what's the point of these systems, compared to just using hot water? The main advantage is probably that you don't need any pumps. This can increase efficienc

what's the point of these systems, compared to just using hot water? The main advantage is probably that you don't need any pumps. This can increase efficiency, and removes a major point of failure. The other big difference is that phase transition from water to steam is very high energy. By weight, steam transports 6x the heat of liquid water, and releases 5/6 of this as soon as it condenses back to water. Depending on the situation, this can be an advantage. In general, I don't think that either system has a tremendous advantage -- they are just different. For modern systems, advantage probably leans to hot water.