Steam Heating Systems
Steam is probably the most fascinating type of heating system. By utilizing the unique nature of super-heated water, turned into a gas, the physics behind this help to enhance the heat transfer to the emitters (radiators). Steam systems are a type of high-temperature, high-mass system and their primary benefit is found in the heat transfer ability due to the latent heat stored within the steam.
Residential steam systems are a type of “semi-open” heating system. Typically with hot water systems, they’re completely enclosed and filled with liquid water; void of any air. With a steam system, the liquid exists in three forms – 1) as a liquid in the boiler; 2) as saturated steam after the water in the boiler has been heated; 3) as condensate (again, liquid) after the hot steam has transferred its energy into the radiator. During operation, the boiler heats the water until the boiling point. As more heat energy is absorbed by the water, and the molecules are excited, steam begins to build in the empty space above the water level. As the steam makes its way through the pipes, displacing the air in the system, small vents on the side of the radiators allow for the air to evacuate. Those who have lived with steam systems will know this sound by heart – a hissing sound. The heated steam makes it’s way into the much cooler radiator. This is where the magic happens. When the water was heated in the boiler to make steam, it underwent what’s referred to as a “phase transition”. In this case, the steam (existing as a gas) has potential energy in the form of “latent heat” and when it comes in contact with a cool surface, it condenses, releasing this energy into the radiator, and ultimately the room. This stored “extra” energy allows a much higher efficiency of heat transfer versus the heat transfer means (conduction) typically found in a static-state hot water system. The order of magnitude is in fact several times greater.
Steam systems are no longer installed in new home construction. They were most prolific during the early part of the 20th century when homes were poorly insulated. The high-mass nature of radiators, coupled with the extreme amount of heat energy found in steam, allowed for the radiators to absorb this energy, and slowly release it into a room even after the system’s demand was satisfied.