are very few houses that run on steam.
Steam is very powerful; when water multiplies it increases in volume 1600 times. A small boiler, half full of water enough to heat a school, is less efficient when it’s no longer sending 140-degree water to a radiator. In order for the boiler to make steam at 212 degrees, it must burn significantly more energy getting the water hot enough to boil, versus a conventional hydronic system that needs lower design temperatures.
It may be fair to say – modern day steam is a ‘dead man’s art’; heating wise, if you’re working on a steam system with radiators, it’s going to be an old system. Today, there are no new construction buildings that will ever be done in steam due to the inefficiency. The only practical applications for steam heating systems nowadays is for processing; generally cheese factories, breweries, dry cleaners, etc., places like these will still install steam systems to operate their equipment.
Steam is pretty old technology. Before the invention of circulating pumps, installers had no efficient way to get water distributed to far away distance in a building. With the original hydronic system, the water was heated and rose through the building by convection. It had large piping diameters, and all pipes were sloped to the radiators and the radiators had a large inlet connection. The water would literally get hot, be less dense than the cold water, and rise through the system by convection. The colder water would then fall back to the boiler on its own.
When churches and schools were designed and constructed, there needed to be a way for pipes large enough or sloped enough, to travel the extent of the buildings and get water there hot. Nowadays we have forced circulation, there’s a pump on the boiler and it can take that hot water from that boiler and send it a thousand feet in seconds. So, with the advent of modern technologies, we’ve kind of obsoleted steam as a heating system.