Residential Steam Systems

Mon, November 11, 2016

Not all hydronic heating systems are water or glycol based. In this area, and on the East Coast, there are still a fair number of steam systems. Many of these systems are residential, and while they are not being designed and installed for new construction, they still represent an important niche in the hydronic market. Just because steam is thought of as an old technology doesn't mean steam systems can't operate efficiently. Steam systems are generally not installed in new residential construction because they require more maintenance than water systems, and have a reputation for being more difficult to control than water based systems. While it is true that steam systems require regular maintenance, and are generally less efficient on the gas side than water systems, they also use significantly less electricity than water systems because they require no distribution pump(s), and gravity systems have no electrical requirement except for the burner and boiler controls. Properly maintained and controlled steam heating systems can be made to operate near the overall system efficiency of high temperature, set-point controlled water based systems. Steam systems operate by boiling water, and when the water turns from liquid to vapor, it wants to expand. One pound of steam occupies roughly 1,787 times the volume of one pound of water at 0 PSI. The piping in the system contains this expansion and pressure is created. Like water, steam goes from high pressure to low pressure, and this differential pressure between the boiler and atmosphere (or vacuum pump) drives the "circulation", or distribution of steam through the system. Steam cannot move anywhere unless the air that was there is removed from the system! Venting is therefore responsible for the rate and path of steam distribution. Vents must be chosen for capacity, application and working pressure. Adjustable vents can be used to "balance" steam systems and ensure even distribution so a system isn't satisfied in one area and starved in another. Thermostatic vents mounted on zone valves, like those for Danfoss can be used to zone steam systems and control overheating in one-pipe steam systems. The energy in steam comes from two components: sensible, and latent heat. Sensible heat is energy that relates to a change in temperature, and it takes 1 BTU to heat 1 lb. of water 1 degree F. Latent heat is the energy that changes the state of the fluid from liquid (water) to vapor (steam). Steam tables tell us information like the ratio of latent heat to sensible heat for steam at any given pressure. For example, one pound of 2 PSI Steam contains 187 BTU sensible heat, and 966 BTU of latent heat, with a temperature of 222 F. The lower the steam pressure, the greater it's ratio of latent heat to sensible heat. Steam does work in heating a system when it condenses, and gives off latent heat. After it has condensed, it is still the same temperature as it was at the pressure of the steam at that point. If steam is not contained without condensing first, the latent heat is wasted up the stack. It is the job of the steam trap to distinguish steam from air (by temperature), and steam from condensate (by state) and keep it in the heat transfer device (radiator or heat exchanger) until it has done it�s work and condensed. Companies like Hoffman manufacture several different styles of traps (thermostatic, float & thermostatic, thermo-disc, bucket and other specialty styles), and it is important that traps are selected properly, not only for load and working pressure, but also for application. When steam condenses, it "shrinks" in size (also roughly 1,787 times) and a vacuum could be pulled if it weren�t for a device called a vacuum breaker. The vacuum breaker prevents the condensate from �hanging up� in the terminal unit. If condensate is left in the terminal unit, noise often results from water hammer, and flashing. In order to enhance efficiency, water based system can operate on "outdoor reset" when the water temperature of the system varies in relation to the outdoor air temperature. Steam boilers can also operate using the principal of outdoor reset, but rather than varying temperature, controls from manufacturers like tekmar vary the timing of the firing cycle in relation to outdoor air temperature. Steam reset controls enhance heating system efficiency by preventing "short-cycling" of the boiler, and reducing the tendency of steam systems to overheat under light-load conditions. Most steam systems bear the signature of those that have designed and worked on them. Unfortunately, much of the knowledge that goes into the design of steam systems is being lost to time. Luckily, there places like Bell & Gossett�s Little Red Schoolhouse that offer you the opportunity to learn about steam applications, design variations, and maintenance. Call Bornquist and your local stocking wholesaler for the steam specialty equipment and knowledge that can maximize the efficiency and operation of your steam heating application.